Electrical activity of muscles of the trunk during walking.

In contrast to the muscles of the lower limbs, little attention has been paid to the muscles of the trunk during ambulation. This neglect is partially understandable since, as compared with limb muscles, trunk muscles are difficult to palpate individually or observe separately during contraction. Moreover, movement of the trunk is brought about by intricately patterned contributions of many different muscles, so that the effect of a single muscle is difficult to demonstrate. Electromyography has proved an effective means of studying the function of muscles of the trunk. Morris, Benner & Lucas (1962) placed electrodes in specific muscles of the back to demonstrate electro-myographically the contributions of muscle groups in flexion-extension, rotation and other movements of the trunk. Similar experiments have been made on abdominal muscles with the subjects performing isolated motions in the supine and standing positions (Walters & Partridge, 1957; Floyd & Silver, 1950). Joseph & McColl (1961), using surface electrodes to record the activity of the posterior vertebral muscles, found that, in the standing-at-ease position, the activity of these muscles varied at different levels. The level of each specific muscle studied by us is given in Table 1. It is recognized that there may be minor variations in activity at other vertebral levels, but our investigation of back muscles was not concerned with the vertebral level but with which muscles (and muscle groups) are active during standing and during walking at two speeds. Our intention was to interpret such activity in terms of phases of the walking cycle. We believed that a pattern might emerge. In contrast to Joseph, we chose to use electrodes placed within the muscle rather than surface electrodes, which, for our purposes, were believed to be too 'noisy' for recording the walking subject. We chose this method of study because we wished to expand it to obtain data on subjects walking with certain spinal supports (a chairback brace and a lumbosacral corset) in order to test their effect on muscle activity. This second study (Waters& Morris, 1970) has been reported in the literature. Walking requires an upset of the delicate balance of the trunk, which is maintained by minimum muscle activity during standing at rest. During ambulation the pelvis undergoes significant translational and rotary motion in the sagittal, coronal and transverse planes. Therefore, the requirements for balancing the trunk by action of the trunk muscles are much more complex than during standing.