The Decrease in Force Steadiness with Aging is associated with Increased Power of the Common but not Independent Input to Motor Neurons

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Abstract

Declines in motor function with advancing age have been attributed to changes occurring at all levels of the neuromuscular system. However, the impact of aging on the control of muscle force by spinal motor neurons is not yet understood. In this study on 20 individuals aged between 24 and 75 years (7 women), we investigated the common synaptic input to motor neurons of the tibialis anterior muscle and its impact on force control. Motor unit discharge times were identified from high-density surface EMG recordings during isometric contractions at forces of 20% of the maximal voluntary effort. Coherence analysis between motor unit spike trains was used to characterize the input to motor neurons. The decrease in force steadiness with age (R2 = 0.6, P < 0.01) was associated with an increase in the amplitude of low-frequency oscillations of functional common synaptic input to motor neurons (R2 = 0.59; P < 0.01). The relative proportion of common input to independent noise at low frequencies increased with the variability (power) in common synaptic input. Moreover, the variability in interspike interval did not change and the strength of common input in the gamma band decreased with age (R2 = 0.22; P < 0.01). The findings indicate that the age-related reduction in the accuracy of force control is associated with increased common fluctuations to motor neurons at low frequencies and not by an increase in independent synaptic input.
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Detaljer

Declines in motor function with advancing age have been attributed to changes occurring at all levels of the neuromuscular system. However, the impact of aging on the control of muscle force by spinal motor neurons is not yet understood. In this study on 20 individuals aged between 24 and 75 years (7 women), we investigated the common synaptic input to motor neurons of the tibialis anterior muscle and its impact on force control. Motor unit discharge times were identified from high-density surface EMG recordings during isometric contractions at forces of 20% of the maximal voluntary effort. Coherence analysis between motor unit spike trains was used to characterize the input to motor neurons. The decrease in force steadiness with age (R2 = 0.6, P < 0.01) was associated with an increase in the amplitude of low-frequency oscillations of functional common synaptic input to motor neurons (R2 = 0.59; P < 0.01). The relative proportion of common input to independent noise at low frequencies increased with the variability (power) in common synaptic input. Moreover, the variability in interspike interval did not change and the strength of common input in the gamma band decreased with age (R2 = 0.22; P < 0.01). The findings indicate that the age-related reduction in the accuracy of force control is associated with increased common fluctuations to motor neurons at low frequencies and not by an increase in independent synaptic input.
OriginalsprogEngelsk
TidsskriftJournal of Neurophysiology
ISSN0022-3077
DOI
StatusUdgivet - 5 jul. 2018
PublikationsartForskning
Peer reviewJa
ID: 282201296