The purpose of this dissertation was to examine the conceptual, operational, and theoretical basis of physical activity (PA) self-efficacy among youth, and in particular adolescent girls. Manuscript one examined the conceptual basis of PA self-efficacy by conducting a concept analysis. A review of the literature identified 276 articles with 55 articles selected for review. Social cognitive theory (SCT) guided the analysis. Several conceptual definitions were found in the literature. Defining attributes of PA self-efficacy were personal cognition/perception, self-appraisal process, related action, and the power to choose PA. The concept was also found to be dynamic and bi-dimensional. Antecedents and consequences were consistent with SCT. This analysis provided clarification of the concept and identified the need to examine PA self-efficacy instruments for consistency between conceptual and operational definitions.Manuscript two investigated the operational basis of PA self-efficacy by comparing the factorial validity, measurement invariance, and composite reliability of two PA self-efficacy instruments. A secondary analysis of data was conducted from Year 1 and 2 of a group randomized controlled trial (RCT) investigating the effect of a 17-week intervention on increasing PA among girls (N=1012) in the 5th-8th grades. Girls completed 6- and 7-item PA self-efficacy instruments at baseline and post-intervention. Confirmatory factor analyses were used to conduct invariance testing for intervention and control groups. Model fit was assessed using the model chi-square test, comparative fit index (CFI), and root mean square error of approximation (RMSEA). For simultaneous cross-group and longitudinal analysis, results demonstrated configural invariance for both instruments but not metric invariance. The 6-item instrument achieved partial metric invariance with one indicator non-invariant (2=170.224, df=112, p<.001, 2=24.308, df=18, p=.145, RMSEA=.032, CFI=.991, CFI= .001). Both instruments demonstrated longitudinal scalar invariance in the control group but not the intervention group. Composite reliability for the 6-item and 7-item instruments ranged from .772 - .842 and .719 - .800, respectively. Results suggest that the intervention influenced how girls responded to indicator items. Neither of the instruments achieved simultaneous metric invariance making it difficult to accurately examine mean differences in PA self-efficacy between groups.Manuscript three examined the theoretical basis of PA self-efficacy. The purpose of the study was to test hypothesized relationships of the Health Promotion Model (HPM) between individual, interpersonal, and situational influences and moderate-to-vigorous PA (MVPA) among adolescent girls. Data from Year 3 of the same group RCT were collected from girls (N=512) in the 5th-8th grades. MVPA was measured using accelerometers. Study hypotheses were analyzed using structural equation modeling (SEM). Mean age of the girls was 11.8 years (SD=1.0). PA self-efficacy predicted commitment to PA (β=.524) and MVPA (β=.343). Commitment to PA suppressed the total effect of PA self-efficacy on MVPA (β=.279). Social support and options for PA were not significant predictors of MVPA. The model predicted 33.5% of the variance in commitment and 10.2% of the variance in MVPA. Consistent with other studies, PA self-efficacy continues to be a significant correlate of MVPA for adolescent girls. While this study is the first to examine HPM relationships using an objective measure of MVPA, the model predicted a small amount of the variance for this outcome. Future theory testing studies should involve longitudinal analyses and include reliable and valid measures based on well-defined concepts.
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