Length dependent recruitment behaviour in fast skeletal muscle

There is evidence that optimum lengths of motor units of a muscle are not uniform, but distributed with respect to muscle length according to motor unit size. For cat flexor digitorum longus it was found that smaller units had their optimum length at lower muscle lengths and larger units at higher muscle lengths. The purpose of this study was to investigate effects of such distribution on isometric recruitment-force characteristics at different muscle lengths by modelling. Recruitment force curves were calculated at different muscle lengths (-6, -3 and 0 mm below muscle optimum length) using a random recruitment order and an recruitment order according to size principle. The results show that although the slope of the isometric recruitment-force curve is less for the size principle condition, there are marked differences in slope at different lengths compared to the random condition. It is concluded that a distribution of motor unit optimum lengths can cause differences in the isometric recruitment curve between different muscle lengths, making grading of force during FES difficult.