Re-evaluation of the amplitude-force relationship of trunk muscles.

Amplitude-force relationships of major trunk muscles are established in terms of curve characteristics, but up to now were not normalized with respect to maximum voluntary contraction (MVC) force levels. The study therefore aims at a re-evaluation of trunk muscle amplitude-force relationship data according to MVC. Surface EMG of five major trunk muscles was taken from 50 healthy subjects of both sexes (age 20-40 years). All tasks were performed in a device where submaximal loads on the trunk were applied by gradually tilting the subjects in sagittal plane to horizontal position. MVC flexion and extension forces were determined in upright position using an additional harness over the subject's shoulders. Furthermore, the subject's upper body mass (UBM) was obtained during forward tilt to horizontal. MVC to UBM ratio was calculated, corrected by the actual tilt angle, and these linearly estimated values compared with the measured relative values according to MVC. All abdominal muscles confirmed the known non-linear amplitude-force relationship. At low load levels the linearly estimated values overestimated the measured ones and, at higher load levels, underestimated the true stress levels considerably. Back muscles confirmed the known linear curve shape, but for the longissimus muscle at L1 level measured data was always below estimated values. With increasing load, muscular stress of abdominal muscles changes from overestimated towards considerably underestimated values if expected stress levels are based on linear interpolation. Major back muscles' activation levels are nearly linear, but the amplitude-force relationship values seem overestimated for longissimus.