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thesis entitled

THE EFFECTS OF A PLYOMETRIC TRAINING PROGRAM
ON THE NEUROMUSCULAR CHARACTERISTICS OF
FEMALE ATHLETES

presented by
Angela Ann DiPasquale

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8/01 cJClRCJDatoDuopes—sz

THE EFFECTS OF A PLYOMETRIC TRAINING PROGRAM ON THE
NEUROMUSCULAR CHARACTERISTICS OF FEMALE ATHLETES

By

Angela Ann DiPasquale

A THESIS

Submitted to
Michigan State University
in partial fulﬁllment of the requirements
for the degree of

MASTER OF SCIENCE
Department of Kinesiology

2003

ABSTRACT

THE EFFECTS OF A PLYOMETRIC TRAINING PROGRAM ON THE
NEUROMUSCULAR CHARACTERISTICS OF FEMALE ATHLETES

By
Angela Ann DiPasquale

Research on the recent popularity and increase in participants in female athletics
has shown that there are a greater number of anterior cruciate ligament (ACL) tears that
females sustain in comparison to the number of ACL tears suffered by males. Prevention
programs are now being implemented in high school and college athletics to attempt to
reduce the occurrence of ACL injuries.

Twenty-ﬁve college-age female participants (height= 66.02, weight: 132.92,
age= 20.32) volunteered as participants. Eleven females were selected to the plyometric
training program, while fourteen were used as control subjects. The training program
consisted of a variety of plyometric exercises, that were performed for 45 to 60 minutes,
3 days a week, on alternating days, for 6 weeks under the supervision of the investigator.
Four testing sessions were held: one prior to the start of the study, and one after weeks
two, four and six of the training program. The testing sessions measured the subject’s
postural control, and hamstring and quadriceps strength.

A repeated measure ANOVA revealed signiﬁcant interactions for average power
and peak torque during concentric ﬂexion at the speed of 120°/sec. However, no
statistical signiﬁcance was found for the postural control or other strength variables. The
increased hamstring strength shows that plyometric training may be a good prevention
program for young females to utilize to aide in the reduction in the number of ACL tears

sustained by female athletes.

ACKNOWLEDGMENTS

I would like to thank my thesis committee members, Dr. Kavin Tsang, Dr. John Powell,
and Dr. Chris Womack, for the time that they put forth, and the guidance that they gave
through out this process. I would also like to thank the participants of my research
project. I couldn’t have done it without their time and dedication. Many thanks to my
family and friends who have always believed in me and supported me through out the

years.

iii

TABLE OF CONTENTS

LIST OF TABLES ........................................................................ v
LIST OF FIGURES ....................................................................... vi
INTRODUCTION ........................................................................ 1
CHAPTER 1: LITERATURE REVIEW ............................................... 5
Anatomy .......................................................... 6
Etiology .......................................................... 8
Risk Factors ...................................................... 9
ACL Injury Prevention Strategies ........................... 13
Plyometrics ...................................................... 16
CHAPTER 2: METHODS ............................................................... 20
Subjects .......................................................... 20
Plyometric training program ................................... 21
Testing ............................................................ 21
Statistical analysis .............................................. 23
CHAPTER 3: RESULTS ................................................................. 25
Group Comaprisons ............................................ 25
Postural Control ................................................. 25
Strength ........................................................... 26
CHAPTER 4: DISCUSSION ........................................................... 36
Effects of Plyometrics on Strength ........................... 36
Effects of Plyometrics on Postural Control ................. 37
Plyometric Training Programs ................................ 39
Clinical Implications ............................................ 40
Consideration for Future Research ............................ 41
Limitations ........................................................ 42
Appendix A: Informed Consent Form ................................................... 43
Appendix B: Plyometric Training Program .............................................. 48
Appendix C: Pictures of Testing Instruments ........................................... 58
Appendix D: ANOVA Tables ............................................................ 6O
BIBLIOGRAPHY ......................................................................... 63

iv

LIST OF TABLES

Table 4.1: Descriptive Statistics for Peak Torque of Extension at 60°/sec ................ 29
Table 4.2: Descriptive Statistics for Peak Torque of Flexion at 60°/sec ................... 29
Table 4.3: Descriptive Statistics for Peak Torque of Extension at 120°/sec ............... 30
Table 4.4: Descriptive Statistics for Peak Torque of Flexion at 120°/sec .................. 30

Table 4.5: Descriptive Statistics for Average Peak Torque of Extension at 60°/sec. . ....31
Table 4.6: Descriptive Statistics for Average Peak Torque of Flexion at 60°/sec. . . . . ...31
Table 4.7: Descriptive Statistics for Average Peak Torque of Extension at 120°/sec. . .32

Table 4.8: Descriptive Statistics for Average Peak Torque of Flexion at 120°/sec ...... 32

Table 4.9: Descriptive Statistics for Average Power of Extension at 60°/sec ............ 33
Table 4.10: Descriptive Statistics for Average Power of Flexion at 60°/sec .............. 33
Table 4.11: Descriptive Statistics for Average Power of Extension at 120°/sec ......... 34
Table 4.12: Descriptive Statistics for Average Power of Flexion at 120°/sec ............ 34
Table 4.13: Descriptive Statistics for Average Velocity .................................... 34
Table 4.14: Descriptive Statistics for Area of the 95% Ellipse ............................. 35
Table 4.15: Descriptive Statistics for Path Length ........................................... 35

LIST OF FIGURES

Figure 4.1: Flexion Average Power at 120 Degrees per Second ................. 27
Figure 4.2: Flexion Peak Torque at 120 Degrees per Second .................... 28

Figure 4.3: Flexion Average Peak Torque at 120 Degrees per Second. . . . . ....28

vi

Chapter 1
INTRODUCTION

As the popularity of female sports continues to rise, an elevation in the intensity
of competition between female athletes has occurred. This increase in the level of play
amongst female athletes places more pressure on the athletes to become quicker, stronger,
and faster to remain competitive in their respective sport. Recently research efforts have
focused on female athletes and their higher injury rates when compared with those of
male athletes. One area of particular concern is the greater rate of anterior cruciate
ligament (ACL) injuries that females sustain in comparison to the number of ACL
injuries suffered by males.

Previous studies have shown that female athletes have a higher incidence of injury
in sport}, 2 High school female basketball players were shown to have more injuries,
more time lost due to injury, require surgery more frequently3, and have a higher rate of
knee injuries.4 Particularly compelling is the number of ACL tears sustained by female
athletes.4'6 The average number of ACL injuries per intercollegiate basketball program
was reported to be 2.37 for females and .71 for males over a four year period, with
incidence rates per 100 players of 3.52 for females, and .98 for males.7 Arendt and Dick5
reported that the ACL injury rate per 1000 athlete exposures for female soccer players
was .31, which was double that of the male players (.13). Also, the female basketball
players had an injury rate (.29), four times the male injury rate (.07).

The most alarming statistic with female ACL injuries is that a majority of the
injuries are caused by noncontact mechanisms, such as landing from a jump and cutting.5

During contact injuries the cause of the injury is known, but with noncontact injuries the

reason for injury in unknown. Various research studies have been conducted to try to
determine the causes of these noncontact injuries and to identify speciﬁc risk factors for
ACL injury.of the injury in unknown.

Two intrinsic factors thought to risk factors for ACL ruptures are hamstring and
quadriceps strength, and postural control.8 The ratio of hamstring strength to quadriceps
strength is important in an athlete because the hamstrings act as a knee stabilizer. The
hamstrings help to resist anterior tibial translation at the knee, which is also the function
of the ACL, so the less the hamstrings are able to reduce the amount of anterior tibial
translation, the more strain and load that is placed on the ACL.

Females were observed by Soderman et al.9 to have a lower hamstring to
quadriceps ratio during concentric action, and that this factor was related to a higher risk
of traumatic injury. Speciﬁcally, of the ﬁve players in the study that sustained ACL
tears, all of them had hamstring to quadriceps ratios lower than 55% on the ACL injured
side than on their noninjured side.9 Female athletes were also found to have a signiﬁcant
decrease in hamstring to quadriceps ratio bilaterally when compared to male athletes.10
Male athletes tend to be hamstring dominant, make a higher hamstring to quadriceps
ratio, while females tend to be quadriceps dominant, resulting in a lower ratio.11

Postural control is controlled by joint stability and proprioceptive mechanisms.
During movements in sport postural control is constantly being altered, and the ACL
plays an important role in supporting postural control. Ankle stability is important at the
knee because the ankle inﬂuences tibial orientation and effects the positioning of the
ACL.12 Increased postural sway values, which indicate poor balance, corresponded with

higher ankle sprain rates. The athletes with poor balance sustained seven times as many

ankle sprains as those with decreased postural sway values, which indicate better
balance.13 While a paucity of research has been conducted on the effects of postural
sway values on incidence of knee injuries, it has been shown that the enhancement of
proprioception and muscular strength are equally effective in promoting joint stability
and balance maintenance. 12

An injury to an athlete’s ACL has a great impact on the person physically,
emotionally and ﬁnancially. There is a great amount of time lost from the sport, time
spent during rehabilitation, and a ﬁnancial burden due to surgery and other costs.14'16
There are numerous theories as to why this trend is occurring in females. Several
studies14'17 have looked at the various intrinsic and extrinsic factors that are different
between the genders that may be causing this increased rate of injury. Only recently has
the research13'21 started to focus on how training programs can prevent the occurrence of
ACL injuries through various techniques to improve strength, impact forces,
proprioception, and identiﬁcation of potential ACL injuring situations.

