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

• Published in: The Journal of physiology Vol. 548; no. 2; pp. 649 - 661
• Main Authors: Scaglioni, G, Narici, M V, Maffiuletti, N A, Pensini, M, Martin, A
• Format: Journal Article
• Language: English
• Published: England The Physiological Society 15.04.2003; Blackwell Science Inc
• Subjects: Action Potentials - physiology; Adult; Aged; Aging - physiology; Electric Stimulation; Electromyography; Electrophysiology; Female; H-Reflex - physiology; Humans; Male; Motor Neurons - physiology; Muscle Contraction - physiology; Muscle, Skeletal - cytology; Muscle, Skeletal - innervation; Muscle, Skeletal - physiology; Original
• ISSN: 0022-3751; 1469-7793
• DOI: 10.1113/jphysiol.2002.032763

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 (H max ) and by the direct muscle compound action potential (M max ). 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 M max (−57 %, P < 0.001) and H max (−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 H max /M max 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 H max (M atHmax ) than did the younger males (young 5 % vs. elderly 29 % of the M max , P < 0.01). This finding suggests an increased homogeneity between the excitability threshold of sensory and motor axons. The twitch torque at H max (Pt H–M ) was subsequently calculated by subtraction from the total twitch torque of the mechanical contamination associated with M atHmax . The resulting Pt H–M was significantly lower in the elderly (−59 %, P < 0.001). Despite a discrepancy of 20 % between the two groups, the mechanical ratio (Pt H–M /Pt M ; Pt M , twitch tension related to the M max 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 (Pt H–M /H max ) 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 (Pt M /M max ).