AGE-RELATED CHANGES IN THE HOFFMANN-REFLEX PATHWAY OF THE FLEXOR CARPI RADIALIS

Ageing is accompanied by numerous changes within the sensory and motor component of the muscle spindle pathway. To further document these age-related changes, this study compared the characteristics of the Hoffmann (H) reflex and M wave, evoked with several pulse durations, between young and old adults. The H-reflex and M-wave recruitment curves were analysed from recordings performed at rest in the flexor carpi radialis of 12 young (21-36 yrs) and 12 old adults (62-80 yrs). For each pulse duration (0.05-ms, 0.2-ms, and 1-ms), the maximal H-reflex amplitude (H MAX ), the associated M wave (M Hmax ), and the H-reflex amplitude for a stimulus intensity evoking an M-wave of 5% M MAX (H M5% ) were measured. The strength-duration time constant and response threshold were estimated from the charge/stimulus-duration relation for H reflex and M wave. The main results indicate that varying pulse duration mainly induces similar effect on H-reflex and M-wave recruitment curves between young and old adults. However, regardless of pulse duration, old adults had lesser H MAX (p = 0.029) but greater H M5% (p<0.001) and M Hmax (p<0.001). The strength-duration time constant was lesser in old than young adults for the H reflex (p=0.048) but not the M wave (p=0.21). The H-reflex and M-wave response thresholds were greater in old than young adults (p=0.003). These results suggest greater age-related changes in the sensory than motor component of the H-reflex pathway, which may be indicative of a greater loss of sensory than motor axons or alterations of synapses between Ia afferents and motor neurones.