Lowered cutaneous sensitivity to nonpainful electrical stimulation during isometric exercise in humans

The effect of isometric exercise on cutaneous sensitivity to nonpainful electric stimulation was studied in human subjects. The exercises consisted of brief (duration: 1-10 s) palmar flexions of the hand or foot against varying loads (10-30% of the maximal force). A visual "go" cue was used to indicate the start and end of the exercise. Isometric hand exercise produced a load-dependent increase of electrotactile thresholds of the fingers. The threshold elevation was rapidly attenuated with prolonged duration of the exercise. The hand exercise-induced threshold elevation was of equal magnitude in the glabrous and hairy skin of the fingers. Thresholds were not changed for the hand contralateral to the exercising hand. Passive static pressure of the hand did not produce threshold changes, whereas activation of afferent inhibition by a vibrotactile stimulus (100 Hz, 0.1 mm) did produce a significant threshold elevation. Exercise-induced threshold elevation was also significant immediately prior to the EMG response of the arm but not at the time of the visual "go" signal, or before it. The threshold increase found during the EMG response was not significantly stronger than that found prior to the EMG response. These results suggest that isometric exercise load-dependently produces a phasic, rapidly attenuating increase in cutaneous tactile thresholds in the exercising limb but not multisegmentally. Corollary efferent barrage from motor to sensory structures of the brain could be underlying the threshold changes produced by isometric exercise, whereas afferent inhibitory mechanisms seem to have only a minor role.