There seems to be considerable individual variation both in paraspinal and trapezius muscle activity according to the wide standard deviations in the mean values of the muscle activities. The left trapezius muscle activity varied from 1 to 12% of MVC and in right side from less than 1% to over 7% of MVC. The average paraspinal muscle activity varied from nearly 0% to over 3% of MVC between subjects of both groups. It was difficult to estimate how largely the muscle loading consists of dynamic and how largely of static load.