A kinesiologic study of erectores spinae activity during trunk flexion and extension

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Abstract
Dynamic flexion and extension of the trunk in the sagittal plane were performed slowly by five normal males with straight knees and bent back under three load conditions. The activity of the erectores spinae muscle was recorded by electromyography and the mean amplitude of the EMG of the muscle per 05 s was calculated. The movements of the acromidn (AC), the trochanter major (TR) and the spina iliaca anterior superior (SIAS) were measured by video analyser, from these, the angle of forward flexion and the inclination of the pelvis were calculated. A sudden decrease of muscle activity during flexion was observed when the angle of flexion coincided with the pelvic inclination, that is, when the AC, the TR and the SIAS were approximately in line. An abrupt increase in the activity of the muscle during extension began before the two angles coincided. The decrease of muscle activity occurred when the flexibility of the lumbar vertebrae was limited, while the abrupt activity increase occurred with the recovery of their limited flexibility.