Effect of ageing on the electrical and mechanical properties of human soleus motor units activated by the H reflex and M wave

Abstract

This study was designed to investigate the effect of ageing on the mechanical and electromyographic (EMG) characteristics of the soleus motor units (MUs) activated by the maximal Hoffmann reflex (Hmax) and by the direct muscle compound action potential (Mmax). Eleven young (mean age 25 +/- 4 years) and ten elderly (mean age 73 +/- 5 years) males took part in this investigation. The senior group presented lower amplitudes of Mmax (-57 %, P < 0.001) and Hmax (-68 %, P < 0.001) waves compared to the younger population. These were associated with a depression of relative twitch torque of the plantar flexors. The average values of the Hmax/Mmax ratio did not statistically differ between the two populations, despite a tendency for lower values (~23 %) in the senior group. However, the older adults showed a greater relative amplitude of the sub-maximal M wave evoked at Hmax (MatHmax) than did the younger males (young 5 % vs. elderly 29 % of the Mmax, P < 0.01). This finding suggests an increased homogeneity between the excitability threshold of sensory and motor axons. The twitch torque at Hmax (PtH-M) was subsequently calculated by subtraction from the total twitch torque of the mechanical contamination associated with MatHmax. The resulting PtH-M was significantly lower in the elderly (-59 %, P < 0.001). Despite a discrepancy of 20 % between the two groups, the mechanical ratio (PtH-M/PtM; PtM, twitch tension related to the Mmax compound action potential), like the EMG ratio, did not statistically differ between the young and older individuals. Nevertheless, the senior subjects exhibited a higher twitch/EMG ratio for the reflexively activated MUs (PtH-M/Hmax) than the younger individuals (+40 %, P < 0.05). This finding suggests an on-going neuromuscular remodelling, resulting in an increased innervation ratio. The neural rearrangement may be viewed as a compensatory adaptation of the motor system to preserve the mechanical efficiency of the surviving MUs, despite the age-related impairment of the segmental reflex system. This phenomenon is confirmed by the maintenance, with senescence, of the approximately constant values of the twitch/EMG ratio for the entire motor pool (PtM/Mmax).

Modulation of reflex responses in activated ankle dorsiflexors differs in healthy young and elderly subjects

“…This observation is in agreement with most of the studies investigating another lower limb muscle (i.e. soleus; Obata et al 2010;Sabbahi and Sedgwick 1982;Scaglioni et al 2003). Recently, Obata et al (2010) also investigated the latency of the tibialis anterior stretch reflex during weak contractions (*10% MVC) and did not observe any significant difference between elderly (66 ms) and young (60 ms) adults.…”

“…The absence of a significant age-related decrement in H-reflex area contrasts with some studies (Angulo-Kinzler et al 1998;Kido et al 2004;Sabbahi and Sedgwick 1982) but not others (Kawashima et al 2004;Scaglioni et al 2003). As already mentioned, this discrepancy may be partly due to the variability of reflex responses, particularly when they are recorded at rest (Toft et al 1991).…”

“…As for the stretch reflex, reports on the effect of ageing on the H-reflex amplitude, normalized to the maximal muscle motor potential (M max ), are divergent. Indeed, some authors reported a decrease (Angulo-Kinzler et al 1998;Kido et al 2004;Sabbahi and Sedgwick 1982) whereas others did not observe any significant difference between young and elderly adults (Kawashima et al 2004;Scaglioni et al 2003).…”

“…Changes in reflex responses with advancing age have been extensively investigated in the ankle plantar flexor muscles (Angulo-Kinzler et al 1998;Kawashima et al 2004;Kido et al 2004;Lin and Sabbahi 1998;Obata et al 2010;Sabbahi and Sedgwick 1982;Scaglioni et al 2003). Surprisingly, very little is known about the adaptation of the dorsiflexor muscles during ageing (Obata et al 2010).…”

Measurement of ageing effects on muscle tone and mechanical properties of rectus femoris and biceps brachii in healthy males and females using a novel hand-held myometric device

“…Tone and mechanical properties of muscles (Blanpied & Smidt 1993) and tendons (Scaglioni et al 2003) change with ageing and are also altered in neurological conditions, such as stroke and Parkinson's disease (PD).…”