Effects of elbow flexion on radial nerve motor conduction velocity.

The position of the elbow while conducting motor studies of the radial nerve is generally extended (15) or slightly flexed (10), but little is known about the effects of elbow flexion on the radial motor conduction velocity. We attempted to measure the effects of the elbow position on the radial motor conduction velocity, as well as directly measure the change in the length of the nerve at different elbow positions in the cadaver. In addition, we established normal values for the radial F-wave with the recording electrode on the extensor indicis proprius muscle. Radial motor nerve conduction was studied in 25 normal subjects to evaluate the effect of 3 different elbow positions (0 degree, 45 degrees, and 90 degrees) on motor conduction velocity (MCV) across the elbow. Direct (in situ) radial nerve measurements were performed on four cadaver specimens to compare changes in nerve length during elbow flexion. Normal values for radial F-wave latencies recording from the extensor indicis proprius muscle were also determined in 23 subjects. The radial MCV decreased significantly as the elbow flexed from 0 degree to 90 degrees. The normal range of values for radial motor nerve conduction at 0 degree of elbow flexion was 71.7 +/- 4.7 m/s, 68.9 +/- 4.9 at 45 degrees of elbow flexion, and 62.0 +/- 6.4 m/s at 90 degrees elbow flexion. Actual (in situ) radial nerve measurements were also found to decrease in length as the elbow flexed. The minimal radial F-wave latency was 19.8 +/- 3.7 milliseconds. For routine determination of motor nerve conduction velocity across the elbow, we recommend that the elbow be fully extended, as the distance measurement most accurately reflects the maximum anatomic length of the nerve. We also feel that the radial nerve F-wave latency is readily obtainable from the extensor indicis proprius muscle.