An exponential model of isometric muscular fatigue as a function of age and muscle groups.

Despite the recommendations that an important design criterion is not to exceed 15% of an operator's maximum voluntary contraction (MVC) with any muscle that must be used for a long period of time, many tasks involve the exertion of much higher forces coupled with long-term contractions. Many studies have investigated the force-time relationship of isometric muscle contractions to determine the endurance time of a given relative force. To date, however, direct studies of muscle performance throughout fatiguing tasks have not been conducted to the same degree. This research was concerned with studying the effects of different muscle groups (biceps vs quadriceps) of subjects with different age groups (20-29 vs 50-59 years of age) on long-term muscular isometric contractions at different levels of %MVC (20%, 40%, 60%, 80%, and 100% MVC), and modelling the functional data to describe the time course of strength decrement. The data revealed that the time course of strength decrement was best modelled by the function: [formula: see text] An experiment, using 20 subjects with each subject performing 10 conditions (two muscle groups x five levels of %MVC), showed that this function accounted for over 95% of the variance of strength decrement. All parameter estimates were statistically significant.