GILLIS, G.B.*; FLYNN, J.P.; BIEWENER, A.A.: In Vivo Patterns of Strain and Activity in Thigh Muscles of Goats During Locomotion

To date, studies of limb muscle function during terrestrial locomotion have focused largely on extensors acting at the ankle joint, and have often emphasized the nearly isometric behavior of these muscles when active. In contrast, our recent recordings of strain and activity in hip and knee extensors of rats have shown that these more proximal muscles undergo substantial length changes while active. In order to investigate more broadly the activation conditions and strain regime of thigh muscles during quadrupedal locomotion, we used sonomicrometry and electromyography to study the function of the vastus lateralis and biceps femoris of goats. Length change and EMG data were collected from four individuals (16-26 kg) trained to exercise over a range of speeds (1-4 m/s). For several individuals, multiple pairs of sonomicrometry crystals were implanted into the vastus to test for spatial heterogeneity in fascicle strain regimes. Both muscles exhibit most of their EMG activity during stance. Fascicles of the biceps shorten substantially during stance (up to 40% of resting length) and re-lengthen during swing. Vastus fascicles are stretched during the first half of stance (mean = 16 % rest length) and shorten a similar amount over the second half of stance. The vastus then undergoes a similar stretch-shortening cycle during swing, but with greater strain levels. Although subtle variations in strain magnitude and timing are observed at different locations in the vastus, general patterns of length change are qualitatively similar in the proximal, middle and distal regions of the muscle. Thus, like the hip and knee extensors of rats, the biceps and vastus of goats undergo substantial strains while active during the stance phase of locomotion.