RELATIONSHIPS BETWEEN RANGE OF MOTION, L(0), AND PASSIVE FORCE IN 5 STRAP-LIKE MUSCLES OF THE FELINE HIND-LIMB

The relationships between range of motion, optimal length for force pr oduction (l(o)), and passive force provide useful insights into the st ructure and function of muscles but are unknown for most individual mu scles. We measured these values and examined their relationships in fi ve strap-like muscles of the cat hind limb: caudofemoralis, semitendin osus, sartorius anterior, tenuissimus, and biceps femoris anterior. Th e range of motion relative to l(o) was found to vary significantly bet ween different muscles and even between different specimens of the sam e muscle. The passive force-length (FL) curve was found to be correlat ed with both l(o) and l(max) (maximal in situ muscle length) but was c orrelated more strongly with l(max). The mean passive force produced b y these muscles at l(max) was less than 7% of estimated maximal isomet ric force, suggesting that passive force may not be important in these muscles during normal activation patterns. The variance in passive FL curves between specimens of the same muscle was found to be significa ntly lower when length was scaled by l(max) as opposed to l(o). These results suggest that l(max) may provide a more useful scaling factor f or generic models of muscle. However, the passive length-tension prope rties of mammalian muscle appear to reflect a complex mix of structure s at both the myofilament and connective tissue levels that may differ depending on muscle-fiber architecture and perhaps on the history of trophic influences on a particular specimen. (C) 1996 Wiley-Liss, Inc.