Correlation between the biceps brachii muscle bulk and the size of its evoked compound muscle action potential.

This study was to determine if the size of the compound muscle action potential (CMAP) recorded from a large muscle, such as the biceps brachii, can be correlated with the muscle mass or volume that is generating the electrical response. Supramaximal CMAPs were obtained from 32 normal biceps brachii muscles to electrical stimulation of the musculocutaneous nerve at the axilla. Upper-arm circumference was measured with biceps muscle relaxed (elbow extended) and during muscle contraction (elbow flexed). For all muscles, mean CMAP duration was 16.45 ms, amplitude was 10.50 mV, and area was 96.13 mVms. Body mass index and absolute upper-arm circumference (biceps muscle relaxed or contracted) did not correlate with CMAP duration, amplitude, and area; Pearson correlation coefficients (r) ranged from -0.24 to 0.16. However, the difference (increment) in upper-arm circumference due to biceps muscle contraction correlated with CMAP duration (r = 0.47), amplitude (r = 0.67), and area (r = 0. 75). CMAP size (amplitude and area) correlated well with biceps brachii muscle volume as estimated by the increase from baseline in upper-arm circumference during muscle contraction. The difference (increment) in upper-arm circumference due to biceps muscle contraction reflects mainly the underlying biceps muscle bulk, and eliminates other variables such as thickness of the subcutaneous tissues and triceps brachii muscle.