A variety of methods have been used in the recent prevention training programs.
One study20 used a video titled “ACL Awareness Program” to help decrease the number
of severe knee sprains suffered amongst experienced skiers.20 Other studies looked at the
effects of proprioception training programs on the number of ACL injuries in male and
female soccer players”, 21. A study by Hewett et a1.18 observed the effect of a jump-
training program on the incidence of knee injury in female athletes. The program had
already been shown to decrease impact forces at the knee and increase hamstring

strength.22 After participating in the plyometric jump training program, trained

particpants had a reduced incidence of knee injury when compared with the untrained,
which the investigators attributed to an increased dynamic stability of the knee joint.

Plyometric training consists of exercises that help improve muscles’ strength and
power. The basic principle of plyometrics is to preload the muscle with an eccentric
contraction, and follow that with a quick concentric contraction. The eccentric
preloading of the muscle induces the myostatic stretch reﬂex of the muscle and caused a
more forceful concentric contraction.23 Hewett et al.22 reported that plyometric training
can safely improve lower body strength and power in female athletes. Plyometric
exercises have also been shown to promote joint stabilizatioin and the use of efﬁcient,
functional motor patterns.24 Trochnanteric bone mineral content, leg strength and
balance in adolescent females all improved after a nine month plyometric training
program.25

The purpose of this study was to evaluate the effects of a plyometric jump training
program on the neuromuscular characteristics of female athletes. It was hypothesized
that the plyometric training program will improve the subjects’ postural control and
strength of the hamstring and quadriceps muscles. Any improvements seen in these
measures may help reduce the number of knee injuries in these athletes. This could lead
to the development and use of programs like this one by female athletes at the high

school and college levels.

Chapter 2
LITERATURE REVIEW

Ever since Title IX was instituted in 1972, there has been an increase in the
amount of females playing sports. The purpose of this act was to ensure equal rights to
male and female athletes at federally ﬁmded institutions.26 Within twenty years of its
enactment, the number of female college athletes rose from 10,000 to 100,000.16 With
the increase in the number of participants, female sports began to be played at a faster,
more precise and powerful skill level. Though female athletes are beginning to play at
earlier ages and have better coaching and improved skills, their injury rate of anterior
cruciate ligament (ACL) tears has not decreased.

Arendt et al.27 studied ACL injury patterns among collegiate men and women
participating in basketball and soccer. Their study reviewed data that was submitted to
the NCAA Injury Surveillance System from 1989-1999. In the latter ﬁve years they
found that the rate, based on the number of injuries per 1000 athlete exposures, of total
ACL injuries in women soccer players was .33, as compared to .12 in males. Female
basketball players had a rate of .29, while the male players had a rate of .10. These rates
are signiﬁcantly different between the genders, and led the authors to conclude that
female athletes involved in basketball and soccer suffer ACL injuries at signiﬁcantly
higher rates than college-age men in the same sports. Their results also suggest that
neither lack of skill, nor lack of years of playing experience are the primary causative
factor for injury.

Messina, Famey and DeLee4 compared the injury rates between male and female

high school basketball players over the course of one season. The results showed that the

females had a signiﬁcantly higher rate of lower extremity injuries and knee injuries, their
rate was 2.3 times greater than the rate of the male athletes. The incidence of knee
injuries in the female athletes was .71 per 1000 player-hours, as compared with the .31
for the males. The female population was also found to have a signiﬁcantly higher
incidence of ACL injury (025%), which was 3.6 times higher than the male athletes
(.007).

ANATOMY

The knee joint is comprised of the tibia, femur, patella, anterior cruciate ligament,
posterior cruciate ligament, lateral collateral ligament, medial collateral ligament, the
menisci, and numerous muscle tendons. Three different joints form the knee complex:
tibiofemoral, patellofemoral, and superior tibioﬁbular. The tibioﬁbular joint is comprised
of the articulations between the condyles of the femur and tibia, the patellofemoral joint
lies between the patella and femoral condyles and the superior tibioﬁbular joint is the
articulation between the head of the ﬁbula and tibia.

The femur is the longest and strongest bone in the body. At the distal end are the
lateral and medial condyles, which are covered with hyaline cartilage so they can
articulate with the tibia. The tibia is the major weight-bearing bone of the leg. The tibial
plateau is located at the top of the proximal end of the tibia and articulates with the
femur. The patella is located in front of the tibiofemoral joint in the patellar tendon. It
covers and protects the anterior portion of the knee.

The medial and lateral collateral ligaments support and stabilize the knee by
reinforcing the tibiofemoral joint capsule and preventing abnormal movement of the knee

ﬁ'om side to side. They provide restraint against valgus and varus stresses to the knee,

respectively. The menisci are pads of ﬁbrocartilage attached to the plateau of the tibia.
They help provide shock absorption, assist in control of normal knee motion and enhance
the total stability of the knee. The posterior cruciate ligament (PCL) is the shortest and
strongest of the cruciate ligaments. It arises from the posterior aspect of the tibia and
then passes medially to the anterior cruciate ligament and attaches to the lateral portion of
the femur’s medial condyle. The function of the PCL is to help restrain against posterior
displacement of the tibia on the femur.

The anterior cruciate ligament is approximately 3.5 cm long and 1.1 cm wide.28
It arises from the anteromedial intercondylar tubercle of the tibia and travels posteriorly
and laterally to attach to the medial wall of the lateral femoral condyle. It has two
discrete segments, an anteromedial bundle and a posterolateral bundle. The anteromedial
band runs from the proximal femoral attachment to the anteromedial aspect of the tibial
attachment, while the posterolateral band inserts on the posterolateral tibial attachment.
In ﬂexion, the anteromedial band is taught, and as the knee moves into extension, this
band becomes slack and the posterolateral band becomes tighter.29 The ACL serves as a
stabilizer against anterior translation of the tibia on the femur, internal rotation of the
tibia on the femur, external rotation of the tibia on the femur, and hyperextension of the
tibia. It also works as a secondary restraint against valgus and varus stresses.

Many muscles contribute to the movement, stability, and support of the knee
joint. The main extensor muscle is the quadriceps, which is comprised of the vastus
medialis, vastus lateralis, vastus intermedius and rectus femoris. Each has a common
attachment to the tibial tuberosity via the patellar tendon. The hamstring muscle group,

sartorius, gracilis, gastrocnemius and popliteus are all knee ﬂexors. The hamstring

muscle group is made up of the semitendinosus, semirnembranosus and biceps femoris.
The gracilis, sartorius and semitendinosus all form a common tendon called the pes
anserine muscle group, which attaches to the medial aspect of the tibia and assists in
medial stability of the knee.

In relation to the ACL, the hamstrings act as a protagonist by reducing anterior
tibial translation and internal tibial rotation during ﬂexion.3O They function with the
ACL to help provide anterior knee stability. The quadriceps also stabilize the knee joint
by contracting eccentrically. A deﬁcit in eccentric hamstring strength relative to
eccentric quadriceps strength could predispose an athlete to an ACL injury-7’l This is
especially important in female athletes who tend to be more quadriceps dominant. 11
ETIOLOGY

The ACL can be injured a number of different ways. It can sustain an injury from
either a direct contact or a noncontact mechanism. Alarmingly, the majority of female
ACL injuries occur from noncontact mechanisms.32 Arendt et al.27 reported that 63% of
women soccer players’ ACL injuries were from no apparent contact, as compared with
48% of the males’ injuries. In the women’s basketball data, no apparent contact
accounted for 80% of the injuries. Noyes et al. 33 found that 4/5 of ACL injuries
occurred from noncontact injuries, with the majority occurring when landing ﬁ'om a
jump.

Three main noncontact mechanisms have been found: planting and cutting,
straight—knee landing, and one-step stop landing with the knee hyperextended.32 These
mechanisms put the knee into hyperextension or externally rotate the tibia with the knee

in a valgus position and therefore place an excessive rate of strain on the ACL.

Noncontact injuries typically occur during deceleration of the body. 16 The “position-of-
no-return” has also been described as a noncontact mechanism. This position is

described as the back in forward ﬂexion, the hip in adduction and internal rotation, 20-

300 of knee ﬂexion, external rotation of the tibia and forefoot pronation.34
RISK FACTORS

There are multiple factors that contribute to the females’ increased rate of
noncontact ACL injuries. These factors are classiﬁed as intrinsic, extrinsic and both.26
Intrinsic factors are uncontrollable by the athlete and are related to the knee’s anatomy,
while extrinsic factors originate outside of the knee joint, and are potentially controllable
by the athlete.35 A risk factor that is categorized as both extrinsic and intrinsic in nature,
is partially controllable by the athlete. Skill and coordination are factors that are
classiﬁed as both intrinsic and extrinsic.26 Extrinsic factors include body movement,
muscular strength, conditioning, type of shoes, motivation, coaching differences, skill
level, the use of braces and deceleration forces during injury. Joint laxity, size of the
ACL, lower extremity malalignments, genu recurvatum, dimensions of the intercondylar
notch, and hormones are intrinsic factors that predispose female athletes to injury.34
Extrinsic Risk Factors

In a survey taken in 1994 by the Chronicle of Higher Education, coaches of
women’s teams were only paid 59 cents on the dollar as compared with men’s coaches-7’6
While compensation of coaches does not correlate with their quality, experience or
expertise, the ability to recruit the best coaches is often affected by the payment offered.
Also, ﬁrnding for Division I female sports is about 40% compared with male sports. This

lack of ﬁnancial support may lead to inferior equipment and athletic shoes, unsafe

playing ﬁelds, and less individualized coaching.36 These factors may contribute to the
greater number of ACL injuries in female athletes by having them participating is sports
with less experienced coaches, and in unsafe conditions and environments.

