Abstract
Aging is associated with reduced maximum force production and force steadiness during low-force tasks, but both can be improved by training. Intermuscular coherence measures coupling between two peripheral surface electromyography (EMG) signals in the frequency domain. It is thought to represent the presence of common input to alpha-motoneurons, but the functional meaning of intermuscular coherence, particularly regarding aging and training, remain unclear. This study investigated knee extensor intermuscular coherence in previously sedentary young (18–30 years) and older (67–73 years) subjects before and after a 14-week strength training intervention. YOUNG and OLDER groups performed maximum unilateral isometric knee extensions [100% maximum voluntary contraction (MVC)], as well as force steadiness tests at 20 and 70% MVC, pre- and post-training. Intermuscular (i.e., EMG-EMG) coherence analyses were performed for all (three) contraction intensities in vastus lateralis and medialis muscles. Pre-training coefficient of force variation (i.e., force steadiness) and MVC (i.e., maximum torque) were similar between groups. Both groups improved MVC through training, but YOUNG improved more than OLDER (42 ± 27 Nm versus 18 ± 16 Nm, P = 0.022). Force steadiness did not change during 20% MVC trials in either group, but YOUNG demonstrated increased coefficient of force variation during 70% MVC trials (1.28 ± 0.46 to 1.57 ± 0.70, P = 0.01). YOUNG demonstrated greater pre-training coherence during 20% and 70% MVC trials, particularly within the 8–14 Hz (e.g., 20%: 0.105 ± 0.119 versus 0.016 ± 0.009, P = 0.001) and 16–30 Hz (20%: 0.063 ± 0.078 versus 0.012 ± 0.007, P = 0.002) bands, but not during 100% MVC trials. Strength training led to increases in intermuscular coherence within the 40–60 Hz band during 70% MVC trials in YOUNG only, while OLDER decreased within the 8–14 Hz band during 100% MVC trials. Age-related differences in intermuscular coherence were observed between young and older individuals, even when neuromuscular performance levels were similar. The functional significance of intermuscular coherence remains unclear, since coherence within different frequency bands did not explain any of the variance in the regression models for maximum strength or force steadiness during 20 and 70% MVC trials.
Highlights
Aging is associated with degenerations in neural functioning that reduce performance during voluntary force production tasks
Pre-training force steadiness did not differ between the age groups during either 20% maximum voluntary contraction (MVC) trials (YOUNG: 0.35 ± 0.15%, OLDER: 0.28 ± 0.09%) or 70% MVC trials (YOUNG: 1.28 ± 0.46%, OLDER: 0.96 ± 0.34%)
YOUNG demonstrated significant worsening of force steadiness during 70% MVC trials (1.28 ± 0.46% to 1.57 ± 0.70%, P = 0.01), and the change was statistically different compared to that in OLDER (0.29 ± 0.34% versus −0.02 ± 0.2%, P = 0.025)
Summary
Aging is associated with degenerations in neural functioning that reduce performance during voluntary force production tasks. Force steadiness during low- and moderate-force isometric contractions is poorer in older individuals (Tracy and Enoka, 2006; Griffin et al, 2009) and so in those with a history of falls (Carville et al, 2007). This is accompanied by less (larger) motor neurons/units (Lexell et al, 1988), which increases the size of each motor unit (Piasecki et al, 2016), and by greater variability in motor unit discharge rates (Tracy et al, 2005; Kallio et al, 2012). Intermuscular coherence methods allow examination of high-force contractions (>70% of maximum), whereas EEG signals may be contaminated by muscular artifacts when examining high-force corticomuscular coherence
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