Effects of external load and abduction angle on EMG level of shoulder muscles during isometric action

Goal of the study was to investigate the effect of muscle length and load on EMG of shoulder muscles. Surface EMG's were taken of seven subjects during isometric abduction. One maximal and three submaximal tasks were performed. The force was always at right angles to the arm. Arm and electrode position were carefully standardized. EMG's for most muscles appeared to rise with increase in abduction angle for submaximal loads. Many different patterns were found at maximal load. Intertrial variation was quite acceptable. For pectoralis major the part from 90 to 135 degrees EMG curve becomes increasingly steeper with load. This is likely to be due to an increase of the lordosis of the lumbar spine as loads become heavier. The pectoralis major gets an adequate leverage to contribute to the task when the upper part of the thorax is tilted backwards. It is suggested that, because we do not know whether movements, relative muscle activation and force-length relation are the same when submaximal and maximal contractions are compared, it is not advisable to use MVC's for normalisation purposes in shoulder EMG research.