Effects of activation pattern on nonisometric human skeletal muscle performance

1 Department of Physical Therapy and 2 Interdisciplinary Graduate Program in Biomechanics and Movement Sciences, University of Delaware, Newark, Delaware; and 3 Department of Mechanical and Aeronautical Engineering, University of California, Davis, California Submitted 28 June 2006 ; accepted in final form 28 December 2006 During volitional muscle activation, motor units often fire with varying discharge patterns that include brief, high-frequency bursts of activity. These variations in the activation rate allow the central nervous system to precisely control the forces produced by the muscle. The present study explores how varying the instantaneous frequency of stimulation pulses within a train affects nonisometric muscle performance. The peak excursion produced in response to each stimulation train was considered as the primary measure of muscle performance. The results showed that at each frequency tested between 10 and 50 Hz, variable-frequency trains that took advantage of the catchlike property of skeletal muscle produced greater excursions than constant-frequency trains. In addition, variable-frequency trains that could achieve targeted trajectories with fewer pulses than constant-frequency trains were identified. These findings suggest that similar to voluntary muscle activation patterns, varying the instantaneous frequency within a train of pulses can be used to improve muscle performance during functional electrical stimulation. nonisometric contractions; doublets; mathematical modeling Address for reprint requests and other correspondence: S. Binder-Macleod, Dept. of Physical Therapy and Interdisciplinary Graduate Program in Biomechanics and Movement Sciences, 301 McKinly Laboratory, Univ. of Delaware, Newark, DE (e-mail: sbinder{at}udel.edu )