Age‐related reductions in the estimated numbers of motor units are minimal in the human soleus

Abstract

The documented impact of contractile level on decomposition-enhanced spike-triggered averaging motor unit number estimates (MUNEs) in young adults demonstrates the importance of selecting an objective contraction intensity that yields the most representative MUNE for a given muscle. Whether the same contraction intensity would be ideal in an altered system (e.g., by aging or disease) has yet to be examined. Thus, the main purpose of this study was to compare the effects of contraction intensity on MUNEs from the soleus muscle in young ( approximately 27 years) and old ( approximately 75 years) men. Using decomposition-enhanced spike-triggered averaging, surface and intramuscular electromyographic signals were collected from the soleus during a range of submaximal isometric plantar-flexion contractions (threshold, 10%, 20%, and 30% of maximum voluntary contraction; MVC). Five MUNEs were calculated, one for each of the four contraction intensities and an ensemble MUNE was derived from all MUs collected. Although MUNE decreased similarly with increased effort in both groups, MUNEs were not significantly reduced in the old men compared to the young men. Consequently, the ensemble MUNE was extrapolated to an intensity of approximately 15% MVC in both young and old. The results suggest that, in the soleus, the use of the same contraction intensity across age groups is a valid comparison.

The altered vestibular-evoked myogenic and whole-body postural responses in old men during standing

“…However, based on numerical modeling, motor unit firing rate cannot explain exclusively the large frequency bandwidth differences between the neck and lower limb muscles of young adults (Forbes et al, 2013). Even though the soleus and its constitutive motor units maintain their properties well into the eight decade of life (Dalton et al, 2008(Dalton et al, , 2009, we also observed a lower frequency bandwidth in the old men than in the young for this muscle. Similar to the muscle-dependent variability in neuromuscular properties and function, the different bandwidth frequencies between age groups are likely not explained solely by the age-related variance in motor unit properties.…”

“…With healthy adult aging, whole muscle contractile properties are slower (Dalton et al, 2009, motor unit firing rates are lower (Dalton et al, 2009(Dalton et al, , 2010Rubinstein and Kamen, 2005) and inherent motor neuron properties are altered (Kalmar et al, 2009;Piotrkiewicz et al, 2007), but these age-related declines cannot be generalized to all muscles and their constitutive motor neurons (Dalton et al, 2008(Dalton et al, , 2009Deschenes et al, 2010;Ishihara et al, 1987;Moran et al, 2005). Further, high-frequency sound and vibrotactile detection thresholds are increased with adult aging (Wells et al, 2003;Willott, 1984).…”

The influence on sarcopenia of muscle quality and quantity derived from magnetic resonance imaging and neuromuscular properties

“…For decomposition-enhanced spike-triggered averaging (DE-STA) measures that include an estimate of the number of functional motor units (MUNE), intramuscular EMG signals were recorded via a disposable concentric needle electrode with a recording surface of 0.03 mm 2 (Model N53153; Teca, Hawthorne, New York) inserted into the TA, 5-10 mm proximal to the active surface electrode. The MUNE technique was performed similarly to what we have reported previously (Dalton et al 2008;McNeil et al 2005b;Power et al 2012aPower et al , 2010. Electromyography data were acquired using customized software on a Neuroscan Comperio system (Neurosoft, El Paso, Texas).…”

Motor Unit Survival in Lifelong Runners Is Muscle Dependent

“…Because this process does not recapture all denervated fibers, there is a gradual loss of muscle rnass leaving the whole muscle composed of fewer and larger Type I MUs (15). The rate of this process may vary by muscle group (9,30), but eventually, some limit in the size of the remaining MUs is reached and the continued loss of MUs, and hence, sarcopenia may occur at a faster rate in very old age (30). The number of functioning MUs in a human muscle group or a MU number estimate (MUNE) can be determined electrophysiologically by dividing the compound muscle action potential (M wave), which represents the "maximal electrical size" of the muscle group, by a representative sample of surface-detected MU potentials (S-MUP), "electrical size" of an individual MU (see Bromberg (4) for review).…”