Trunk muscle activity while lifting objects of unexpected weight.

OBJECTIVE To determine trunk muscle activity when lifting an object of greater weight than expected, which may contribute to the development of low back pain. DESIGN Electromyographic evaluation of trunk muscle activity. SETTING University of Tsukuba, Spine laboratory. PARTICIPANTS Eleven healthy men with a mean age of 24 (SD 2) years. INTERVENTIONS Trunk muscle activity was measured when subjects lifted an object with their right arm in immediate response to a light stimulus. Surface and wire electrodes were used to measure the activity of the rectus abdominis, external oblique and erector spinae muscles, and the transversus abdominis and lumbar multifidus muscles, respectively. The lifting tests were performed in three different settings: lifting an expected 1-kg object, lifting an unexpected 4-kg object (erroneously expected to weigh 1 kg), and lifting an expected 4-kg object. MAIN OUTCOME MEASURES The muscle activity induced when subjects lifted objects of different weights was compared by calculating the root mean square (RMS) of muscle activity at rest and % maximum voluntary contraction. RESULTS When the subjects were aware of the weight of the object to be lifted, the activity of the external oblique, transversus abdominis, erector spinae and lumbar multifidus muscles increased immediately after lifting. When the subjects were not aware of the weight of the object to be lifted, the increase in muscle activity was delayed (P<0.05). CONCLUSIONS Trunk muscles may not be able to function appropriately when individuals lift an object that is much heavier than expected.