MARSH, R.L.*; ELLERBY, D.J.; BUCHANAN, C.I.: Mechanical function of thigh muscles in running Guinea fowl.

When animals run on the level they perform no net work. This observation has led to the hypothesis that most muscles active during running simply produce force to support the body and prevent collapse of the limb during the stance phase. Measurements of the function of muscles in the lower leg of birds and mammals support this hypothesis. However, more limited data on the thigh muscles of mammals suggest a more complicated picture, with some muscles shortening and producing work, while others may be stretched significantly while active. We measured the function of several strap-like thigh muscles in running Guinea fowl using sonomicrometry and electromyography. Muscles active during both swing and stance were found to undergo significant active shortening. We hypothesize that a significant fraction the hindlimb muscles in running birds actively shorten to provide both internal and external work in each stride. Internal work may be relatively high because of co-contracting muscles that function to provide stability during critical phases of the stride. Supported by NIH grant AR47337.