Exp Brain Res (1992) 89:447-452 Experimental BrainResearch 9 Springer-Verlag1992 Lowered cutaneous sensitivity to nonpainful electrical stimulation during isometric exercise in humans Antti Pertovaara 1, Pentti Kemppainen l'z, and Heikki Lepp~inen1 t Department of Physiology and 2 Department of Prosthetic Dentistry, University of Helsinki, Siltavuorenpenger 20 J, SF-00170 Helsinki, Finland Received June 6, 1991 / Accepted November 25, 1991 Summary. The effect of isometric exercise on cutaneous sensitivity to nonpainful electric stimulation was studied in human subjects. The exercises consisted of brief (dura- tion: 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-depen- dent 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 exer- cising hand. Passive static pressure of the hand did not produce threshold changes, whereas activation of affer- ent inhibition by a vibrotactile stimulus (100 Hz, 0.1 mm) did produce a significant threshold elevation. Exercise- induced threshold elevation was also significant im- mediately 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 exer- cise load-dependently produces a phasic, rapidly at- tenuating 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 inhibi- tory mechanisms seem to have only a minor role. Key words: Isometric exercise Modulation Somato- sensory system - Human Introduction Effect of voluntary movement of the finger or limb on cutaneous sensitivity has been studied by numerous in- Offprint requests to." A. Pertovaara vestigators (eg. Angel et al. 1985; Chapin and Woodward 1981; Chapman et al. 1988; Coquery 1978; Coulter 1974; Dyhre-Poulsen 1978; Ghez and Lenzi 1971 ; Jiang et al. 1991) and it is known that movement produces a segmental phasic decrease of skin sensitivity which be- gins before the start of the movement. Also passive movement of the finger and limb reportedly decreases skin sensitivity (Chapman et al. 1987; Milne et al. 1988; Rushton et al. 1981) indicating that joint and skin affer- ents may have a significant contribution to the modula- tion produced by voluntary isotonic movement of the limb or finger, at least under some experimental con- ditions. The modulation of skin sensitivity by voluntary isometric activation of the limb muscles has been studied in less detail. In isometric contractions (i.e. when the muscle is kept at a constant length during the contrac- tion) the activation of joint afferents should be negligible. Furthermore, the skin and muscle afferents may be ac- tivated in a different manner than during isotonic con- ditions (Vallbo et al. 1979). The purpose of the present study was to determine in more detail than previously the effect of varying experi- mental parameters on the electrotactile threshold eleva- tions induced by isometric exercise in humans. Specific attention was paid to the possible role of the efference copy of the motor command signal and the activation of afferent inhibitory mechanisms as a cause of the thresh- old changes. Methods Subjects The total number of human volunteers tested in the psychophysical experiments of this study was 8 (all males, age range 22-42 years). The subjects were healthy university graduates. Informed consent was obtained before the experiments. Two of the subjects par- ticipated in every experiment. Isometric hand exercise Three different levels of isometric exercise (10, 20 or 30% of the maximal force) were produced by pressing the fingers (palmar