Paradoxical variation of strength determinants with different rotation axes in trunk flexion and extension strength tests.

The aim of this study was to illustrate the influence of different levels of the fulcrum (the axis of sagittal rotation) on measured trunk flexion and extension strength and compare force and torque as a unit of measure. The isometric trunk strength was measured in 16 healthy female subjects. The dynamometer was kept at the shoulder level and the moment arm was lengthened step by step by moving the fulcrum caudally from the level of the posterior superior iliac spine to the level of the gluteal fold. The moment of force (torque) increased from 117.0 to 208.5 N.m in flexion and from 182.2 to 292.5 N.m in extension, P < 0.0001. An attempt to quantify this change was made. Paradoxically, the measured force remained at a constant level (in flexion) or slightly decreased (in extension). We concluded that torque as a measure of trunk flexion and extension strength is highly dependent on the level of the rotation axis and force appears to be less sensitive for variations with the height of the fulcrum. We would suggest that the observed increase in torque is physiological and reflects to what extent hip flexor or extensor muscles are recruited. The force, on the other hand, may better characterize a person's capability to perform functional tasks. Force and torque should strictly be distinguished from one another.