Torque resistance of the passive tissues of the trunk at axial rotation.

Sitting in a twisted working posture is common when driving an agricultural tractor in the field. To be able to twist backwards, the driver has to overcome the torque from the passive tissues (passive resistance). The purpose of this investigation was to determine the relationship between passive resistance at axial rotation and twisting angle of the trunk when a person is seated. Ten healthy tractor drivers and 10 healthy office workers were passively twisted about the cranio-sacral axis with simultaneous measurements of the torque needed to twist them and the twisting angle. Surface electromyogram was used to control the relaxation of the muscles of the subjects. An exponential function was fitted to the data, which shows that passive resistance increases progressively with twisting angle. No significant differences in trunk passive resistance due to the subject's occupation, direction of twist or their interactions could be found. The fitted function is useful when assessing the load in the trunk in tractor driving, but also for other occupational activities where the trunk is twisted.