THE ROLE OF CUTANEOUS INFORMATION IN A CONTACT CONTROL TASK OF THE LEG IN HUMANS

The aim of this study was to examine the effect of loss of sensation i n 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 t he net knee and hip moments. Based on literature which demonstrates st rong projections of cutaneous afferents on the motoneurones of biartic ular muscles, it has been assumed that this regulation is controlled o n the basis of information from the plantar mechanoreceptors of the fe et. Subjects with diabetic neuropathy often exhibit marked sensory los s in the distal extremities and thus provide a useful model for studie s of the role of afferent input in motor control. Twenty subjects with diabetes mellitus were divided equally between those who had signific ant peripheral neuropathy and those with no apparent signs of neuropat hy. 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 pl atform. Visual feedback specifying the direction and magnitude of the ground reaction force was provided on a TV screen, Once subjects had l earned to apply the correct force, they were required in the second pa rt of the experiment to maintain a given force direction without visua l feedback. Force data and position data were collected and EMG activi ty was recorded for six muscles of the upper leg. The results showed t hat all subjects from both groups increased the time they stayed on th e targets as they repeated the task (p < 0.01). The accuracy of the ta sk was significantly lower for the neuropathic group than the group wi th 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 a ctivation patterns of the thigh muscles. The activation of the biartic ular muscles was increased as performance of the task improved and the correlation coefficients between the activation pattern and the net j oint moments improved. The results suggest that mechanoreceptors of th e plantar surface of the feet play an important role in the accurate c oordination of contact control le tasks, although the role of other af ferent inputs, which may also be degraded in diabetic neuropathy, cann ot be excluded. However, this study could not demonstrate that the reg ulation of the net joint moments by the biarticular thigh muscles is b ased on information from the mechanoreceptors of the feet. The strong projections of cutaneous afferents on the motoneurones of biarticular muscles do nor appear to play a role in the organization of the activa tion of muscles on an ongoing basis and may be primarily used to allow fast adjustments as a response to unexpected disturbances. (C) 1998 E lsevier Science B.V.