Sarcomere Length-Joint Angle Relationships of Seven Frog Hindlimb Muscles

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Abstract
The sarcomere length-joint angle relationship was measured in 7 different muscle-joint complexes (n = 43 muscles) of the frog hindlimb (Rana pipiens). Muscles studied included the cruralis, iliacus internus, gastrocnemius, gluteus magnus, gracilis major, semimembranosusand the semitendinosus. Muscle-joint complexes were mounted in a jig and submerged in chilled Ringer’s solution. Joints were rotated throughout their range of motion, while sarcomere length was measured by laser diffraction. Muscles were then formalin fixed and architectural properties determined by microdissection of individual muscle fibers. Sarcomere length change per degree of joint rotation (dLs/dΘ) ranged from a low of 3.7 nm/degree for the cruralis muscle acting at the knee to a high of 12.5 nm/ degree for the semitendinosus muscle acting at the hip. Values for dLs/dΘ were significantly different between all muscles (p < 0.001), and dLs/dΘ values for muscles acting at the hip were significantly greater than those for muscles acting at the knee (p < 0.005). dLs/dΘ was negatively correlated with fiber length, suggesting a balance between fiber length and moment arm in most muscle-joint systems. However, many exceptions to this generalization were noted. These data suggest that different muscle-joint systems are ‘designed’ for differential contribution of muscle force production to the joint torque profile. The low variability of these data also suggests that sarcomere number is tightly regulated in these muscle-joint systems but not simply as a result of the total in vivo muscle excursion.

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