Background: With the growing need to promote muscle strengthening activity (MSA) for improved health-related quality of life (HRQOL) comes the growing need for proper measurement of MSA behavior. The purpose of this study was to examine test and item functioning of the MSA scale (MSAS) using item response theory (IRT). Methods: The current research fit data from a sample of N = 400 respondents to two different graded response models (GRMs), a three-item muscular strength scale and a three-item muscular endurance scale. For each GRM, model-data fit was examined and IRT assumptions assessed. Results: An unconstrained GRM was found to fit the data better than the constrained model (Δ G²_Strength = 10.3, p = .006, RMSEA = .043 & Δ G²_Endurance = 7.0, p = .031, RMSEA = .021). GRM boundary location parameters covered the latent trait scale well for both strength (bs: -4.26 to 2.58) and endurance (bs: -3.86 to 1.79) scales with each item showing adequate fit to the data (all RMSEAs < .05). Test information was approximately evenly distributed around a theta of zero with summed information from theta ranges ±4 of 92.8 (strength) and 93.5% (endurance). Only 2.3 and 1.5% of persons misfit the strength and endurance GRMs, respectively. Conclusion: The MSAS has shown to be a valid tool for measuring MSA behavior in adults using modern psychometric theory.

Muscle strengthening activity (MSA), Item response theory (IRT), Graded response model (GRM), test information