Trunk muscle activation: The effects of torso flexion, moment direction, and moment magnitude

This study was performed tp quantify the electromyographic trunk muscle activities in response- to variations in moment magnitude and direction while in forward-flexed postures. Methods. Recordings were made over eight trunk muscles in 19 subjects who maintained forward-flexed postures of 30° and 60°, in each of the two flexed postures external moments of 20 Nm and 40 Nm were applied via a chest harness. The moment directions were varied in seven 30° increments to a subject's right side. such that the direction of the applied load ranged from the upper body's anterior midsagittal plane (0°) to the posterior midsagittal plane (180°). Results. Statistical analyses yielded significant moment magnitude by moment-direction interaction effects for the EMG output from six of the eight muscles. Trunk flexion by moment-direction irrteractions were observed in the responses from three muscles:. Conclusions. In general, the primary muscle supporting the torso and the applied load was the contralateral (left) erector spinae. The level o f electromyographic activity in the anterior muscles was quite low, even with the posterior moment directions.