A nerve stimulation method to selectively recruit smaller motor-units in rat skeletal muscle.

: J Neurosci Methods 2001 May 30;107(1-2):87-92 Related Articles, Books, LinkOut A nerve stimulation method to selectively recruit smaller motor-units in rat skeletal muscle.van Bolhuis AI, Holsheimer J, Savelberg HH.Department of Movement Sciences, University of Maastricht, P.O. Box 616, 6200 MD, Maastricht, The Netherlands. annette.vanbolhuis@bw.unimaas.nlElectrical stimulation of peripheral nerve results in a motor-unit recruitment order opposite to that attained by natural neural control, i.e. from large, fast-fatiguing to progressively smaller, fatigue-resistant motor-units. Yet animal studies involving physiological exercise protocols of low intensity and long duration require minimal fatigue. The present study sought to apply a nerve stimulation method to selectively recruit smaller motor-units in rat skeletal muscle. Two pulse generators were used, independently supplying short supramaximal cathodal stimulating pulses (0.5 ms) and long subthreshold cathodal inactivating pulses (1.5 s) to the sciatic nerve. Propagation of action potentials was selectively blocked in nerve fibres of different diameter by adjusting the strength of the inactivating current. A tensile-testing machine was used to gauge isometric muscle force of the plantaris and both heads of the gastrocnemius muscle. The order of motor-unit recruitment was estimated from twitch characteristics, i.e. peak force and relaxation time. The results showed prolonged relaxation at lower twitch peak forces as the intensity of the inactivating current increased, indicating a reduction of the number of large motor-units to force production. It is shown that the nerve stimulation method described is effective in mimicking physiological muscle control.