Evidence that maximum muscle stress is not a constant: differences in specific tension in elbow flexors and extensors.

The specific tension of muscle (or maximum muscle stress) is the maximum force developed per unit cross-sectional area and is a frequently used parameter by investigators estimating muscle force. Generally, it is assumed to be a constant value for all muscles and, when multiplied by a muscle's cross-sectional area, is used to provide a measure of a muscle's maximum force production. In this study, the specific tension for elbow flexors and for extensors were compared to evaluate the validity of this assumption. Maximum muscle stress was determined using maximum joint moments measured as a function of joint angle and using anatomical parameters reported in the literature. It was observed that the specific tension for elbow flexors was considerably larger than for extensors when measured a variety of ways. The exact reasons for the differences are unknown, but variations in specific tension of individual fibers may play a role. It was concluded that the use of a constant value for specific tension in muscle models is questionable in studies that demand accurate results.