Tibialis anterior response to sudden ankle displacements in normal and parkinsonian subjects

It is well known that in Parkinsonian subjects with akinesia, reaction times are increased but reflex latencies remain normal. We have attempted to use this knowledge to distinguish between 'reflex' and 'voluntary' components of the electromyographic (EMG) response to ankle displacement. The EMG and torque responses of tibialis anterior (TA) to randomly applied servo-controlled plantar-flexing displacements of the ankle with and without the subject's intentional opposition were examined in 9 Parkinsonian and 9 age-matched normal humans. To obtain a measure of akinesia, the response latency to a visual stimulus was subsequently measured in the same subjects. Three principal findings emerged. (1) The intermediate latency EMG component (PSR) of the response evoked by ankle displacement with the subject instructed to relax was more regularly evoked and of lower threshold in Parkinsonians than in normals. This finding corresponds to the enlarged M2 component in upper limb muscles. However, the facilitation of PSR was not found to be associated with an increase in torque. In fact, the patients did not exhibit more stiffness than normals under our experimental conditions. (2) Mean latency estimate of the PSR was indistinguishable between Parkinsonians and normals. This finding puts the PSR in the nature of a reflex. Indeed, in accordance with reflex behaviour which is proportional to input characteristics, its area increased linearly with increase in the magnitude of displacement velocity. (3) In contrast, the 'late' EMG response (FSR) evoked by opposing sudden ankle displacement exhibited a significantly longer latency in 6 out of 8 Parkinsonians than normals. In the same patients, the EMG response latency to a visual signal was similarly increased. The delay of FSR in akinesia patients thus argued against its being a stereotyped reflex. The result is discussed with reference to the recent finding that preprogrammed responses are delayed in Parkinsonians.