Our estimates of SDTC and response threshold do not strictly reflect the biophysical properties of afferent and motor axons. The M wave results from the recruitment of motor axons, the activation of the muscle fibres at the neuromuscular junction, and the propagation of action potentials along the sarcolemma. However, in the steepest portion of the RC (around I_M50), the M-wave amplitude can be assumed to be indicative of the recruitment properties of motor axons (Funase et al., 1994). Furthermore, the neuromuscular junction should not be a limitation as the safety margin permits an action potential conveyed by the axon to trigger an action potential on the sarcolemma (Paton & Waud, 1967). Other factors than the number of active motor units influence the M-wave amplitude (dispersion of their innervation zones, the thickness of the subcutaneous tissue layers, the orientation of the detection system with respect to the muscle fibres, and the intracellular action potential shape). However, these factors should only marginally influence the effect of pulse duration on the M wave as the EMG recording conditions were similar for the three pulse durations. Therefore, we expect that the response threshold and SDTC for the M wave provide an indirect estimate of the biophysical properties of motor axons.

The H-reflex amplitude is a more complex response, which depends on the recruitment of afferent axons, as well as presynaptic and postsynaptic mechanisms. As the SDTC measured from the H reflex in the median nerve did not differ when recorded at rest and during contraction (Lin et al., 2002), it is unlikely that mechanisms influencing the excitability of the motor neurone pool (recurrent inhibition and non-reciprocal group I inhibition) affect the measured parameter as these mechanisms change during voluntary contraction. Furthermore, Ia presynaptic inhibition should not influence our measures because it lasts less than 500 ms (Mizuno et al., 1971), whereas the inter-stimulus interval used in the present study was 10 s. Accordingly, the SDTC and response threshold for the H reflex likely represents Ia afferents’ biophysical properties, as suggested by Lin and colleagues (Lin et al., 2002).