When motor execution is selectively impaired: control of manipulative finger forces in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis is a degenerative motor neuron disorder with progressive and exclusive loss of motor neurons in the spinal cord, brainstem, and motor cortex. Five patients with amyotrophic lateral sclerosis, and 5 age-matched, healthy control subjects performed vertical point-to-point arm movements with an instrumented hand-held object. In between the movements, the object was held stationary. Compared with healthy controls, all patients generated greater grip forces during the phase of stationary holding of the object and greater ratios between grip and load force maximums during the arm movements. We conclude that in amyotrophic lateral sclerosis, the ability to scale the grip force magnitude efficiently according to the actual loading requirements is impaired. When performing upward movements, controls increased grip force in parallel with load force right from the movement onset; during downward movements, controls anticipated an early decrease of load force by constant or decreasing grip forces. In contrast, 3 of 5 patients showed an early increase of grip force during both upward and downward movements, indicating that in amyotrophic lateral sclerosis, the differential regulation of the grip force output according to the direction-dependent load force profile may be impaired. In motor neuron disease, the inaccurate grip force scaling and the impaired temporal coupling between grip and load force profiles may either directly result from deficient motor execution or be secondary to accompanying symptoms, such as dyscoordination of hand and finger muscles due to spasticity.