This study was designed to assess the clinical usefulness of the H reflex in cases of unilateral S₁ radiculopathy.

Sixty-two low back pain patients were divided into 3 groups which consisted of L₅ radiculopathy, S₁ radiculopathy and normal EMG groups. These groups of 23, 16 and 23 patients respectively were matched for sex, age, duration between onset and electrodiagnostic study and the level of root involvement.

H reflex latency was measured in both legs. The mean difference of each group was 0.23 msec (-0.9 msec~1.3 msec), 1.55 msec (-0.5 msec~5.0 msec) and 0.49 msec (-0.9 msec~1.5 msec) respectively.

This data supported the hypothesis of prolongation or absence of the H reflex latency on the affected side of the unilateral S₁ radiculopathy. In 75% of S₁ radiculopathy cases reviewed, the mean difference was more than 1.0 msec, 43.8% patients more than 1.5 msec and only 37.5% patients showed a mean difference of more than 2.0 msec.

Therefore in patients with S₁ radiculopathy a mean difference of more than 1.0 msec would indicate an abnormality.