MUSCLE-FIBER CONDUCTION-VELOCITY IN AMYOTROPHIC-LATERAL-SCLEROSIS ANDTRAUMATIC LESIONS OF THE PLEXUS BRACHIALIS

Muscle fiber conduction velocity (MFCV) in biceps brachii was studied in traumatic brachial plexus lesions (16 patients) and amyotrophic lat eral sclerosis (ALS) (22 patients) by means of an invasive (S-MFCV) an d a surface (S-MFCV) method. After complete denervation an exponential decrease of the mean I-MFCV was found, T-1/2 =1.1 month. After 4-5 mo nths this resulted in severely reduced conduction velocities (mean 1.4 m/sec, range from 0.5 to 2 m/sec). Simultaneous with signs of reinner vation, fibers with faster conduction velocities were seen. In ALS, a decrease of the mean I-MFCV was found, and slow conducting fibers were found in every patient on at least one side. At the same time muscle fibers with increased I-MFCVs were found. This increased range of velo cities seems based on a combination of slow conducting, atrophic fiber s, with fast conducting, hypertrophic fibers, compensating the force l oss. In some subjects we found these disturbances without clear abnorm alities with concentric needle EMG, and with unimpaired muscle force. The surface EMG measurements in the ALS patients revealed increased S- MFCV values in combination with a decrease of the median frequency (F- med). We suggest that the opposite finding of an increased S-MFCV is a consequence of the muscle fiber hypertrophy in the surviving, volunta rily recruited, motor units. The simultaneous decrease of the F-med se ems to be caused mainly by the change in shape of the motor unit poten tial.