VISCOELASTIC STRESS-RELAXATION DURING STATIC STRETCH IN HUMAN SKELETAL-MUSCLE IN THE ABSENCE OF EMG ACTIVITY

The present study sought to investigate the role of EMG activity durin g passive static stretch. EMG and passive resistance were measured dur ing static stretching of human skeletal muscle in eight neurologically intact control subjects and six spinal cord-injured (SCI) subjects wi th complete motor loss. Resistance to stretch offered by the hamstring muscles during passive knee extension was defined as passive torque ( Nm). The knee was passively extended at 5 degrees/s to a predetermined final position, where it remained stationary for 90 s (static phase) while force and integrated EMG of the hamstring muscle were recorded. EMG was sampled for frequency domain analysis in a second stretch mane uver in five control and three SCI subjects. There was a decline in pa ssive torque in the 90-s static phase for both control and SCI subject s, P < 0.05. Although peak passive torque was greater in control subje cts, P < 0.05, there was no difference in time-dependent passive torqu e response between control (33%) and SCI (38%) subjects. Initial and f inal 5-s IEMG ranged from 1.8 to 3.4 mu V . s and did not change durin g a stretch or differ between control and SCI subjects. Frequency doma in analysis yielded similar results in both groups, with an equal ener gy distribution in all harmonics, indicative of 'white noise'. The pre sent data demonstrate that no measurable EMG activity was detected in either group during the static stretch maneuver. Therefore, the declin e in resistance to static stretch was a viscoelastic stress relaxation response.