Motor units estimates (MUEs) of the brachial biceps muscles of chronic motor complete quadriplegic patients (C5-C7) were compared to those of normal subjects using a spike-triggered averaging technique. In each subject, a surface-recorded maximum compound muscle action potential (M potential) was first evoked by stimulation of the musculocutaneous nerve at the axillary fold. Ten different surface single motor unit action potentials (S-MUP) were then computed by triggering an average of surface EMG with the motor unit action potential recorded by a needle electrode inserted to different locations within the muscle. The MUE was calculated by division of the amplitude of the M potential by the mean amplitude of the SMUPs.
The influences of surface electrode shape and location on MUE in normal muscles were evaluated, and it was also verified that the technique was sufficiently reproducible to evaluate partially denervated muscles of quadriplegic patients. In the main study, 102 normal muscles and 12 muscles of quadriplegic patients (of which three were fair, six were weak and three were normal in strength) were examined. The MUEs of the normal muscles were relatively constant (average MUE±1 standard deviation, 631±219) up to the age of 60, but thereafter decreased in proportion to age. The MUEs of the quadriplegic patients were reduced according to muscle strength; for example, the MUEs of the muscles with fair strength were all less than 100.
The method utilized in this study can be used to quantitatively evaluate partially denervated brachial biceps muscles of quadriplegic patients. It should be useful for studying the mechanisms of motor recovery in this population, for estimating eventual strength at an early stage following injury, and for evaluating the effects of muscle training.

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