Fig. 4. Function of muscles 177c and 179 during the preferred-speed running. (A,B) Simultaneous coxa–femur joint kinematics of the hindlimb and EMG recordings of muscles 177c and 179 during preferred-speed running. The coxa–femur joint angle increased with extension during stance (shaded areas) and decreased with flexion. Both muscles were activated during shortening. Note the overlap in muscle activity. (C,D) Individual muscle strain and stress during isolated muscle experiments. The shortening and lengthening patterns were determined from the joint kinematics during running. Muscle forces were measured as the muscles were stimulated to simulate preferred-speed running. (E) Work loops generated using in vivo conditions. The counterclockwise direction of the muscle 177c work loop illustrates that this muscle generated higher forces during shortening than during lengthening, resulting in positive work or power output (P). The clockwise direction of the work loop for muscle 179 illustrates that this muscle generated higher forces during lengthening than during shortening, resulting in negative work or energy absorption when operating under preferred-speed running conditions. The shaded areas indicate the stance phase of running when the joint angle increases and the muscles shorten. Squares represent the timing of muscle action potentials.
