Age-related changes in sensory and motor components of the Hoffmann-reflex pathway of the flexor carpi radialis.

Ageing is accompanied by numerous changes within the sensory and motor components 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 recorded at rest in the flexor carpi radialis of 12 young (21-36 years) and 12 older adults (62-80 years). For each pulse duration (0.05, 0.2 and 1 ms), the maximal M-wave (MMAX ) and H-reflex (HMAX ) amplitude, the M-wave amplitude associated with HMAX (MHmax ) and the H-reflex amplitude for a stimulus intensity evoking an M-wave of 5% MMAX (HM5% ) were measured. The strength-duration time constant and response threshold were estimated from the charge/stimulus-duration relation for the H reflex and M wave. Results indicate that varying pulse duration mainly induces a similar effect on H-reflex and M-wave recruitment curves between young and older adults. Regardless of pulse duration, old adults had lesser HMAX (p = 0.029) and HM5% (p < 0.001) but greater MHmax (p < 0.001). The H-reflex and M-wave response thresholds were greater in old than young adults (p = 0.003), but 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). These results suggest greater age-related changes in the sensory than the 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.