The response threshold was greater in old than young adults for the H reflex and M wave (p = 0.003). It was also greater for the M wave than the H reflex (p < 0.001; Figure 4A) in young [M wave: 6.0 (1.4); H reflex: 3.1 (0.5) mA] and old adults [M wave: 12.3 (1.8); H reflex: 7.5 (1.1) mA]. SDTC was briefer in old than young adults for the H reflex [young: 860.5 (71.7); old: 617.9 (49.2) µs; p = 0.009] but not the M wave [young: 555.4 (48.9); old: 436.1 (33.4) µs; p = 0.24]. The SDTC was shorter for the M wave than the H reflex in both groups (p < 0.001; Figure 4B).

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This study aimed to compare the effect of pulse duration on the H reflex and M wave between young and old adults with the rationale that it may contribute to further understanding the effect of ageing on the H-reflex pathway. The results indicate that varying pulse duration has mainly a similar effect on H-reflex and M-wave RC in young and old adults. However, if these results do not support our hypothesis (lesser effect of pulse duration on H-reflex recruitment curve in old adults), the analysis of specific parameters of the H-reflex RC (H_max, H_M5%, M_Hmax and I_H50) for the three pulse durations allows to step further in the effect of ageing on the neural part of the muscle spindle pathway. Indeed, the decrease in H_max and H_M5% and the increase in M_Hmax regardless of pulse duration suggest greater age-related changes in the sensory than the motor component of the H-reflex pathway that may reflect a greater loss of sensory than motor axons or alterations of synapses between Ia afferents and motor neurones.