Stimulation level-dependent length-force and architectural characteristics of rat gastrocnemius muscle

Length-force characteristics of maximally and submaximally stimulated rat medial gastrocnemius muscle were studied in relation to muscle geometry. Maximal stimulation was obtained by stimulating the nerve innervating the muscle with supramaximal current. Submaximal stimulation was obtained by using 600-Hz blocking stimuli in addition to the 100-Hz excitation stimuli. Geometry of the active muscle was recorded by photography. Length-force characteristics during progressively developing blocks showed muscle optimum lengths occurring at increasing lengths. A significant correlation (r = -0.9) was found between decrease of optimum force as a consequence of the block and the length shift for optimum length. For the length range in which optimum lengths were found muscle geometry could not be distinguished on the basis of stimulation protocol used. At shorter lengths, however, differences of geometry were encountered. It is concluded that any differences of fiber length and/or shortening velocity could not explain the shifts of optimum lengths of up to 14%. Two major factors for explaining the stimulation level-dependent length-force characteristics were indicated, but could not be distinguished unequivocally as yet: (a) The occurrence of a distribution of fiber optimum lengths with respect to muscle optimum length in such a way that fibers of large motor units reach their optimum at shorter muscle lengths and fibers of motor units of decreasing size reach their optimum length at progressively higher muscle lengths. (b) If the blocking technique involved slowing of firing of motor units before derecruitment, a dependence of optimum length on firing frequency could have played a major role in the stimulation level-dependent shifts of this length. Possible functional significance of these phenomena for in vivo performance of movement is discussed.