Ageing is accompanied by numerous changes within the sensory and motor component 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 analysed from recordings performed at rest in the flexor carpi radialis of 12 young (21-36 yrs) and 12 old adults (62-80 yrs). For each pulse duration (0.05-ms, 0.2-ms, and 1-ms), the maximal H-reflex amplitude (H_MAX), the associated M wave (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 H reflex and M wave. The main results indicate that varying pulse duration mainly induces similar effect on H-reflex and M-wave recruitment curves between young and old adults. However, regardless of pulse duration, old adults had lesser H_MAX (p = 0.029) but greater H_M5%(p<0.001) and M_Hmax (p<0.001). 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). The H-reflex and M-wave response thresholds were greater in old than young adults (p=0.003). These results suggest greater age-related changes in the sensory than 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.

The Hoffmann (H) reflex is an electromyographic response triggered by the electrical stimulation of muscle spindle afferents, mainly Ia afferents, which recruit spinal motor neurones (Burke, 2016). The H reflex is assumed to assess the effectiveness of Ia afferents to discharge motor neurones. A classical approach in motor control studies consists of recording the recruitment curves (RC) of the H-reflex and compound muscle action potential (M-wave) by progressively increasing the current intensity from an intensity that does not induce an H reflex to an intensity that evokes the maximum amplitude of the M wave (M_max). Paillard already reported in the soleus that a 1-ms pulse duration favoured the H-reflex amplitude relative to M-wave amplitude compared with a briefer (0.1 ms) pulse duration (Paillard, 1955). This was later confirmed for the same muscle (Lagerquist & Collins, 2008; Lin et al., 2002) and the flexor carpi radialis (FCR) in young adults (Lin et al., 2002; Panizza et al., 1989). Furthermore, increasing pulse duration from 0.05 to 1 ms induces a leftward shift of the H-reflex RC relative to the M-wave RC (Lagerquist & Collins, 2008; Panizza et al., 1989).

Lin and colleagues investigated the biophysical properties of afferent and motor axons involved in the H reflex and M wave, as well as the properties of cutaneous afferents, by stimulating the median nerve at the wrist with different pulse durations (Lin et al., 2002). Their results indicated a longer strength-duration time constant (SDTC) and a lower rheobase for the H reflex and cutaneous afferents compared with the M wave, with no difference between the H reflex and cutaneous afferents. The SDTC measures the rate of decline of the threshold current as the stimulus duration increases, whereas the rheobase is the threshold current for a stimulus of infinite duration (Bostock et al., 1983; Mogyoros et al., 1998). A lower rheobase implies that less current is required to generate action potentials, while a longer SDTC indicates that a long pulse duration (1 ms) is more effective in triggering action potentials. Such differences result in a greater excitability of Ia axons compared with motor axons for long pulse duration (Lagerquist & Collins, 2008; Panizza et al., 1989).

Mogyoros and colleagues showed a briefer SDTC for cutaneous afferents in the median nerve in old compared with young adults, without age-related difference for motor axons (Mogyoros et al., 1998). Considering that cutaneous and Ia afferents share biophysical properties (Lin et al., 2002), a similar decrease in SDTC of Ia axons with ageing should reduce their excitability to long pulse duration and minimise the difference between the excitability of sensory and motor axons for briefer pulse duration. Accordingly, a lesser leftward shift of the H-reflex RC relative to the M-wave RC should be observed in old adults when increasing the pulse duration. Indices which document H-reflex amplitude relative to M-wave response – such as the H-reflex maximal amplitude (H_max) relative to M_max(H_MAX/M_MAX ratio), the amplitude of the H reflex for a given amplitude (5% of M_max) of the M wave (H_M5%) and the amplitude of the M wave associated with H_max (M_Hmax) (Lagerquist & Collins, 2008; Scaglioni et al., 2003) – should be particularly suitable to document age-related changes of sensory relative to motor components of the spindle-reflex pathway. Indeed, a lesser change in these parameters from 0.05- to 1-ms pulse duration should indicate a lesser leftward shift of the H-reflex RC with increasing pulse duration, suggesting thereby more changes in the sensory relative to the motor component of the pathway. This approach is partly supported by the greater amplitude of the M wave evoked at the intensity eliciting the maximal H-reflex amplitude (M_Hmax) in old compared with young adults when using a 1-ms pulse duration (Scaglioni et al., 2003).

Therefore, this study compared the effect of pulse duration on the H reflex and M wave between young and old adults. To this end, the H-reflex and M-wave RC were recorded in the FCR by stimulating the median nerve at rest with pulse durations of 0.05, 0.2 and 1 ms. We hypothesised a lesser leftward shift of the H-reflex RC relative to the M-wave RC in old adults when increasing the pulse duration.

Twenty young [mean (SD); 27 (5) yrs] and 22 old adults [72 (7) yrs] participated in this study. They did not report Parkinson’s disease, multiple sclerosis, diabetes, stroke, cardiac history or neuropathology. In addition, participants with orthopaedic problems in their dominant upper limb within 12 months prior to the study were not included. Approval for the study was obtained from the local Ethics Committee, and all procedures used in this project conformed to the Declaration of Helsinki.