The Journal of Neurophysiology Vol. 88 No. 3 September 2002, pp. 1308-1317
Copyright ©2002 by the American Physiological Society

Mutability of Bifunctional Thigh Muscle Activity in Pedaling due to Contralateral Leg Force Generation

 ¹Rehabilitation R and D Center (153), Veterans Affairs Palo Alto Health Care System, Palo Alto, California 94304;  ²Department of Functional Restoration and  ³Department of Mechanical Engineering, Biomechanical Engineering Division, Stanford University, Stanford, California 94305; and  ⁴Department of Physical Therapy and Human Movement Sciences and Department of Physical Medicine and Rehabilitation, Northwestern University Medical School, Chicago, Illinois 60611-2814

Kautz, S. A., D. A. Brown, H.F.M. Van der Loos, and F. E. Zajac. Mutability of Bifunctional Thigh Muscle Activity in Pedaling due to Contralateral Leg Force Generation. J. Neurophysiol. 88: 1308-1317, 2002. Locomotion requires uninterrupted transitions between limb extension and flexion. The role of contralateral sensorimotor signals in executing smooth transitions is little understood even though their participation is crucial to bipedal walking. However, elucidating neural interlimb coordinating mechanisms in human walking is difficult because changes to contralateral sensorimotor activity also affect the ipsilateral mechanics. Pedaling, conversely, is ideal for studying bilateral coordination because ipsilateral mechanics can be independently controlled. In pedaling, the anterior and posterior bifunctional thigh muscles develop needed anterior and posterior crank forces, respectively, to dominate the flexion-to-extension and extension-to-flexion transitions. We hypothesized that contralateral sensorimotor activity substantially contributes to the appropriate activation of these bifunctional muscles during the limb transitions. Bilateral pedal forces and surface electromyograms (EMGs) from four thigh muscles were collected from 15 subjects who pedaled with their right leg against a right-crank servomotor, which emulated the mechanical load experienced in conventional two-legged coupled-crank pedaling. In one pedaling session, the contralateral (left) leg pseudo-pedaled (i.e., EMG activity and pedal forces were pedaling-like, but pedal force was not allowed to affect crank rotation). In other sessions, the mechanically decoupled contralateral leg was first relaxed and then produced rhythmic isometric force trajectories during either leg flexion or one of the two limb transitions of the pedaling leg. With contralateral force production in the extension-to-flexion transition (predominantly by the hamstrings), rectus femoris activity and work output increased in the pedaling leg during its flexion-to-extension transition, which occurs simultaneously with contralateral extension-to-flexion in conventional pedaling. Similarly, with contralateral force production in the other transition (i.e., flexion-to-extension; predominantly by rectus femoris), hamstrings activity and work output increased in the pedaling leg during its extension-to-flexion transition. Therefore rhythmic isometric force generation in the contralateral leg supported the ongoing bifunctional muscle activity and resulting work output in the pedaling leg. The results suggest that neural interlimb coordinating mechanisms fine-tune bifunctional muscle activity in rhythmic lower-limb tasks to ensure limb flexion/extension transitions are executed successfully.