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

Abstract 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 (M MAX ) and H‐reflex (H MAX ) amplitude, the M‐wave amplitude associated with H MAX (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 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 H MAX ( p = 0.029) and H M5% ( p < 0.001) but greater M Hmax ( 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.