We recently reported a case study of chronic spinal cord injury (SCI), in which motor evoked potentials (MEPs) with normal amplitude and latency could be recorded from the extensor carpi radials (ECR) muscle, even though the motor power of this muscle was just 1/5.1

Calancie and Alexeeva2 have hypothesized that volume conduction from the unaffected brachioradialis (BR) muscle explains this observation, and suggested that this could be tested by recording the surface electromyography (EMG) from both muscles during a resisted elbow flexion with the forearm in mid-pronation and wrist relaxed.

We tested this hypothesis in the SCI participant from the original report. We observed strong EMG activity from the BR, whereas there was no EMG signal detected from the ECR (Figure 1). The root-mean-square EMG for BR was 97 μV compared to 4 μV for ECR (background noise level).

Figure 1
figure 1

Surface EMG (1 s) from brachioradialis (BR) and extensor carpi radialis (ECR) during resisted elbow flexion. No volume conduction from BR was detected in the ECR recording.

Full size image

These results indicate that volume conduction from the BR does not explain our original report. The case study adds to previous literature3 that small, delayed MEPs can be recorded from weak muscles in people with SCI, by demonstrating that in some cases these MEPs can be remarkably normal.