Muscular strength is important in maintaining knee stability. Studies have shown
that women have a deﬁcit in hamstring strength relative to quadriceps strength when
compared to male athletes. 10 It has been hypothesized that a hamstring to quadriceps
ratio lower than 60% in males and females can predispose an athlete to ACL injury.37
Since the hamstrings act to help control anterior tibial translation, any diminished
strength of the hamstrings could predispose an athlete to an ACL injury, by increasing the
amount of strain and load placed on the ACL.

Females also have different muscle activation patterns than males. Huston and
Wojtys11 observed that female athletes tend to activate their quadriceps ﬁrst in response
to anterior tibial translation, while the male and control subjects in the study recruited
their hamstrings ﬁrst. Shultz et a1.38 did not ﬁnd the same results in muscle recruitment
patterns between male and female athletes. The male and female subjects in their study
demonstrated similar muscle-recruitment patterns in response to a perturbation, but the
females activated their quadriceps earlier than the male subjects. The added anterior
force caused by the quadriceps can stress the ACL further by pulling anteriorly on the
tibia via the patellar tendon at small angles of knee ﬂexion.39 The added strain from the
quadriceps, in combination with weak hamstrings, places the ACL under great strain.

Male and female athletes have been observed to have different jumping and
landing characteristics. Females tend to have a more erect posture, with an extended

knee and hip, which increases the amount of forces felt by the body during landing.4o If

10

landing forces are applied over a short period of time, without accommodating joint
movement, there is less opportunity for the body to absorb the forces. Full range of
motion of a joint allows enough time for the forces to be absorbed by the body.41 Gray-
McNitt“2 found that the peak vertical ground reaction forces could be decreased if the
knee is in the maximum degree of ﬂexion.
Intrinsic Risk Factors

Anatomic alignment differences have also been indicated as possible causes of
ACL injury. The Q-angle is the angle formed by the intersection of a line from the
anterior superior iliac spine to the center of the patella and a line from the center of the
patella to the tibial tubercle. A larger than normal Q-angle is often associated with tibial
internal rotation43 and the ACL functions to prevent internal rotation. If internal tibial
rotation is increased in females by their Q-angles then this may compromise the integrity
of the ACL. The large Q-angles of females also place medial stress on the knee because
of the pull of the rectus femoris on the patella. Q-angles have been found to be greater in
female athletes when compared with Q-angles of male athletes.10

Other anatomical considerations can create positions that are common to ACL
injury mechanisms. Females have wider pelvic bones, increased femoral anteversion,
increased genu valgum, increased genu recurvatum, increased external tibial torsion and
more forefoot pronation.44 These malalignments increase the energy consumption and
mechanical stresses of the body.29 Faulty alignments place the joint into a position that
has preloaded stress onto the ligaments and therefore, cause structural failure under

external forces well below the critical stress limits”, 45, 46

ll

Hormonal inﬂuences are another internal factor possible for putting females at
greater risk for ACL ruptures. Female athletes are exposed to rhythmic variations in
endogenous hormones during their menstrual cycles or via oral contraceptives. More and
more research is being done on how a female’s menstrual cycle and its ﬂuctuating
hormones may affect the laxity of the ACL, therefore predisposing females for injury at
certain times in their cycle. There are three phases of the menstrual cycle: menstrual
(days 1-5), follicular (days 6-14) and luteal (days 15-28).47 These phases are a series of
cyclic changes that the uterine endometrium undergoes, month after month, as it responds
to changing levels of ovarian hormones, estrogen and progesterone, in the blood.

Only a few studies have been performed to study the correlation between the
menstrual cycle and its hormonal changes, and ACL injuries and laxity. Heitz et al.47
observed signiﬁcant differences in ACL laxity in conjuction with estrogen and
progesterone surges during their menstrual cycles. The greatest amount of ACL laxity
was found during the luteal phase, where progesterone levels increase. From the results,
it was concluded that female ACL laxity signiﬁcantly increases in conjunction with
surging levels of estrogen and progesterone during the normal menstrual cycle.

Another study, done by Wojtys et al.43, investigated the variation in ACL injury
rates during the menstrual cycle. The results of their study found a signiﬁcant association
between the stage of the menstrual cycle and the likelihood for an ACL injury. There
were more injuries during the ovulatory (follicular) phase than was expected. This
increased rate of ACL injury in females during this phase suggests that the frequent

occurrence of ACL tears may be related to hormonal ﬂuctuations. The study concluded

12

that the association found between the noncontact ACL injury rates in female athletes and
the reported phase of their menstrual cycle is a cause for concern.

If the hormones do not directly affect the ACL, they may have some effect on the
elements that predispose females to ACL tears. The role that the hormones play on the
risk factors of ACL injuries is a topic that has yet to be extensively studied. The intrinsic
factors, such as muscle strength, joint laxity and proprioception, may become more
apparent during certain phases of the menstrual cycle or at speciﬁc levels of the sex
hormones circulating through the body, making female athletes more susceptible to injury
at certain times. If a correlation could be found between the level of hormones
circulating throughout the body and another intrinsic factor, more prevention measures
could take place so that many of the ACL tears could be avoided.

ACL INJURY PREVENTION STRATEGIES

At Vermont ski resorts, Ettlinger et al.20 observed an increasing rate of severe
knee sprains, while the overall rate for lower extremity injury decreased. They attributed
the decline to the introduction of the releasable ski binding, but then noted that there are
no products that have been implemented to sense and respond to potential injury causing
loads on the knee. This led them to design an experimental program for advanced skiers
with the purpose of developing an awareness of events leading to severe knee sprains in
these skiers. They h0ped that the “ACL Awareness Training” would reduce the number
of severe knee sprains to the ACL among experienced skiing professionals.

The training program was divided into three sections. The ﬁrst part dealt with
avoiding high-risk behavior. The subjects watched a video of skiers falling, and at

various points were asked to record answers to questions in the workbooks provided.

13

The questions asked about the positioning of the skiers limbs and skis up to the point of
injury. The video was followed by a discussion session, where the leader and participants
developed proﬁles for the mechanism of injury, and then devised strategies as to how to
respond quickly and effectively to the risk of ACL injury. To conclude the session, all
the participants contributed to the development of guidelines for avoiding high-risk
behavior. Section two assisted the subjects in recognizing potentially dangerous
situations by presenting them with a list of the six elements of the phantom-foot injury
mechanism. Part three recommended speciﬁc strategies for responding to these
potentially high risk situations.

The “ACL Awareness Training” program was implemented among the on-slope
ski staff at 20 ski areas. Data was also recorded at 22 ski areas, where the program was
not utilized, to be used as control data. The analyzing of the data showed that there was a
decrease of 62% in ACL injuries among the ski staff that had completed the training
program, while no reduction in the number of injuries occurred in the control group. The
investigators concluded that an effort should be made to teach skiers how and when to
fall, and how to stop after falling. They suggest that an education program would be a
cost-effective means for helping to reduce the risk of ACL injuries among experienced
professional alpine skiers.

Caraffa et a1.19 examined the use of proprioception as an intervention to decrease
the rate of ACL injury. Twenty male semi-professional and amateur soccer teams
received proprioception training, while twenty did not. The teams involved with the
training were instructed to train for at least 20 minutes every day during the preseason,

and three days a week during their active season. The proprioceptive training was

14

divided into ﬁve phases of increasing difﬁculty and was utilized over the course of three
seasons. The injury data recorded indicated that the trained teams had ten ACL injuries
over the three seasons and an injury rate of .15 injuries per team per season, while the
untrained group had 70 injuries and an injury rate of 1.15 injuries per team per season.
From this data, the authors were able to conclude that proprioceptive training can reduce
the number of ACL injuries in soccer. The authors also suggested that proprioceptive
training should become a standard practice in the preseason and in season conditioning of
teams.

Soderman et a1.21 conducted a research study on the effects of balance board
training on female soccer players. Seven teams from the second and third Swedish
divisions served as the experimental group, while six teams were used as controls. The
experimental participants were given a balance board and a training program, so they
could complete the exercises at home. Participants were told to perform the exercises
each day for the ﬁrst 30 days, and then 3 days a week during the rest of the soccer season

(about 6 months). The program took about 10 to 15 minutes to complete.

In contradiction to Caraffa et al.19, the investigators of this article found no effect
of the balance board training on the number, incidence, or type of traumatic injuries of
the lower extremity. When comparing the experimental and control groups, the balance
training group showed a signiﬁcantly higher incidence rate of major injuries than the
controls. Knee sprains were the most common major injury in the intervention group,
and 4 out of the 5 ACL injuries occurred in this group as well. Therefore, the authors
concluded that balance board training did not prevent ACL injuries in female soccer

players.

