Proceedings of The Physiological Society

Ageing and Degeneration (Edinburgh, UK) (2015) Proc Physiol Soc 33, PC17

Poster Communications

Age-related loss of motor units in the vastus lateralis

A. Ireland1, M. Piasecki1, D. W. Stashuk2, D. Jones1, J. S. McPhee1

1. School of Healthcare Science, Manchester Metropolitan University, Manchester, United Kingdom. 2. Department of Systems Design Engineering, University of Waterloo, Waterloo, Ontario, Canada.

A progressive loss of motor units has been documented in small and peripheral muscles, with around half of the motor unit pool lost by the age of around 80 yrs (Power et al. 2010) but very little is known about motor unit changes during healthy ageing in large locomotor muscles that are important for maintaining mobility. After ethical approval and with informed written consent, 29 active, healthy young men (age 26±5y) and 17 active, healthy older men (age 73±6y) were recruited. Proximal and distal motor points of the vastus lateralis muscle were identified by percutaneous electrical stimulation and the recoding of surface electromyographuic (SEMG) signals from those locations during isometric knee extension held at 25% of maximal voluntary contraction. Intramuscular EMG signals were also recorded close to the motor points at three depths within the muscle and turning the needle through 90o between contractions so that 12 separate recordings were made. Single motor unit potentials (SMUPs) were estimated from SEMG signals using a decomposition-enhanced spike-triggered averaging technique developed from that described by Parsaei et al (2012) and an average SMUP area determined using between 20-40 SMUPs for each subject. A motor unit number estimate (MUNE) was calculated by dividing the area of a compound surface muscle action potential, obtained by supra-maximal stimulation of the femoral nerve, by the average SMUP area. MUNE in the older men was 75% of the value for the young (255±118 vs 339±128, P = 0.02). While it is possible for all fibres of a small distal muscle to make significant contributions to a recorded SEMG signal this is not likely with large muscles such as the quadriceps. If the SEMG signals were recorded from the same volume of muscle in each subject then it can be speculated that the total number of MUs in a muscle is proportional to the product of muscle volume and MUNE. We have recently shown that in older men of this age, quadriceps volume was approximately 70% that of the young (Maden-Wilkinson, 2014) suggesting that the total number of MUs in the older subjects relative to the younger subjects may be as low as 55%, depending on how much of the muscle wasting is due to fibre atrophy. Loss of MUs may contribute to the loss of strength which is characteristic of aging but the disruption of the normal recruitment patterns could also have important consequences for the ability to make smooth and coordinated movements.

Where applicable, experiments conform with Society ethical requirements