Activity in torso muscles during relaxed standing.

Electromyographic activity of erector spinae, external oblique, and rectus abdominis muscles was studied during relaxed standing compared to lying down. Activity in the forearm extensors and forearm flexors was also studied. Surface electrodes were used. Each of the torso muscles exhibited 0.2 microV of activity and the forearm muscles 0.1 microV while subjects were relaxed and lying down. During quiet standing the erector spinae, external oblique, and rectus abdominis muscles showed a median activity of 1.0 microV, 2.5 microV, and 0.7 microV respectively (for a minimum of ten 10-sec samples per subject). Examination of the integrated records during standing revealed no periods without increased muscle activity in the torso muscles. By contrast, activity in the forearm muscles did not increase during standing. The major superficial muscles of posture in the torso appear to act as guy wires, being continually active during standing. There is no support for hypotheses of passive support for the torso, nor do torso muscles act in either/or fashion; both anterior and posterior muscles are active at once. There is no sign of generally increased muscle tone in all muscles or in extensors; only the postural muscles are continuously active.