15

Hewett et al.18 studied the effect of a jump-training program on the incidence of
knee injuries in female athletes. The program was designed to increase joint stability by
increasing the strength of the muscles surrounding the joint, and to decrease the amount
of landing forces by teaching neuromuscular control of the lower limb during landing.
The program was employed amongst ﬁfteen female high school volleyball, basketball
and soccer teams. The program was completed 3 days a week, 60-90 minutes a day for 6
weeks. Results aﬂer the 6 week program indicated that the training had a signiﬁcant
effect on the incidence of serious knee injury as the incidence of knee injury (per 1000
exposures) was 0.43 in the untrained group, 0.12 in the trained group, and 0.09 in the
male control group. The trained group also had a signiﬁcantly lower rate of noncontact
injuries (0) than the untrained group (0.35). Results from these studies indicate that
neuromuscular training may decrease injury risk in female athletes.

PLYOMETRICS

The practice of using plyometric exercises to improve strength and power has
been utilized for a long time. However their beneﬁts to athletes has only recently started
to be studied in the past three decades. The term plyometrics has only been used since
the mid-1960’s to 1970’s, but the pioneers of plyometrics were probably track and ﬁeld
athletes in the 1920’s and 30’s.49 Any exercise can be considered plyometric if the
muscle is preloaded with an eccentric contraction and then follow it quickly with a
concentric contraction. Plyometric exercises cover a wide range of difﬁculties and
intensities.25 They require little equipment, time and they are safe for both children and

adults to perform.23

16

Plyometric exercises are based on the premise that increasing the eccentric
preload on a muscle induces the myostatic stretch reﬂex and potentially cause a more
forceful contraction.25 The mechanism underlying all plyometric exercises is the stretch-
shortening cycle. This cycle is divided into three phases. Phase I is the
prestretch/eccentric power phase, where there is forceful lengthening of the contracted
muscle, and joint loads are absorbed and stored in the tenomuscular unit. During the
second phase there is a brief period of time after the eccentric loading where the muscle
length does not change. Then a concentric muscle contraction occurs during the third
phase. The resulting powerful muscle contraction is developed by combining the stored
elastic energy with voluntary and reﬂexive muscle activation.50

Studies have shown that plyometric jump-training can safely and effectively
improve lower body strength and power in female athletes.22 Plyometrics have also been
used successfully in rehabilitation programs for lower extremity musculoskeletal injuries.
Using plyometric exercises in rehabilitation promotes the use of efﬁcient, functional
motor patterns and promotes dynamic joint stabilization.24

Hewett et a1.22 tested the effects of a plyometric exercise program on female
athletes and their landing mechanics and lower extremity strength. The authors designed
the exercise program to teach neuromuscular control to the lower extremities, and
therefore decrease the amount of landing forces felt by the body, and to increase the
strength of the muscles surrounding the knee joint, to increase the stability of the joint.
Eleven high school volleyball players participated in the jump-training program. The

training sessions lasted 2 hours a day, 3 days a week, over the course of 6 weeks. The

17

athletes were also required to perform a stretching program before performing the
plyometric exercises, and weight-training program after completing the exercises.

Before and after completion of the 6-week jump-training program, the subjects
underwent vertical jump height testing, muscle strength testing and force testing. The
force testing was completed by having the athletes complete a simulated volleyball block
jump and landing on a force plate. The jumps were analyzed for peak forces at landing,
jump height, joint angles, and joint moments. Muscle strength was recorded on an
isokinetic dynarnometer at a speed of 360 °/sec. A control group of nine male subjects
were also tested in the same manner.

The results of the testing indicated that ten of the eleven subjects decreased their
landing forces by an average of 456 N. Hamstring peak torque and average power, and
hamstring to quadriceps ratio all increased after training. The subjects also exhibited a
9.2% increase in vertical jump height after completing the training program.

Knee abduction and adduction moments decreased signiﬁcantly after training,
which suggests that the muscular control of the lower extremity was altered in the coronal
plane. However, peak landing ﬂexion and extension moments did not change with
training, nor did the knee ﬂexion angles. When compared with the male control group
angles, the males did not differ as much as the investigators thought they would. Landing
forces have been thought to decrease with increased knee ﬂexion, the results of this study
do not indicate that the angle of knee ﬂexion is the most important factor in decreasing
peak landing forces after training.

Witzke and Snow25 investigated the effects of a nine month plyometric training

program on the bone mineral content, lower extremity performance and static balance in

18

adolescent girls. Twenty-ﬁve girls performed the plyometric training program three
times a week for 30-45 minutes. The plyometrics consisted of hopping, jumping,
bounding and box depth jumps. Data on bone mineral content, strength, and static
balance was recorded at baseline and nine months. The results indicated that plyometric
training improved trochanteric bone mineral content, leg strength, and balance in
adolescent girls. The percent change in static balance from zero was signiﬁcantly greater
in those girls who were in the plyometric training program. The medial/lateral direction
increased 29%, and the anterior/posterior direction increased 17%, in the girls
participating in the jump training program. The authors attributed this difference to the
plyometric drills activating the muscles and neural pathways involved in hip abduction
and adduction, and knee and ankle stabilization.

Other research studies that examined the effects of plyometric training programs
used various techniques, durations and populations, and had different purposes. Two
studies observed improved running economy in participants of a plyometric training
program“, 52 Plyometrics also brought about improvements in proprioception,
kinesthesia and contractile strength of the rotator muscles of the shoulder in female

swimmers.53 The onset of fatigue during vertical jumping was delayed in the plyometric

training group particpants when compared to a group that completed a weight training

program.54 Improvement was also found after completing a plyometric training program
with pefomring drills that measured acceleration, speed, coordination, dynamic balance,

agility, lateral movement, and explosive power.55 However, no differences were found

between the plyometric and control groups in performing a freestyle tumble turn.

19

Chapter 3
METHODS
Subjects

Twenty-ﬁve female subjects (height= 167.69 : 6.91 cm, weight= 59.81 i 9.37 kg,
age= 20.32 i 1.99 years) volunteered to participate in this research study. Participants
had no history of diagnosed knee or ankle injury in either limb, no coexisting
concussions, and were required to be physically active 3 days a week, but not currently
participating in strength training for the past 2 months. Fifteen participants were selected
to participate in the plyometric training program, while the other ﬁfteen participants were
placed in the control group. Randomization of group placement was not feasible due to
availability of some participants. Four of the participants in the plyometric group
dropped out over the course of the study for various reasons, such as an outside injury
and time. Also, one control subject could not complete all of the testing sessions because
of illness. This gave a total of 11 participants in the plyometric group, and 14 in the
control group. The subjects that participated in the plyometric training program were
compensated ﬁfty dollars for their time, while the control group subjects received ten
dollars.

The plyometric training program started with an information session held for all
the potential subjects to explain the purpose and process of the study. Upon agreement of
participation, the subjects were asked to read and sign an informed consent form, which
was approved by the Michigan State University Committee on Research Involving

Human Subjects (see Appendix A). All subjects were given a copy of the Paffenbarger

Physical Activity Questionnaire56 to complete before and after the completion of the

20

study to determine their level of physical activity. The questionnaire collects information
about the type and intensity of activities done in the past 2 weeks, and produces a kcal
total. A questionnaire was used to obtain height, weight, leg dominance, and previous

sport experience.

Plyometric training program

The training program consisted of thirteen plyometric exercises, varying in
difﬁculty and intensity. Appendix B includes a detailed description of the program and
individual jumps. This plyometric training program was adapted from Hewett et al.9-2
An instructor taught the ﬁrst session, and explained and demonstrated the proper
technique for performing each jump. Emphasis was placed on identifying the correct
posture, and the correct take-off and landing positions. A handout detailing the program
was given to each subject. The contents of the paper described each jump, and gave
instructions on the length of time for each jump and how to increase the intensity each
week. The subjects were required to perform the training program for 45 to 60 minutes, 3

days a week, on alternating days, for 6 weeks under the supervision of the investigator.

Testing

All of the subjects were required to attend four testing sessions: one prior to the
start of the training program, and one after weeks two, four and six of the training
program. Before testing, all participants went through a warm-up of 5 minutes of jogging

and skipping, and then lower body stretching.

21

Postural control measurements were made using an AMTI Accusway force plate
(AMTI Inc., Watertown, MA) utilizing SWAYWIN software (AMTI Inc., Watertown,
MA.). Subjects performed 3 trials for 30 seconds each on their dominant leg. The
participant’s dominant leg was determined by the asking which foot they would use to
kick a soccer ball. The subjects were positioned on the force plate with the foot of their
dominant leg in the middle, and then stood on their dominant leg with eyes open and
arms crossed in front of them. They were told to focus on a target on the wall in front of
them and remain as still as possible. (Appendix C)

Data were collected at 200 Hz, and the force plate was calibrated before each
testing session. Dependent measures were mean center of pressure excursion velocity in
the frontal and sagittal planes (VAvg), total length of path pressure (PLength), and the
area of the 95% conﬁdence ellipse formed by the center of pressure excursion.(Area95).
These variables are deﬁned as:

Average velocity (Vavg): the distance of the center of pressure traveled divided

by the time interval

Area of the 95% ellipse (Area95): area of the 95% conﬁdence ellipse formed by

the center of pressure excursion

Path length (Plength): total length of the center of pressure during the time

interval
Hamstring and quadriceps strength of each subject’s dominant leg was assessed

using an isokinetic dynamometer (Biodex Inc., Shirley, NY). Participants were

positioned on the dynamometer with their trunk perpendicular to the ﬂoor, hips ﬂexed to

