Correlates of motor unit size, recruitment threshold, and H-reflex jitter

The aim of this research was to study the neurophysiology of the anterior horn cell (AHC) using single-fiber EMG (SFEMG) study of the flexor carpi radialis (FCR) H-reflex. Twelve men and 7 women, ages 20-80 years, were studied. The mean H-jitter was 138 +/- 59 microseconds. H-jitter increased with age (while the M-jitter did not) and was greater in men than in women. There was a direct correlation between the H-jitter and H-latency which was used as an indirect measure of the AHC's size. Given that small AHCs have a higher input resistance than large ones, the H-jitter can be used as an indirect indicator of the AHC's input resistance and therefore its size. When subjects fell asleep, the H-jitter increased over tenfold the baseline value, raising the possibility of an alternative, oligosynaptic pathway. H-reflex jitter studies provide a useful clinical neurophysiological tool for the study of AHC physiology.