Abstract
PurposeCurrent methods for estimating muscle motor unit (MU) number provide values which are remarkably similar for muscles of widely differing size, probably because surface electrodes sample from similar and relatively small volumes in each muscle. We have evaluated an alternative means of estimating MU number that takes into account differences in muscle size.MethodsIntramuscular motor unit potentials (MUPs) were recorded and muscle cross-sectional area (CSA) was measured using MRI to provide a motor unit number estimate (iMUNE). This was compared to the traditional MUNE method, using compound muscle action potentials (CMAP) and surface motor unit potentials (sMUPs) recorded using surface electrodes. Data were collected from proximal and distal regions of the vastus lateralis (VL) in young and old men while test–retest reliability was evaluated with VL, tibialis anterior and biceps brachii.ResultsMUPs, sMUPs and CMAPs were highly reliable (r = 0.84–0.91). The traditional MUNE, based on surface recordings, did not differ between proximal and distal sites of the VL despite the proximal CSA being twice the distal CSA. iMUNE, however, gave values that differed between young and old and were proportional to the muscle size.ConclusionWhen evaluating the contribution that MU loss makes to muscle atrophy, such as in disease or ageing, it is important to have a method such as iMUNE, which takes into account any differences in total muscle size.
Highlights
Loss of muscle mass and strength are primary features of ageing and while there may be a number of causes, including atrophy or the reduction in specific force of individual fibres, there is good evidence that a reduction in the number of fibres (Lexell et al 1988) as a consequence of the loss of motor neurons (Tomlinson and Irving 1977) plays an important part in the ageing process
There are a range of motor unit number estimates” (MUNE) methods which differ largely in the way the average surface recorded motor unit potentials (sMUPs) is obtained, the most commonly used in recent years is ‘spike-triggered averaging’ (STA) in which individual sMUPs are identified and averaged using a trigger from an intramuscular needle electrode
Vastus lateralis cross-sectional area (CSA) was larger at the proximal site, and larger in the young compared to the old
Summary
Loss of muscle mass and strength are primary features of ageing and while there may be a number of causes, including atrophy or the reduction in specific force of individual fibres, there is good evidence that a reduction in the number of fibres (Lexell et al 1988) as a consequence of the loss of motor neurons (Tomlinson and Irving 1977) plays an important part in the ageing process. Current methods for estimating numbers of motor units in larger muscles, often referred to as “motor unit number estimates” (MUNE), rely on comparing the average size of surface recorded motor unit potentials (sMUPs) with a maximal electrically stimulated compound muscle action potential (CMAP) The rationale for this is that the size of the maximal CMAP is a measure of the electrical activity of the whole muscle and dividing this by the size of an average sMUP provides an estimate of the number of MUs (Brown et al 1988). This was first proposed by Brown et al (1988) and further developed with improved recording and analytical methods (Daube 2006; Bromberg 2007; Gooch et al 2014) that have been used to characterize MU loss in disease (Gooch et al 2009; Allen et al 2015) and ageing (McNeil et al 2005; Power et al 2012; Piasecki et al 2015)
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