22

90°, and the knees ﬂexed to 90°. The rotational axis of the dynamometer was aligned
with the lateral femoral condyle. The participants were secured with a thoracic strap,
waist strap, thigh strap, and a shin strap that was positioned one ﬁnger length above the
lateral malleolus. (Appendix C)

Gravity correction was obtained by measuring the amount of torque placed on the
dynamometer with the knee in a relaxed state at 45° of ﬂexion. Two sets of ten
repetitions of concentric ﬂexion and extension at 60°/sec and 120°/sec were performed
on the subject’s dominant leg, with one minute rest given in between sets. Five practice
repetitions were performed before the testing of each new speed. During the testing, the
participants were required to fold their arms across their chest, were given verbal
encouragement from the researcher. Dependent measures were peak torque (PeakTq),
average peak torque (AngkTq), and average power (Angower) generated by the
muscle groups during concentric ﬂexion and extension. These variables are deﬁned as:

Peak torque- PeakTq- highest amount of force generated by muscle contraction

through the range of motion

Average peak torque- AngkTq- average torque produced through the range of

motion

Average power- Angower- total amount of work divided by the total time of the

work

Statistical Analysis

For each dependent variable of postural control (VAvg, Area95, and PLength),

and isokinetic strength (PeakTq, AngkTq, and Angower), a one-between (group) and a

23

one-within (time) repeated-measures analysis of variance (AN OVA) was used. The
between subjects factor was grouped with 2 levels (experimental, control), while the
within-subjects factor of time contained 4 levels (baseline, 2 weeks, 4 weeks, 6 weeks).
Tukey post hoc analyses were performed for all signiﬁcant interactions. Level of
signiﬁcance (P < .05) was set a priori for all statistical analyses. Analyses were
performed with SPSS 10.0 (SPSS Inc., Chicago, IL) statistical software package.

The subject number was ﬁrst calculated using a power of .8 and a moderate effect
size of .6, and the subject number came out to be 63 subjects per group, but due to cost
and time constraints this number would not be feasible. Previous research studies that
studied ACL prevention programs used various subject group sizes. Hewett et al.’s ﬁrst
study consisted of 11 females in the test group and nine males in the control group. The
second study used 15 trained female high school teams, 15 untrained female high school
teams and 13 untrained male teams. Caraffa et al.19 had 20 soccer teams in both their test
and control groups, while Ettlinger et a1.20 had 20 ski areas in their test group and 22 ski
areas in their control group. Other related studies used smaller subject populations, such
as Rozzi et al.’s study57 on knee joint laxity and neuromuscular characteristics consisted

of 17 female and 17 male subjects. Also, Aagaaard et al.’s study58 on isokinetic

hamstring to quadriceps ratio used 4 female and 5 male subjects.

24

Chapter 4
RESULTS

Group Comparisons

ANOVA revealed no signiﬁcant main effects or interaction effects for the amount
of kcal burned per week (F133 = 2.851, P = .105, 1-0 = .634) which was calculated from
the Paffenbarger Physical Activity Questionnaire. This indicates that the groups were
equal in their activity levels at the beginning and end of the study. Also, there were no
statistically signiﬁcant differences between the plyometric and control groups at the ﬁrst
testing session of the strength and postural control variables. Therefore, the groups were
equivalent in their quadriceps and hamstring strength, and balance at the beginning of the
research study.
Postural Control

AN OVA revealed that there were group x time interaction for all of the postural
control variables, VAvg (F146 = .137, P = .873, 1-13 = .930), Area95 (F2,46 = .718,P =
.493, 1-[3 = .836) and PLength (F 2,46 = .145, P = .866, 1-0 = .929). Since no signiﬁcant
interactions found for these variable, this indicates that the values for average velocity,
area of the 95% ellipse and path length were similar between pre and post measurements
for both the plyometric and control groups. However there were some trends seen in the
means of the data. The descriptive statistics show an overall slight decrease in velocity
average and path length values across both groups, while the values for the area of the

95% ellipse slightly increased over the testing sessions for both groups.

25

Strength

For the strength variables at 60 °/sec, ANOVA revealed no signiﬁcant differences
between the groups over time, for PeakTq- extension (F3459 = .080, P = .971, 1-0 = .936),
PeakTq- ﬂexion (F339 = .774, P = .512, 1-[3 = .792), AngkTq- extension (F3459 = .822, P
= .486, 1-0 = .781), AngkTq- ﬂexion (F169 = .748, P = .527, 1-[3 = .798), Angower-
extension (F169 = .177, P = .912, 1-0 = .919), and Angower-ﬂexion (F169 = 1.553, P =
.209, l-[3 = .608).

No signiﬁcant interactions were found at 120 °/sec for PeakTq- extension (F169 =
1.397, P = .251, 1-[3 = .645), AngkTq-extension (F 3,69 = .572, P = .636, 1-0 = .838),
AngkTq- ﬂexion (F359 = 2.138, P = .103, l-B = .477), and Angower- extension (F359
= 2.706, P = .052, 1-0 = .366).

There was a signiﬁcant group x time interactions for the strength values at
120°/sec of PeakTq- ﬂexion (F3459 = 3.162, P = .030) and Angower- ﬂexion (F169 =
3.487, P = .020). Effect sizes were calculated as .38 for PeakTq and .72 for Angower.
Post hoc analysis revealed that there were signiﬁcant differences between the plyometric
and control groups for average power at testing sessions two, three and four.(Figure 1.1)
A signiﬁcant difference also existed between the ﬁrst and last testing sessions of the
plyometric group.

There was a signiﬁcant difference between the means of the ﬁrst and last testing
sessions for the plyometric group for peak torque-ﬂexion.(Figure1.2) The plyometric
group was able to increase their peak torque with each session, while the control group’s

peak torque decreased with the ﬁrst 3 sessions and then increased slightly. This same

26

trend can be seen in average peak torque of ﬂexion at 120 °/sec, however there was not a

signiﬁcant difference.(Figure 1.3)

Figure 4.1

Flexion Average Power at 120 Degrees per Second

8

AveragePowerMetts)
a 8 Pa 8 8 8 a

.s
0

0|

 

0

Testing Smlon

 

II Plyometric Group DControl Group ]

 

* Signiﬁcantly different than the control group (P = .020)

27

Figure 4.2

Flexlon Peak Torque at 120 Degrees per Second

Peak Torque (FLLbeJBody Weight)
8 at B 8 8 8%

0|

 

O

Testlng Seselon

 

[I3 Plyometric Group UControl Group I

 

* Signiﬁcant difference between plyometric group Session 1 vs. Session 4 (P = .030)

 

Figgre 4.3
Flexion Average Peak Torque at 120 Degrees per Second
40
35

 

 

8

 

 

 

Average Peek Torque (Ft. Lbs.)
8

 

 

 

 

 

 

 

Testing Seeelon

 

I 1:! Plyometric Group Cl Control Group

 

28

Descriptive Statistics

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Table 4.1
Descriptive Statistics for Peak Torque of Extension at 60°/sec
Test PlyometricGroup( 1 )/ Mean Standard Number

Session Control Group(2) Deviation

l 1 74.3091 10.8385 11

2 72.2250 10.9181 14

Total 73.1420 10.7066 25

2 1 66.8636 11.1093 11

2 66.1607 11.3582 14

Total 66.4700 1 1.0195 25

3 1 64.5773 9.2379 11

2 63.6214 8.0072 14

Total 64.0420 8.3977 25

4 1 63.0000 9.8426 1 l

2 62.0464 10.1944 14

Total 62.4660 9.8434 25

Table 4.2
Descriptive Statistics for Peak Torque of Flexion at 60°/sec
Test PlyometricGroup( l )/ Mean Standard Number

Session Control Group(2) Deviation

1 1 38.2000 5.4453 11

2 36.7607 6.4891 14

Total 37.3940 5.9746 25

2 1 36.6000 7.7359 11

2 35.4214 5.8536 14

Total 35.9400 6.6220 25

3 1 35.8182 6.7663 11

2 33.6571 5.2549 14

Total 34.6080 5.9357 25

4 1 37.6182 5.8745 11

2 33.9321 5.6383 14

Total 35.5540 5.9234 25

 

29

 

 

Table 4.3

Descriptive Statistics for Peak Torque of Extension at 120°/sec

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Test P1yometricGroup( 1)/ Mean Standard Number

Session Control Group(2) Deviation

1 1 46.4318 8.0625 11

2 47.6143 9.6321 14

Total 47.0940 8.8147 25

2 1 46.6364 5.1655 11

2 46.5821 8.9485 14

Total 46.6060 7.3819 25

3 1 48.1500 6.0548 11

2 45.3964 7.9106 14

Total 46.6080 7.1496 25

4 1 47.9545 4.5856 11

2 44.9143 7.4745 14

Total 46.2520 6.4340 25

Table 4.4
Descriptive Statistics for Peak Torque of Flexion at 120°/sec
Test PlyometricGroup(1)/ Mean Standard Number

Session Control Group(ZL Deviation

1 1 24.7182 4.8561 11

2 26.2250 5.5731 14

Total 25.5620 5.2185 25

2 1 24.8273 5.8097 1 1

2 25.3464 4.1007 14

Total 24.1180 4.8209 25

3 1 25.5091 6.4140 11

2 24.4464 3.8821 14

Total 24.9140 5.0591 25

4 1 27.2000 5.1688 11

2 24.9607 5.8520 14

Total 25.9460 5.5650 25

 

3O

 

 

