Figure 1. Schematic illustration of the main variables of interest obtained from the recruitment curves of one old adult. A. Maximal amplitude of the M wave (M_MAX) and H reflex (H_MAX), M-wave amplitude associated with H_MAX (M_Hmax), H-reflex amplitude associated with an M wave of 5% M_MAX(H_M5) and current intensity associated with a M-wave and H-reflex amplitude of 50% M_MAX (I_M50) and H_MAX (I_H50). These data were extracted from the H-reflex and M-wave recruitment curves fitted by a Boltzmann sigmoid function. B, C and D. Illustration of the M_MAX, H_MAX, M_Hmax) and H_M5%. E. Response threshold and strength-duration time constant (SDTC), extracted from the threshold-charge vs stimulus duration relation.

Figure 2 . H-reflex and M-wave recruitment curves (RC) in one young (23 yrs; left panels) and one old adult (67 yrs; right panels) recorded with pulse duration of 0.05 ms (top panels), 0.2 ms (middle panels) and 1 ms (bottom panels). The H-reflex and M-wave amplitude are expressed as percentage of the maximal amplitude of the M wave (M_MAX), while the current intensity is expressed relative to the intensity evoking an M wave of 50% of M_MAX (I_M50). Open and filled circles represent H-reflex and M-wave data, respectively. Continuous lines represent the fitting of the experimental data (ascending part only for the H reflex) by a Boltzmann sigmoid function.

Figure 3 . Effect of pulse duration on M_MAX (A), H_MAX (B), M_Hmax (C) and H_M5% (D) in young and old adults. # indicates a difference between young and old adults (p ≤ 0.05). *, ** and *** indicates significant differences between pulse durations at p ≤ 0.05, p ≤ 0.01 and p ≤ 0.001, respectively. Bars and error bars represent means and SEM, respectively.

Figure 4 – Effect of age on the response threshold and SDTC of the H reflex and M wave of young and old adults. # indicates a difference between young and old adults (p ≤ 0.05). *** indicates significant differences between the M-wave and H-reflex pathways (p ≤ 0.001). Bars and error bars represent means and SEM, respectively.