Neural Control of Rhythmic Human Arm Movement: Phase Dependence and Task Modulation of Hoffmann Reflexes in Forearm Muscles

1 Motor Control Research Laboratory, School of Physical Education, University of Victoria, Victoria, British Columbia V8W 3P1, Canada; 2 Neurophysiology Laboratory, Faculty of Physical Education and Recreation, University of Alberta, Edmonton, Alberta T6G 2H9, Canada; and 3 Department of Biophysics, Katholieke Universiteit 6500 HB, Nijmegen, The Netherlands Zehr, E. Paul, David F. Collins, Alain Frigon, and Nienke Hoogenboom. Neural Control of Rhythmic Human Arm Movement: Phase Dependence and Task Modulation of Hoffmann Reflexes in Forearm Muscles. J. Neurophysiol. 89: 12-21, 2003. Although we move our arms rhythmically during walking, running, and swimming, we know little about the neural control of such movements. Our working hypothesis is that neural mechanisms controlling rhythmic movements are similar in the human lumbar and cervical spinal cord. Thus reflex modulation during rhythmic arm movement should be similar to that seen during leg movement. Our main experimental hypotheses were that the amplitude of H-reflexes in the forearm muscles would be modulated during arm movement (i.e., phase-dependent) and would be inhibited during cycling compared with static contraction (i.e., task-dependent). Furthermore, to determine the locus of any modulation, we tested the effect that active and passive movement of the ipsilateral (relative to stimulated arm) and contralateral arm had on H-reflex amplitude. Subjects performed rhythmic arm cycling on a custom-made hydraulic ergometer in which the two arms could be constrained to move together (180° out of phase) or could rotate independently. Position of the stimulated limb in the movement cycle is described with respect to the clock face. H-reflexes were evoked at 12, 3, 6, and 9 o'clock positions during static contraction as well as during rhythmic arm movements. Reflex amplitudes were compared between tasks at equal M wave amplitudes and similar levels of electromyographic (EMG) activity in the target muscle. Surface EMG recordings were obtained bilaterally from flexor carpi radialis as well as from other muscles controlling the wrist, elbow, and shoulder. Compared with reflexes evoked during static contractions, movement of the stimulated limb attenuated H-reflexes by 50.8% ( P