Table 4.5

Descriptive Statistics for Average Peak Torque of Extension at 60°/sec

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Test PlyometricGroup(l)/ Mean Standard Number
Session Control Group(2) Deviation

1 1 86.8818 16.9320 11

2 79.5321 18.1206 14

Total 82.7660 17.6403 25

2 1 76.3091 16.6114 11

2 73.0393 16.3586 14

Total 74.4780 16.2071 25

3 1 76.1636 16.2831 11

2 69.9571 14.5243 14

Total 72.6880 15.3176 25

4 1 77.1864 12.3436 11

2 69.9893 14.8307 14

Total 73.1560 13.9972 25

Table 4.6
Descriptive Statistics for Average Peak Torque of Flexion at 60°/sec
Test PlyometricGroup( l )/ Mean Standard Number
Session Control Group(2) Deviation

1 1 44.5000 7.9678 11

2 40.7000 9.7219 14

Total 42.3720 9.0197 25

2 1 41.1273 6.6255 11

2 37.3464 8.0385 14

Total 39.0100 7.5472 25

3 1 40.5000 8.0277 11

2 35.9214 8.3912 14

Total 37.9360 8.3888 25

4 1 42.9500 6.4281 1 1

2 36.7714 9.0229 14

Total 39.4900 8.4329 25

 

 

 

 

 

 

31

 

 

Table 4.7

Descriptive Statistics for Average Peak Torque of Extension at 120°/sec

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Test PlyometricGroup(l)/ Mean Stande Number
Session Control Group(2) Deviation

1 1 53.0273 15.8463 1 l

2 48.9821 14.1807 14

Total 50.7620 14.7564 25

2 1 52.9091 14.0126 11

2 48.2071 11.6908 14

Total 50.2760 12.7091 25

3 1 53.0409 12.4472 1 1

2 46.5036 11.6744 14

Total 49.3800 12.2209 25

4 1 54.2909 9.0728 1 1

2 47.0571 9.3090 14

Total 50.2400 9.7298 25

Table 4.8
Descriptive Statistics for Average Peak Torque of Flexion at 120°/sec
Test PlyometricGroup( 1 )/ Mean Standard Number
Session Control Group(2) Deviation

l 1 26.8909 6.9059 1 1

2 27.1536 7.0529 14

Total 27.0380 6.8435 25

2 1 27.0136 7.1367 11

2 25.9000 5.8479 14

Total 26.3900 6.3296 25

3 1 28.2409 8.1387 11

2 24.9821 6.0551 14

Total 26.4160 7.0841 25

4 1 29.4045 5.4693 1 1

2 26.0107 7.5721 14

Total 27.5040 6.8174 25

 

32

 

 

Table 4.9

Descriptive Statistics for Average Power of Extension at 60°/sec

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Test PlyometricGroup( 1)/ Mean Standard Number
Session Control Group(2) Deviation
1 1 58.4136 11.4264 11
2 53.7786 12.1238 14
Total 55.8180 11.8124 25
2 1 51.8409 10.2496 1 1
2 47.5893 11.0197 14
Total 49.4600 10.6859 25
3 1 51.3636 9.7106 11
2 47.0464 10.1631 14
Total 48.9460 10.001 1 25
4 1 51.8273 7.6993 11
2 46.3321 9.4870 14
Total 48.7500 9.0112 25
Table 4.10
Descriptive Statistics for Average Power of Flexion at 60°/sec
Test PlyometricGroup( l )/ Mean Standard Number
Session Control Group(2) Deviation
1 1 32.2773 6.2209 11
2 27.7393 7.0592 14
Total 29.7360 6.9572 25
2 1 29.5818 5.5766 11
2 25.5679 5.8730 14
Total 27.3340 5.9814 25
3 1 29.5227 5.8344 11
2 23.5429 5.5100 14
Total 26.1740 6.3092 25
4 1 31.1182 5.2427 11
2 24.3214 6.2223 14
Total 27.3120 6.6544 25

 

33

 

 

Table 4.11

Descriptive Statistics for Average Power of Extension at 120°/sec

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Test PlyometricGroup( l )/ Mean Standard Number
Session Control Group(2) Deviation
l 1 62.9591 11.6672 11
2 61.5393 17.1328 14
Total 62.1640 14.7049 25
2 1 68.0455 16.8551 11
2 59.4429 13.9794 14
Total 63.2280 15.5956 25
3 1 68.2864 13.6838 11
2 58.3000 15.6601 14
Total 62.6940 15.3770 25
4 1 68.4773 9.4694 1 1
2 58.0750 12.5607 14
Total 62.6520 12.2717 25
Table 4.12
Descriptive Statistics for Average Power of Flexion at 120°/sec
Test PlyometricGroup( 1)/ Mean Standard Number
Session Control Group(2) Deviation
l 1 30.7636 7.6975 1 1
2 30.6964 9.4118 14
Total 30.7260 8.5247 25
2 1 33.7636 9.9717 11
2 33.7636 7.8741 14
Total 31.1340 8.9819 25
3 1 34.4000 10.2464 1 l
2 27.5786 7.3306 14
Total 30.5800 9.2085 25
4 1 35.5591 7.9293 11
2 28.3857 9.9483 14
Total 31.5420 9.6443 25
Table 4.13
Descriptive Statistics for Average Velocity
Test PlyometricGroup( l )/ Mean Standard Number
Session Control Group(2) Deviation
l 1 1.6127 .3554 11
2 1.6607 .2892 14
Total 1.6396 .3139 25
2 1 1.5021 .4427 11
2 1.5598 .1786 14
Total 1.5344 .3159 25
4 1 1.4045 .2178 11
2 1.4986 .2625 14
Total 1.4572 .2437 25

 

34

 

 

 

Table 4.14

Descriptive Statistics for Area of the 95% Ellipse

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Test PlyometricGroup( l )/ Mean Standard Number
Session Control Group(2) Deviation

1 1 .8888 .1924 1 l

2 .8426 .2086 14

Total .8629 .1989 25

2 l .8827 .2587 1 l

2 .9571 .3634 14

Total .9244 .3176 25

4 l .9373 .2934 1 1

2 .9174 .3276 14

Total .9261 .3067 25

Table 4.15
Descriptive Statistics for Path Length
Test PlyometricGroup( 1 )/ Mean Standard Number
Session Control Group(2) Deviation

l 1 48.4276 10.5785 11

2 49.8379 8.6512 14

Total 49.2173 9.3636 25

2 1 45.0494 13.2827 11

2 46.7776 5.3626 14

Total 46.0172 9.4793 25

4 1 42.1436 6.5158 11

2 44.9764 7.8950 14

Total 43.7300 7.3152 25

 

35

 

 

Chapter 5
DISCUSSION

Previous research has looked at the trends in, possible risk factors for, and
potential prevention programs for female ACL injuries. The purpose of this research
study was to examine the effects of a plyometric jump training program on the
neuromuscular characteristics in female athletes, speciﬁcally postural control, and
hamstring and quadriceps strength. After the completion of a six week plyometric
program, the plyometric group participants signiﬁcantly increased their average power
and peak torque during concentric ﬂexion at 120°/sec. No differences were found for the
postural control variables of VAvg, Area95, PLength, or other strength variables
AngkTq, Angower, and PeakTq at 60°/sec, and AngkTq at 120°/sec.
Effects of Plyometrics on Strerth

The results of this research study support the hypothesis that plyometric jump
training exercises improved the strength of female athletes. Signiﬁcantly increased peak
torque and average power values were found during ﬂexion for the plyometric group at
the speed of 120°/sec. The same trend was also seen for average peak torque, although it
did not reach statistical signiﬁcance. However, no signiﬁcant differences were seen in
the strength variables between the plyometric and control groups at 60°/sec and extension
at 1 20°/ sec.

The plyometric group improved their hamstring average power over the course of
the plyometric training program, while the control group decreased slightly. Average
power is the total amount of work divided by the total movement time, and indicates how

quickly the muscle can produce force. The plyometric jumps seem to have resulted in the

36

females’ hamstring muscles to being able to produce more work in the same amount of
time.

A signiﬁcant difference was also observed between the ﬁrst and last testing
sessions in the plyometric group for peak torque at 120°/sec. The plyometric group
increased their peak torque during the plyometric training program, while the control
group slightly decreased their peak torque. Peak torque refers to the highest amount of
force the joint was able to generate by muscle contraction throughout the range of
motion. The females in the plyometric group were able to generate a greater force using
their hamstring muscles as they progressed through the plyometric training program.

The same trend was also seen for average peak torque for ﬂexion at 120°/sec.
The plyometric group improved over time, while the control group experienced a slight
decrease in average peak torque. Though the differences were not statistically signiﬁcant
across the testing sessions, these values show that the plyometric group participants were
able to generate more hamstring force as they advanced in the plyometric training.

The strength gains observed in this study can be compared to the results found by
Hewett et al.22, which showed increases in hamstring isokinetic peak torque and average
power after plyometric training. Concentric hamstring strength was measured on a
Biodex isokinetic dynamometer at a speed of 360°/sec. The participants in their study
performed the plyometric program in conjunction with a weight training program, which
may have contributed to the strength gains that were seen. The current study did not use
a weight training program, and signiﬁcant strength gains were still able to be seen in the-

hamstrings.

