A preliminary analysis of the validity of gravity correction procedures applied in trunk dynamometry

Abstract

The validity of gravity correction procedures was examined using a KinCom trunk testing dynamometer. Gravity correction measures were expressed as a percentage of the anthropometric estimate of head, arms and trunk weight at 25° of trunk flexion and extension. Polynomial regression was used to describe gravity correction scores across range and compared to cosine curves generated by the supplier's gravity correction software (GCS) at 25°. The GCS method least reflected the percentage body weight estimate although none of the gravity correction methods closely matched the anthropometric estimate. Eccentric passive force curves and isometric curves were highest in both trunk flexion and trunk extension directions. Trunk extension curves for all gravity correction modes were consistently lower than trunk flexion curves and scores were higher for both directions between 5 and 20°. Cosine curves were different from real curves for all modes. Attenuation of forces may have occurred in the spinal structures for both extension and flexion. These preliminary results suggest caution in the use of gravity correction in trunk testing. Methods which represent all passive forces acting on the force pad throughout range of motion should he employed to validate actions being tested.