In musculoskeletal modeling, isometric muscle strength has been primarily determined based on muscle size. Specifically, the maximum force a muscle can produce may be calculated as:  
where FMAX is maximum isometric muscle force, MMS is maximum muscle stress, and PCSA is muscle physiological cross-sectional area. In general, modeling studies have selected a constant value of MMS, and applied it to all muscles in the model. However, the values reported in the literature for MMS vary widely [1, 2], from as little as 23 N/cm2 up to 137 N/cm2. Furthermore, MMS is likely lower in older adults than young adults, as age-related declines in muscle strength are significantly greater than declines in muscle mass [3], and the specific tension of gastrocnemius fascicles is 30% lower in elderly men than young men [4]. In addition, MMS is not constant between muscle groups. For example, the MMS of the elbow flexors is much greater than that of the elbow extensors [1], while the MMS of the ankle dorsiflexors is more than twice that of the ankle plantar flexors [5]. Thus, the use of a single constant for MMS in musculoskeletal models does not account for differences between individuals or muscle groups, and there is a need for a quantitative approach to assign different values of MMS to muscles in musculoskeletal models.
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