Surface compound action potentials recorded from different locations on the anterior tibial muscle

Supramaximal CMAPs to peroneal nerve stimulation at the knee were recorded from 5 locations on the anterior tibial muscle in 24 patients (30 muscles). The active recording locations were: midpoint of the muscle belly, at 4 cm distal and proximal to it, and at 2 cm lateral and medial to it. Reference electrode was at the medial malleolus. CMAP duration, amplitude, and area were measured, and ratios of their corresponding minimum to maximum values were computed. Thirteen patients (15 muscles) had normal nerve conduction and needle EMG studies. Mean ratios of minimum to maximum values were duration = 0.89, amplitude = 0.67, and area = 0.75. Eleven patients (15 muscles) had abnormal studies, and the mean ratios of minimum to maximum values were duration = 0.87, amplitude = 0.66, and area = 0.71. CMAP duration, unlike amplitude and area, appears least likely to be influenced by the recording electrode location, and is a more stable and reproducible measure during nerve conduction studies. The maximum (or minimum) values in the CMAP duration, amplitude, and area frequently do not coincide to one electrode recording location. A majority of the CMAPs recorded from different muscle locations had an initial negative phase, suggesting that the endplate zones of the anterior tibial muscle are dispersed rather than concentrated in a small region.