The I_M50 decreased with the increase in pulse duration in both groups, indicating the increase in electrical charge with longer pulse durations. However, I_M50 was significantly greater for old compared with young adults for the 0.05-ms pulse duration. Furthermore, the response threshold for the M wave was greater in old adults while STDC did not change, as already observed in the median nerve when stimulated at the wrist (Jankelowitz et al., 2007). These age-related differences may be indicative of thicker subcutaneous tissue in old adults, thereby reducing the current flow from the electrode to the nerve, more so for brief pulse duration (0.05 ms) (Petrofsky, 2008). Changes in axonal excitability can also result from the preferential loss of large-diameter motor axons (more excitable), which belong to high-threshold motor units characterised by a greater innervation ratio, with ageing (Hepple & Rice, 2016; Larsson et al., 2019). Basser and Roth (Basser & Roth, 1991) predicted that the response threshold is inversely proportional to the square of the fibre diameter by using a simulated model. Accordingly, the increase in the response threshold for the M wave in old adults may support a loss of large-diameter motor axons in the aged group. This is further supported by the consistent observation (Baudry et al., 2010; Kido et al., 2004; Scaglioni et al., 2003), of a lesser M_max amplitude in older adults, even though a decline in the integrity of the neuromuscular junction (Hepple & Rice, 2016) can also contribute to decreasing the M_max amplitude.

A common observation reported in the present study and previous work (Baudry et al., 2010; Kido et al., 2004; Scaglioni et al., 2003) is the lesser H_max amplitude in old compared with young adults. Increased presynaptic inhibition of Ia afferences could reduce H_MAX amplitude, although the lack of consensus on an increased presynaptic inhibition with ageing questions this possibility (Butchart et al. , 1993; Morita et al. , 1995; Baudry et al. , 2010; Baudry & Duchateau, 2012). The age-related decrease in H_MAX amplitude can also result from alterations in the synaptic transmission between Ia afferents and motor neurones. In rhesus monkeys and mice, ageing is accompanied by a loss of synaptic input onto motor neurones (Maxwell et al., 2018), while a decrease in the rate of rise of the Ia-induced excitatory postsynaptic potentials was observed in cats (Chase et al. , 1985; Boxeret al. , 1988). Together, these changes should reduce the effectiveness of Ia afferents in causing action potential in motor neurones (Fetz & Gustafsson, 1983). Finally, an age-related loss of Ia afferents has been observed, more so for the large-diameter afferents (Kim et al., 2007; Vaughan et al., 2016). In addition to decreasing the H_max, this may explain the increase in the H-reflex response threshold in old compared with young adults (figure 4). This is further suggested by the greater H-reflex latency in old adults, which may account for a slower conduction velocity. Even though nerve demyelination decreases conduction velocity (Ian McDonald, 1962), it is unlikely to be a major alteration of afferent fibres in this study, as it should increase rather than decrease the SDTC (Bostock, 1983). Furthermore, if the greater I_M50 in older adults could partly be due to thicker subcutaneous layers, a similar effect on I_H50 is unlikely as the difference remains even after being normalised to I_M50, supporting age-related changes involving the sensory components of the H-reflex pathway.

Interestingly, the change in H-reflex SDTC is not accompanied by a lesser leftward shift of the H-reflex RC relative to the M-wave RC. A decrease in SDTC suggests that sensory axons are less excitable for a 1-ms duration but relatively more excitable for a briefer pulse duration. Accordingly, the leftward shift induced by longer pulse durations should have been less pronounced in old adults as biophysical properties between sensory and motor axons were expected to be closer (no change in SDTC for motor axons). However, the almost similar changes in H_M5% and M_Hmax from RC_0.05 to RC₁ between young and old adults do not support a lesser leftward shift of the H-reflex RC relative to M-wave RC. Nonetheless, the lesser H_M5%, greater M_Hmax and I_H50/I_M50, and briefer SDTC in old adults regardless of pulse duration highlight a rightward shift of the H-reflex RC relative to the M-wave RC with ageing. Although changes in the M-wave pathway were also observed (decreased M_max, increased I_M50), this rightward shift suggests a greater age effect on the sensory than motor components of the H-reflex pathway (Bouche et al., 1993; Scaglioni et al., 2003). A loss of large-diameter sensory axons and synaptic alterations can support the absence of age-related changes in the effect of pulse duration on H_M5% and M_Hmax. In contrast with changes in biophysical properties of Ia afferent axons, a decreased number of Ia afferents and a decrease in synaptic efficacy would have a constant effect on the H-reflex amplitude (decrease in H_max and H_M5%) across pulse durations.