Turning on the central contribution to contractions evoked by neuromuscular electrical stimulation

• Published in: Journal of applied physiology (1985) 2007-07, Vol.103 (1), p.170-176
• Main Authors: Dean, J. C, Yates, L. M, Collins, D. F
• Format: Article
• Language: English
• Subjects: Adult; Biological and medical sciences; Central Nervous System - cytology; Central Nervous System - physiology; Electric Stimulation - methods; Excitatory Postsynaptic Potentials; Female; Fibers; Frequencies; Fundamental and applied biological sciences. Psychology; Humans; Male; Motor Neurons - physiology; Muscle Contraction; Muscle Fatigue; Muscle, Skeletal - innervation; Muscular system; Nervous system; Neuromuscular Junction - physiology; Neurons; Neurons, Afferent - physiology; Synaptic Transmission - physiology; Time Factors; Torque
• ISSN: 8750-7587; 1522-1601
• DOI: 10.1152/japplphysiol.01361.2006

Faculty of Physical Education and Recreation, Centre for Neuroscience, University of Alberta, Edmonton, Alberta, Canada Submitted 30 November 2006 ; accepted in final form 25 April 2007 Neuromuscular electrical stimulation can generate contractions through peripheral and central mechanisms. Direct activation of motor axons (peripheral mechanism) recruits motor units in an unnatural order, with fatigable muscle fibers often activated early in contractions. The activation of sensory axons can produce contractions through a central mechanism, providing excitatory synaptic input to spinal neurons that recruit motor units in the natural order. Presently, we quantified the effect of stimulation frequency (10–100 Hz), duration (0.25–2 s of high-frequency bursts, or 20 s of constant-frequency stimulation), and intensity [1–5% maximal voluntary contraction (MVC) torque generated by a brief 100-Hz train] on the torque generated centrally. Electrical stimulation (1-ms pulses) was delivered over the triceps surae in eight subjects, and plantar flexion torque was recorded. Stimulation frequency, duration, and intensity all influenced the magnitude of the central contribution to torque. Central torque did not develop at frequencies 20 Hz, and it was maximal at frequencies 80 Hz. Increasing the duration of high-frequency stimulation increased the central contribution to torque, as central torque developed over 11 s. Central torque was greatest at a relatively low contraction intensity. The largest amount of central torque was produced by a 20-s, 100-Hz train (10.7 ± 5.5 %MVC) and by repeated 2-s bursts of 80- or 100-Hz stimulation (9.2 ± 4.8 and 10.2 ± 8.1% MVC, respectively). Therefore, central torque was maximized by applying high-frequency, long-duration stimulation while avoiding antidromic block by stimulating at a relatively low intensity. If, as hypothesized, the central mechanism primarily activates fatigue-resistant muscle fibers, generating muscle contractions through this pathway may improve rehabilitation applications. plateau potentials; posttetanic potentiation; recruitment order; rehabilitation Address for reprint requests and other correspondence: D. F. Collins, Human Neurophysiology Laboratory, Faculty of Physical Education and Recreation, E-435 Van Vliet Centre, Univ. of Alberta, Edmonton, Alberta, Canada T6G 2H9 (e-mail: dave.collins{at}ualberta.ca )