The perception of motor commands or effort during muscular paralysis

Abstract

An observation by Mach (1906) on the sense of effort during hemiplegia has been confirmed. Two patients, who became suddenly hemiplegic, without sensory symptoms, noted that attempts to move when first paralysed were not accompanied by a sense of effort, but that attempts to move when movement first returned were accompanied by distinct sensations of effort or heaviness. This absence of a sense of effort during upper motor neuron paralysis without sensory signs did not occur either in a group of patients with pure lower motor neuron paralysis or in patients with clinically complete spinal transection. Previous studies have suggested that signals of motor command or effort, which arise at levels rostral to spinal motoneurons, contribute to the estimation of weights and tensions (see McCloskey et al., 1974; Gandevia and McCloskey, 1977a). One interpretation of the return of a sensation of effort as hemiplegia progresses to paresis is that activity in corticofugal paths contributes to generation of the sense of effort. During complete hemiplegia there is no neural traffic in motor corticofugal paths below the internal capsule and a sense of effort is absent. But, during paresis, there is increased neural traffic in the uninterrupted corticofugal fibres and there is a strong sense of effort or heaviness. Alternatively, a subcortical structure with a critical ascending projection to motor cortical areas may co-operate in generation the sense of effort. Afferent information, while important for calibrating and scaling the sense of effort which usually signals force or weight, is not essential for generation of a crude signal of descending motor command or effort.