TRUNK MUSCLE ACTIVATION - THE EFFECTS OF TORSO FLEXION, MOMENT DIRECTION, AND MOMENT MAGNITUDE

Objectives. This study was performed to quantify the electromyographic trunk muscle activities in response to variations in moment magnitude and direction while in forward-flexed postures. Methods. Recordings w ere made over eight trunk muscles in 19 subjects who maintained forwar d-flexed postures of 30-degrees and 60-degrees. In each of the two fle xed postures, external moments of 20 Nm and 40 Nm were applied via a c hest harness. The moment directions were varied in seven 30-degrees in crements to a subject's right side, such that the direction of the app lied load ranged from the upper body's anterior midsagittal plane (0-d egrees) to the posterior midsagittal plane (180-degrees). Results. Sta tistical analyses yielded significant moment magnitude by moment-direc tion interaction effects for the EMG output from six of the eight musc les. Trunk flexion by moment-direction interactions were observed in t he 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 of electromyographic activity in the anterior muscles was quite low, even with the posterior moment direct ions.