Objectives Age has been shown in previous studies to affect axonal membrane polarisation. The aim of this study was to examine the effect of age on skeletal muscle membrane properties in normal patients as assessed by muscle velocity recovery cycles (MVRC).
Methods Informed consent was obtained from 74 healthy control patients (aged 18 to 84yo, median age 35 and IQR 29−55 yo) with intact neuromuscular strength (MRC score 5). MVRC recordings were obtained from tibialis anterior (n=74) and rectus femoris (n=32) muscles as representative distal and proximal muscles. Data was captured using QTRACS threshold tracking software employing the M3RC3 recording protocol with subsequent data analysis through the MAnal8 program.
Results Regression analysis on tibialis anterior MVRC recordings demonstrated that increasing age was associated with longer muscle relative refractory period (MRRP: r=0.61, p<0.001). There was a reduction in early supernormality (r=−0.57, p<0.001) and a smaller reduction in additional late supernormality with multiple conditional stimuli as age increased (r=−0.45, p<0.001). Analysis of rectus femoris MVRCs revealed a similar association of increased age with longer MRRP (r=0.55, p=0.002) and reduced early supernormality (r=−0.43, p=0.01), but no significant association with late supernormality was apparent.
Conclusions The changes suggest resting muscle membrane potential appears relatively more depolarized with increasing age in adulthood. Possible reasons for this finding include the increasing predominance of type 1 muscle fibres with age (1), or age-related Na+/K+ ATPase and mitochondrial dysfunction (2). These findings have implications for the accurate interpretation of MVRC research into the future and our knowledge of normal skeletal muscle ageing.
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