37

No effect was seen on the concentric strength of the quadriceps muscle group
throughout the plyometric program, which is different than the ﬁndings of Witzke and
Snow”, who observed that knee extensor strength improved in adolescent girls who
participated in a plyometric training program. However, the improvement in strength
was not signiﬁcantly different from the improvements in the control subjects. The
plyometric were performed over the course of a 9-month school year, and the participants
were adolescent girls with a mean age of 14.6. The longer duration of the plyometric
program and younger ages of the participants may have produced the improved
quadriceps strength that was not seen in this study.

The plyometric jumps had a positive effect on the concentric strength of the
hamstring muscles, even though the jumps repeatedly used the eccentric actions of the
hamstring muscles. Eccentric muscle actions can develop greater maximum forces than
concentric or isometric muscle actions”, and therefore have been thought to be able to be
more successful at increasing overall strength.6O Various research studies have compared
the effectiveness of eccentric and concentric training on increasing muscular strength. In
relation to the current study where eccentric training resulted in concentric strength gains,
eccentric training has been shown to produce similar61'63, greater“, 55, or no“: 67
increase in concentric strength. The beneﬁts from eccentric training have been attributed
to increased neural activation“: 67 and greater muscle hypertrophy“, and more
speciﬁcally hypertrophy of type II muscle ﬁbers“.

The increase in concentric hamstring strength combined with the maintained
quadricep strength increases the females’ hamstring to quadriceps ratio. A decreased

hamstring to quadriceps ratio has been shown to be more common in female athletes, and

38

associated with a higher injury risk.9 By improving a female’s hamstring to quadriceps
ratio, this may also decrease her risk of injury. The hamstrings are important in
controlling anterior tibial translation. Female athletes have been shown to activate their
quadriceps muscle group ﬁrst when responding to anterior tibial translation.ll This
action adds ﬁrrther strain to the ACL by pulling more anteriorly on the tibia. Improved
hamstring strength can help to counteract this action and reduce the strain placed on the
ACL.

Hewett et al.22 also saw an increase in hamstring and quadriceps ratio after their
plyometric training program. When compared to the ratio of the male controls in their
study, the trained females’ ratios were almost equivalent. The plyometric training
program was used in conjunction with weight training program, which may have
inﬂuenced the strength gains.

Effects of Plyometrics on Postural Control

This study suggests postural control is not improved in female athletes who
participate in a plyometric training program. There was an overall decrease in mean
scores across the variables for both the plyometric and control groups, which indicates
improved postural control in all participants. However, this trend may have been caused
by a familiarity with the testing procedures.

Unlike the results of this research study, Witzke and Snow25 observed signiﬁcant
improvements in static balance in the adolescent girls who participated in the 9 month
plyometric training program. Static balance was determined by using a Biodex
Stabilometer, which was assessed in degrees of deviation in the anterior/posterior and

medial/lateral directions. The medial/lateral balance improved 29%, while the

39

anterior/posterior stability increased 17%. The authors attributed this improvement to the
plyometric drills that incorporated lateral movements, which exercised the muscles and
neural pathways involved in controlling hip abduction and adduction, and knee and ankle
stabilization. The different postural control testing measures, length of plyometric
training programs, or mean ages of participants may have resulted in conﬂicting results
between the two studies.

Similar to the current study, Blackburn et al.12 found that the effects of
proprioception and strength training programs had little effect on static balance. No
improvements were seen in static balance after 6 weeks of training. Balance was
measured by using a force plate, as it was in the current study. The authors attributed
these results to static balance not being very challenging to healthy subjects. All of their
subjects, as well as the subjects in this research study, had properly functioning visual,
vestibular, and somatosensory systems, which contribute to postural control.

In the same Blackburn et a1. study”, signiﬁcant improvements were found in all
of the training groups for semidynamic and dynamic balance. Semidynamic balance was
assessed using a Biodex Stability System, and dynamic balance with a modiﬁed version
of the Bass Test of Dynamic Balance. All of the training groups showed signiﬁcant
differences between pre- and post- testing. The current research study did not employ
testing measures for semidynamic or dynamic balance, only static balance measures were
taken.

Clinical Implications
To date, only a couple of research studies”, 20, 21 have investigated the effects of

ACL prevention programs on female athletes. This study observed the effects of a

40

plyometric training program on two possible ACL risk factors, postural control, and
hamstrings and quadriceps strength. The plyometric training program had a signiﬁcant
effect on the hamstring strength of the female participants. The increased hamstring
strength shows that plyometric training may be a good program for young females to
utilize to improve hamstring strength, a risk factor for ACL injury.

The results of this research study support the implementation of a plyometric
training program by coaches, athletic trainers, and strength training coaches in the daily
routine of their female athletes. The plyometric training program also exposes the
females to landings from a jump during the exercises. Noncontact mechanisms, such as
landing from a jump, are the most common mechanism for ACL injury.32 This aspect of
plyometric training is also important in addition to the increased hamstring to quadriceps
ratio.

The plyometric training program used in this study is relatively easy and quick to
apply to a female athlete’s exercise program. The program consisted of a warm-up, a
series of 7 to 8 jumps, and a cool-down, and was about 45 minutes long in duration. The
series of jumps would be simple to add to the pre-practice routine of, and to teach to
female athletes. The drills do not require any special equipment, other than a cone, and
can easily be done in a gymnasium.

Considerations for Future Research

Research should be continued on the various prevention programs and their effect
on female ACL injuries. Suggestions for further research are:

1) The effects of plyometrics on other ACL risk factors, such as ground reaction

forces and muscle activation patterns;

41

2) The effects of a plyometric training program on the eccentric strength of the
quadricep and hamstring muscle groups;
3) The effects of a plyometric training program on semidynamic and dynamic
balancing;
4) Comparison of female and male hamstring strength gains after participation in
a plyometric training program;
5) Comparison of female strength gains between plyometric training
programs of varying lengths and jumps.
Limitations
The small sample size contributes to the low power of the study. Each of the
groups started out with ﬁfteen subjects, but due to dropouts, eleven ﬁnished the
plyometric training program and fourteen ﬁnished in the control group. A larger sample
size would have been ideal, but due to time and cost restrictions, was not feasible.
Balance data could not be measured during the third testing session for all of the
participants in the study. The force plate was not functioning properly during each of the
trials; therefore about half of the participants did not get tested. The statistical analyses

were run on the data from the ﬁrst, second and fourth testing sessions.

42

APPENDIX A

43

INFORMED CONSENT FORM

Title of proj ect: The Effects of Plyometric Training on Neuromuscular Characteristics in

Female Athletes

1. This section provides an explanation of the study in which you will be
participating:
a. The study in which you will be participating is part of research intended to

develop a plyometric jump training program that may prevent anterior cruciate
ligament tears sustained by female athletes. Plyometric training programs are
used to develop an explosive movement over a short period of time. Plyometric
exercises involve hops, bounds, and jumps to strengthen the lower extremity and
to develop explosive power. The exercises induce a rapid stretch on the muscles,
followed immediately by a rapid concentric contraction.

If you agree to take part in this research, you will undergo a battery of
tests on 4 separate days. Testing will be performed at the beginning of the study,
after week 2, after week 4, and after week 6. If you are assigned to the
experimental training group, you will be required to perform a plyometric jump
training program 3 days a week, on alternating days, for 6 weeks. This training
program will consist of a variety of plyometric exercises and will take
approximately 45 minutes to complete.

Performance testing will consist of assessment of postural control and
isokinetic strength. The protocol for each of these speciﬁc assessment modalities
will be described.

Postural control will be assessed using an AMTI Accusway balance
platform. Subjects will perform 3 trials, 30 seconds each, of single leg balance,
on their dominant limb, with their eyes open. Postural control measures the
ability of the subject to balance. The platform will measure the changes in
position of the subject’s center of gravity as they balance on a single foot.

Hamstring and quadriceps strength will be assessed using a Biodex
isokinetic dynamometer. Subjects will perform 3 sets of 10 repetitions of

concentric ﬂexion and extension at 60°/sec and 120°/sec. During these

44

repetitions, maximum resistance will be provided through out the range of motion
by the machine, while the muscle contraction being performed is at a constant
velocity, set by the machine. During an isokinetic contraction, the muscle length
is changing while the contraction is performed at a constant speed.

Your participation in this research will take approximately 13 hours over
the course of 6 weeks.

You should not experience any discomfort while participating in this study
other than that which normally accompanies intense exercise.

You will receive ﬁnancial compensation for participating in this study if
you complete all of the required training and testing sessions. If you are selected
to be in the plyometric group you will receive $50, and $10 if you are in the
control group.

This section describes your rights as a research participant:

You may ask questions about the research procedures at any time, and/or
discuss any questions regarding research related injuries. These questions and
concerns should be directed to Angela DiPasquale at (517) 353-0892 (mornings)
or (517) 203-4468 (evenings), or to Kavin Tsang at (517) 353-2010 (business
hours). If you have questions regarding your role and rights as a research subject,
you may contact the Institutional Review Board: Dr. Ashir Kumar, M.D. Chair,
University Committee on Research Involving Human Subjects at (517) 355-2180.

Your participation in this study is conﬁdential. Only the investigators will
have access to your identity and to information that can be associated with your
identity. If this research becomes published, no personally identifying
information will be disclosed. To make sure your participation is conﬁdential
only a subject number will appear on your data collection sheet. The researchers
will be the only ones that can match your name with your subject number. Your
privacy will be protected to the maximum extent allowable by law.

Your participation is voluntary and you may stop participating at any time,
or decline to answer any speciﬁc questions.

