Tibialis anterior R-1 response: physiological behaviour, normative data and clinical utility in L4-L5 radicular...

A modified H-reflex, known as R-1 response, was recorded in healthy subjects by averaging 200 responses from the moderately contracting tibialis anterior muscle (TA) after repetitive submaximal stimulation of common peroneal nerve. Physiological changes to different modes of stimulation and recording were studied and normative data obtained at the most reliable parameters. Subsequently, the test is applied to 36 patients with clinical and myelographic evidences of L5 or L4-L5 radicular compression (with or without S1 radicular compression). Increasing the force of muscle contraction resulted in increase in amplitude up to a certain limit, with no change in latency. Increasing the current from subliminal to submaximal stimulation caused initial increase and subsequent fall in the amplitude without change in the latency. The change in the stimulus frequency from 0.5 to 3 Hz did not alter latency or amplitude but the higher frequencies sometimes did. On changing the position of the foot from plantarflexed to dorsiflexed, an increase in amplitude and reduction in duration of the response was noted. Fifty subjects were studied using stimulus intensity just enough to produce 200-400 microV M-response, stimulus duration of 1.0 msec, stimulus rate of 3 Hz and the muscle force sufficient to keep the foot in complete dorsiflexion. The tibialis anterior R-1 response (TAR-1) could be recorded reliably from both legs in all the subjects. The take-off point was sharp and in no subject was there any difficulty in measuring the latency to onset point. On repeating the test, the onset point of the response was superimposable. Latency showed linear correlation with height and age. Among the patients, 21 out of 25 cases of unilateral (84.0%) and 10 out of 11 cases of bilateral (90.9%) radicular compression had abnormal findings. Abnormalities included significant a right-left latency difference within the normal range of absolute latencies, unilateral or bilateral prolongation of latency to onset point and attenuation or absence of the response. It was concluded that the TAR-1 is an important and reliable test for the study of functional integrity of L4-L5 spinal segments in lumbo-sacral sensorimotor root lesions.