EVIDENCE OF A PREPROGRAMMED DEACTIVATION OF THE HAMSTRING MUSCLES FORTRIGGERING RAPID CHANGES OF POSTURE IN HUMANS

Normal subjects were asked to make rapid flexions of the legs from a s tationary initial standing posture in a self-paced mode. Because this movement implicates a rapid change in posture, questions were asked ab out the type of central command which must include the rupture of the erect posture and the accomplishment of the goal directed movement. Mo vements of the different segments of the body were recorded and analyz ed using the optoelectronic ELITE system. Electromyographic (EMG) acti vities of 8 muscles of the lower limb on one side were recorded, recti fied and integrated. The time relationships of the different EMG signa ls (activation or deactivation) were analyzed with respect to selected kinetic measures of the related segments of the body. In the majority of the subjects, before the movement onset, EMG events included a spe cific deactivation of the tonic EMG activity of the semimembranous (SM ) and semitendinous (ST) muscles (time onset relative to the onset of the legs flexion: -196.9 +/- 96.4 ms and -180.5 +/- 89.7 ms, respectiv ely). A second event was a phasic activation of the tibialis anterior (TA) muscle (time onset: -60.5 +/- 117.6 ms). Conjugate cross-correlat ion analysis of these EMG signals demonstrated the existence of a comm on coordinated strategy between the deactivation of the hamstring and the TA activation. Even though a small horizontal displacement of the head was recorded prior to leg movement, it occurred too late to induc e deactivation of the hamstring muscles. These results demonstrate tha t for rapid legs flexion, where the gravity forces are the main source of joint angle acceleration, the deactivation of the SM and ST muscle s acts in conjunction with the phasic activation of the TA. The prepro grammed deactivation of the SM and ST muscles represents the early pha se of the central command to switch from the standing to the squatting posture. (C) 1997 Elsevier Science Ireland Ltd.