Background: The 2018 (2^nd edition) Physical Activity Guidelines for Americans states that adults should participate in muscle strengthening activity (MSA) of at least moderate intensity using all major muscle groups on two or more days a week. However, these guidelines do not promote specific types of MSA such as muscular strength training or muscular endurance training. This ambiguity, in part, is due to the lack of evidence linking specific types of MSA to health outcomes. And this lack of evidence, in part, is due to the inability to measure varying MSA behavior. This study reports the construct validity evidence for the MSA Scale (MSAS). Methods: The following research consists of a second development stage presenting validity evidence for the MSAS. Previous research indicates that seven items can measure three MSA dimensions: a three-item muscular strength dimension, a three-item muscular endurance dimension, and a single-item body weight exercise dimension. The current research used both exploratory factor analysis (EFA) and confirmatory factor analysis (CFA) to examine the MSAS construct validity. Results: EFA indicated a two-factor structure explained 100% of the common variance among the 6 strength and endurance items (3 items per factor with all loadings > .52). The first factor was defined as strength and the second endurance. CFA indicated the two-factor MSAS measurement model had adequate fit (χ²/df = 4.24, GFI = 0.97, CFI = 0.92, and RMSEA = 0.09) with strength and endurance significantly (p < .001) predicting all observed variables. Factor strength scores were strongly correlated with strength sum scale scores and weakly correlated with endurance and body sum scale scores. Similarly, factor endurance scores were strongly correlated with endurance sum scale scores and weakly correlated with strength and body sum scale scores. Conclusion: The seven-item MSAS is a simple and valid tool for measuring MSA behavior in adults. Two additional items are included in the MSAS to quantify MSA participation.

Muscle strengthening activity (MSA), Construct validity, Exploratory factor analysis (EFA), Confirmatory factor analysis (CFA)