Gillis, G.B.*; Biewener, A.A.: Plasticity in Limb Muscle Function During Locomotion

Studies of musculoskeletal function in vivo during vertebrate locomotion have typically focused on muscles performing a specific role over a limited range of activities. As a result we currently have little appreciation for the breadth of function some muscles might exhibit over a broader range of activities. Using sonomicrometry and electromyography, we investigated patterns of activation and strain in the vastus lateralis and biceps femoris of rats during walking, trotting and galloping across different grades, and during swimming. During locomotion over ground, EMG bursts generally occupy the first 60-70% of the stance phase, and increase in intensity with increased speed or grade. Decreased grades lead to decreases in EMG intensity, particularly in the biceps, which exhibits little or no activity during downhill locomotion at any speed. During swimming, both muscles exhibit substantially shorter (about 30% of the propulsive phase), but more intense, EMG bursts. Total fascicle strain is higher during walking, trotting and swimming (0.21-0.26) than during galloping (0.15-0.17). Moreover, patterns of length change vary from nearly sinusoidal during swimming, to more complex waveforms on land. Finally, alterations in strain and stimulation in the vastus appear to change its mechanical role during locomotion. During walking, trotting and galloping (non-lead limb), the vastus is active mostly while isometric or during lengthening, however, during swimming or galloping (lead limb), it is active largely during shortening. Thus, the same muscle can shift from absorbing energy to generating energy, depending on gait or environment. These results indicate that some muscles in the rat hindlimb can exhibit substantial plasticity with respect to their gross function during locomotion.