This study presents no risks to you beyond those associated with intense

exercise.

45

Your participation in this study will potentially beneﬁt those that may be
predisposed to sustaining various injuries.

If you are injured as a result of your participation in this research project,
Michigan State University will provide emergency medical care if necessary. If
the injury is not caused by the negligence of MSU you are personally responsible
for the expense of this emergency care and any other medical expenses incurred
as a result of this injury.

This section indicates that you are giving your informed consent to participate in

the research:

a.

You agree to participate in the scientiﬁc investigation described above, as
an authorized part of the education and research program of the Michigan State
University.

You have been given the information, and have received answers to any
questions that you may have had about the research procedure. You agree to the
conditions of this study as described.

To the best of your knowledge and belief, you are not pregnant and have
no physical or mental illness, or difﬁculties that would increase the risk of
participation in this study. If at any time in the study, you feel that you might
have become pregnant, please notify the investigators immediately and your
participation in the research study will be discontinued.

You will receive ﬁnancial compensation for participating in this study.

Your participation is voluntary and that you may withdraw from this study
at any time by notifying the person in charge.

You are 18 years of age or older, and/or a full-time student of the
Michigan State University.

You will receive a signed copy of this consent form.

 

 

Signature Date

Researcher:

46

I certify that the informed consent procedure has been followed, and I

have answered any questions from the participant above as fully as possible.

 

 

Signature Date

47

APPENDIX B

48

W
(adapted from Hewett et al.22)

WEEK 1

Wall jumps

Tuck jumps

Broad jumps stick landing
Squat jumps

Double leg cone jumps
180° jumps

Bounding in place

WEEK 2

Wall jumps

Tuck jumps

Broad jumps stick landing
Squat jumps

Double leg cone jumps

1 80° jumps

Bounding in place

WEEK 3

$899991“?

Wall jumps

Tuck jumps

Jump, jump, jump, vert. jump
Squat jumps

Double leg cone jumps
Scissor jump

Bounding for distance

Hop, hop, stick

WEEK 4

9°N9‘P'PP‘N?‘

Wall jumps

Tuck jumps

Jump, jump, jump, vert. jump
Squat jumps

Double leg cone jumps
Scissor jump

Bounding for distance

Hop, hop, stick

20 sec.
20 sec.
5 reps

10 sec.

30 see. each (side/side, front/back)

20 sec.
20 sec.

25 sec.
25 sec.
10 reps
15 sec.

30 sec. each (side/side, front/back)

25 sec.
25 sec.

30 sec.
30 sec.
5 reps

20 sec.

30 see. each (side/side, front/back)

30 sec.
1 run

5 reps/leg

30 sec.
30 sec.
8 reps

20 sec.

30 see. each (side/side, front/back)

30 sec.
2 runs

5 reps/leg

49

WEEK 5

HP‘MFP’PT‘

Wall jumps

Cone jumps

Jump, jump, jump, vert. jump
Squat jumps

Single-legged jumps distance
Jump into bounding
Single-legged hop, hop, stick

WEEK 6

SP‘MPP’PI‘

Wall jumps

Cone jumps

Jump, jump, jump, vert. jump
Squat jumps

Single-legged jumps distance
Jump into bounding
Single-legged hop, hop, stick

30 sec.

30 sec. each (side/side, front/back)
8 reps

25 sec.

5 reps/leg

3 runs

5 reps/leg

30 sec.

30 see. each (side/side, front/back)
10 reps

25 sec.

10 reps/leg

5 runs

10 reps/leg

50

DESCRIPTION OF JUMPS

1. 180° jumps- Two-footed jump. Rotate 180° in midair. Hold landing 2 seconds, then

repeat in reverse direction.

2. Bounding for distance- Start bounding in place and slowly increase distance with

each step, keeping knees high.

3. Bounding in place- Jump from one leg to the other straight up and down,

progressively increasing rhythm and height.

4. Broad-jumps, stick landing- Two-footed jump as far as possible. Hold landing for 5

seconds.

5. Cone jumps- Double-leg jump with feet together. Jump side-to-side over cones

quickly. Repeat forward and backward.

6. Hop, hop stick- Single-legged hop. Stick landing for 5 seconds. Increase distance of

hop as technique improves.

7. Jump into bounding- Two-footed broad jump. Land on single leg, then progress into

bounding for distance.

8. Jump, jump, jump, vertical jump- Three broad jumps with vertical jump immediately

after landing the third broad jump.

9. Scissors jump- Start in stride position with one foot well in front of the other. Jump

up, alternating foot positions in midair.

10. Single-legged jumps distance- One-legged hop for distance. Hold landing (knees

bent) for 5 seconds.

51

11. Squat jumps- Standing jump raising both arms overhead, land in squatting position

touching both hands to the ﬂoor.

12. Tuck jumps- From standing position jump and bring both knees up to chest as high

as possible. Repeat quickly.

13. Wall jumps- With knees slightly bent and arms raised overhead, bounce up and

down off toes.

52

PICTURES OF PLYOMETRIC JUMPS

1. 180° jumps-

 

53

4. Broad jumps

amen:

 

54

7. Jump into bounding

 

8. Jump, jump, jump, vertical

 

9. Scissors jump

 

55

10. Single-legged jumps distance

 

11. Squat jumps

___...:\

 

12. Tuck jumps

 

56

13. Wall jumps

 

57

APPENDIX C

58

PICTURES OF TESTING INSTRUMENTS

 

AMTI Accusway Force Platform

Chair-”i

.talzrh'l
it i. a:
.1 "1) ‘
. at?

l l

 

Biodex Isokinetic Dynamometer

59

APPENDIX D

60

ANOVA TABLES

ANOVA Table for the Relationship of Time and Treatment to Postural Control Variables

 

 

 

 

 

 

 

 

 

 

 

Source Measure Sum of df Mean F Sig. Observed
Squares Square Power
Factor VAvg .425 2 .213 8.007 .001 .944
Area95 .05579 2 .02790 .808 .452 .180
PLength 385.673 2 192.837 8.120 .001 .947
F actor*TX VAvg .007262 2 .003631 .137 .873 .070
Area95 .04954 2 .02477 .718 .493 .164
PLength 6.868 2 3.434 .145 .866 .071
Error VAvg 1.222 46 .02656
Area95 1.587 46 .03451
PLength 1092.376 46 23 .747

 

ANOVA Table for the Relationship of Time and Treatment to Strength Variables for Extension at 60°/sec

 

 

 

 

 

 

 

 

 

 

 

Source Measure Sum of df Mean F Sig. Observed
Squares Square Power
Factor PeakTq 1659.027 3 553.009 18.744 .000 1.000
AngkTq 1677.693 3 559.231 20.814 .000 1.000
Angower 850.779 3 283 .593 24.905 .000 1.000
F actor*TX PeakTq 7.062 3 2.354 .080 .971 .064
AngkTq 66.227 3 22.076 .822 .486 .219
Angower 6.046 3 2.015 .177 .912 .081
Error PeakTq 2035 .694 69 29.503
AngkTq 1853 .884 69 26.868
Angower 785 .690 69 1 1 .387

 

AN OVA Table for the Relationship of Time and Treatment to Strength Variables for Flexion at 60°/sec

 

 

 

 

 

 

 

 

 

 

 

Source Measure Sum of (If Mean F Sig. Observed
Squares Square Power
Factor PeakTq 94.510 3 31.503 3.124 .031 .703
AngkTq 260.229 3 86.743 8.309 .000 .990
Angower 158.642 3 52.881 8.110 .000 .998
Factor*TX PeakTq 23.431 3 7.810 .774 .512 .208
AngkTq 23 .423 3 7.808 .748 .527 .202
Angower 30.387 3 10.129 1.553 .209 .392
Error PeakTq 695 .901 69 10.086
AngkTq 720.363 69 10.440
Angower 449.937 69 6.521

 

61

 

 

 

ANOVA Table for the Relationship of Time and Treatment to Strength Variables for Extension at 120°/sec

 

 

 

 

 

 

 

 

 

 

 

Source Measure Sum of df Mean F Sig. Observed
Squares Square Power
Factor PeakTq 4.635 3 1.545 .082 .969 .064
AngkTq 20.156 3 6.719 .276 .842 .100
Angower 29.581 3 9.860 .246 .864 .094
Factor*TX PeakTq 78.753 3 26.251 1.397 .251 .355
AngkTq 41.693 3 13.898 .572 .636 .162
Angower 324.924 3 108.308 2.706 .052 .634
Error PeakTq 1296.242 69 18.786
AngkTq 1677.641 69 24.314
Angower 2761.905 69 40.028

 

ANOVA Table for the Relationship of Time and Treatment to Strength Variables for Flexion at 120°/sec

 

 

 

 

 

 

 

 

 

 

 

Source Measure Sum of df Mean F Sig. Observed
Squares Square Power
Factor PeakTq 18.339 3 6.1 13 1.137 .340 .294
AngkTq 23.088 3 7.696 .855 .469 .227
Angower 21.801 3 7.267 .385 .764 .122
Factor‘TX PeakTq 50.984 3 16.995 3.162 .030 .709
AngkTq 57.725 3 19.242 2.138 .103 .523
Angower 197.615 3 65.872 3.487 .020 .756
Error PeakTq 370.824 69 5 .374
AngkTq 621.072 69 9.001
Angower 1303.274 69 18.888

 

62

 

 

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