Timing of activation of the erector spinae and hamstrings during a trunk flexion and extension task

Study Design: Timing of activation of the hamstrings and erector spinae was assessed using surface electromyography. Objectives: To investigate the influence of posture and movement speed duri ng trunk flexion-extension on the flexion-relaxation response and trunk mus cle activation patterns. Summary of Background Data: The literature contains numerous reports on coa ctivity and synergistic behavior of major muscle groups during trunk flexio n-extension. There are few reports on the timing of muscle activation. Methods: Six subjects were recruited for a training Session and six biweekl y test sessions. Ten surface electromyogram electrodes and a lordosimeter w ere used to record timing of lumbar motion and muscle recruitment in the ha mstrings and at four sites in the thoracolumbar region. A 3 X 2 within-subj ect factorial design was used to test the effects of posture and speed on a ctivation patterns. Results: Patterns of muscle activation were found to be dependent on postur e and the direction of movement. The flexion-relaxation response was pervas ive in the lumbar region but was less consistent at the T9 and hamstring si tes. Significant differences in the delay between electromyogram activation and lumbar motion were found for the standing postures at initiation of ex tension, in which activation progressed in the caudad-to-cephalad direction . Conclusions: The flexion-relaxation response is ubiquitous in the lumbar er ector spinae and is present in the hamstrings and lower thoracic erector sp inae, although not consistently in all subjects. In standing, timing of act ivation differed significantly by site in extension but not in flexion. Mus cle activation patterns and flexion-relaxation were consistent over six biw eekly test sessions.