ABSTRACT
Electrical 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.
Subject(s)
Electrophysiology/methods , Motor Neurons/physiology , Muscle Contraction/physiology , Muscle, Skeletal/physiology , Neurophysiology/methods , Peripheral Nerves/physiology , Recruitment, Neurophysiological/physiology , Action Potentials/physiology , Animals , Electric Stimulation/instrumentation , Electric Stimulation/methods , Electrophysiology/instrumentation , Isometric Contraction/physiology , Male , Motor Neurons/cytology , Muscle Fibers, Fast-Twitch/cytology , Muscle Fibers, Fast-Twitch/physiology , Muscle Fibers, Slow-Twitch/cytology , Muscle Fibers, Slow-Twitch/physiology , Muscle, Skeletal/innervation , Neurophysiology/instrumentation , Peripheral Nerves/cytology , Rats , Rats, Inbred WKY , Sciatic Nerve/cytology , Sciatic Nerve/physiologyABSTRACT
OBJECTIVE: To evaluate the effects of one 10-minute stretch on muscle stiffness in subjects with short hamstrings. DESIGN: Randomized control trial. SETTING: Laboratory for human movement sciences in the department of rehabilitation of a university hospital. SUBJECTS: Sixteen students from the Department of Human Movement Sciences participated with informed consent in the experiment. Subjects were limited to men and women without a history of neurological and orthopedic disorders. To select subjects with short hamstrings, the finger-ground distance had to be greater than 0cm (unable to touch the floor when bending forward) and the manual leg lifting was not to exceed 80 degrees. One group of 10 subjects performed static stretching exercises during 10 minutes interspersed with relaxing, whereas the untreated group of 6 subjects was used as a control. MAIN OUTCOME MEASURES: The instrumental straight-leg-raising set-up enables the measurement of the force needed to lift the leg, range of motion (ROM), pelvic-femoral angle, and the electromyogram of the hamstrings. These variables provide information about the stiffness, elongation, and state of activity of the hamstring muscles. RESULTS. One 10-minute sport stretch resulted in a significant increase in passive muscle moment, ROM, and elongation of the hamstrings. There was no significant change in the course of the passive muscle stiffness curve with respect to the prestretch stiffness curve. CONCLUSIONS: One session of static stretching does not influence the course of the passive muscle stiffness curve. The increased ROM, i.e., the extensibility of the hamstrings, results from an increase in the stretch tolerance.
