The health benefits of muscle strengthening activity (MSA) include the improvement of bone health, body composition, muscular strength, physical function, as well as reduced risk for many chronic diseases and improved health-related quality of life (HRQOL). However, the types of MSA associated with these benefits are less understood. This study reports the development and item analysis of a multidimensional scale to measure MSA behavior. Methods: The following procedures were followed in this measurement study: (1) formation of conceptual definitions for MSA and its subdomains through literature review and collaboration with content experts, (2) development of a large set of MSA items for each subdomain, (3) pilot testing items to a focus group with collected feedback, (4) pilot testing items for a pilot item analysis, (5) administration of an initial MSA instrument to a large sample of participants, (6) formal reliability and item analyses, and 7) formation and naming of a developed instrument. Results: The MSA instrument was initially defined to have two subdomains: muscular strength and muscular endurance. A 20-item scale was reduced to an 8-item scale through initial item analysis and focus group feedback. A convenience sample of 400 adult MSA participants completed the 8-item scale via electronic survey methods. Formal reliability and item analyses indicated that 7 items could measure three MSA subscales: muscular strength behavior, muscular endurance behavior, and body weight exercise behavior. Two additional items are included in the newly developed scale to quantify MSA participation. Conclusion: The seven-item MSA scale (MSAS) may be a simple and valid tool for measuring MSA in adults.