Impaired coordination between grip force and load force in amyotrophic lateral sclerosis: a case-control study.

Abstract

Moving a grasped object involves predictive grip force adjustments for fluctuations in inertial load force. The anticipatory grip force adjustment to movement-induced load fluctuations suggests that motion planning is based on an internal model of both the limb dynamics and the physical object properties. In the present study we analysed grip force control in a patient with amyotrophic lateral sclerosis and in three healthy sex- and age-matched control subjects. Amyotrophic lateral sclerosis is a progressive degenerative disorder of motor neurons in the spinal cord, brainstem and motor cortex. All participants performed discrete vertical arm movements with an instrumented hand-held object. In between the discrete up and down movements the object was held stationary. The patient produced a significantly greater ratio between maximum grip and load forces and a greater grip force to hold the object stationary than the control subjects. When performing downward directed arm movements, the control subjects anticipated an early decrease of load force by constant or decreasing grip forces. The patient showed an early increase of grip force during both upward and downward movements. We conclude that amyotrophic lateral sclerosis impairs the ability to adjust the grip force magnitude relative to the actual loading requirements in an economical way. In addition, there may be a particular problem in adjusting the temporal grip force profile relative to the movement-induced load force profile. Thus, in amyotrophic lateral sclerosis an undifferentiated grip force increase is produced when grasped objects are accelerated, irrespective of actual loading conditions and this force increase may override the precise motor output from an internal forward model.