The role of cutaneous information in a contact control task of the leg in humans

Abstract

The aim of this study was to examine the effect of loss of sensation in the plantar surface of the feet on the learning of a contact control task. It has previously been shown that the biarticular thigh muscles control the external force exerted by the feet by finely regulating the net knee and hip moments. Based on literature which demonstrates strong projections of cutaneous afferents on the motoneurones of biarticular muscles, it has been assumed that this regulation is controlled on the basis of information from the plantar mechanoreceptors of the feet. Subjects with diabetic neuropathy often exhibit marked sensory loss in the distal extremities and thus provide a useful model for studies of the role of afferent input in motor control. Twenty subjects with diabetes mellitus were divided equally between those who had significant peripheral neuropathy and those with no apparent signs of neuropathy. By means of quantitative sensory and motor testing it was possible to select two relatively homogeneous groups matched for age, gender, height, medication, leg strength, weight and duration of diabetes. The task was to learn to push on a force platform with the right foot in four given directions. The foot was not moved relative to the force platform. Visual feedback specifying the direction and magnitude of the ground reaction force was provided on a TV screen. Once subjects had learned to apply the correct force, they were required in the second part of the experiment to maintain a given force direction without visual feedback. Force data and position data were collected and EMG activity was recorded for six muscles of the upper leg. The results showed that all subjects from both groups increased the time they stayed on the targets as they repeated the task ( p < 0.01). The accuracy of the task was significantly lower for the neuropathic group than the group with normal sensation ( p < 0.05). There were no differences between the two groups in the rate of improvement and both groups showed the same amount of drift of the applied force when visual feedback was removed. There were no significant differences between the two groups in the activation patterns of the thigh muscles. The activation of the biarticular muscles was increased as performance of the task improved and the correlation co-efficients between the activation pattern and the net joint moments improved. The results suggest that mechanoreceptors of the plantar surface of the feet play an important role in the accurate coordination of contact control leg tasks, although the role of other afferent inputs, which may also be degraded in diabetic neuropathy, cannot be excluded. However, this study could not demonstrate that the regulation of the net joint moments by the biarticular thigh muscles is based on information from the mechanoreceptors of the feet. The strong projections of cutaneous afferents on the motoneurones of biarticular muscles do not appear to play a role in the organization of the activation of muscles on an ongoing basis and may be primarily used to allow fast adjustments as a response to unexpected disturbances.