Abstract
Fluctuations during isometric force production tasks occur due to the inability of musculature to generate purely constant submaximal forces and are considered to be an estimation of neuromuscular noise. The human sensori-motor system regulates complex interactions between multiple afferent and efferent systems, which results in variability during functional task performance. Since muscles are the only active component of the motor system, it therefore seems reasonable that neuromuscular noise plays a key role in governing variability during both standing and walking. Seventy elderly women (including 34 fallers) performed multiple repetitions of isometric force production, quiet standing and walking tasks. No relationship between neuromuscular noise and functional task performance was observed in either the faller or the non-faller cohorts. When classified into groups with either nominal (group NOM, 25th –75th percentile) or extreme (either too high or too low, group EXT) levels of neuromuscular noise, group NOM demonstrated a clear association (r2>0.23, p<0.05) between neuromuscular noise and variability during task performance. On the other hand, group EXT demonstrated no such relationship, but also tended to walk slower, and had lower stride lengths, as well as lower isometric strength. These results suggest that neuromuscular noise is related to the quality of both static and dynamic functional task performance, but also that extreme levels of neuromuscular noise constitute a key neuromuscular deficit in the elderly.
Highlights
The order of recruitment as well as the variability in the firing of multiple motor units during voluntary isometric submaximal muscular contraction results in an inability of the musculature to generate constant forces, causing oscillations or fluctuations in the resulting force output [1,2,3,4,5,6,7]
The rotated component matrix (Table 2) indicated that force fluctuation principal components (ffPCs) 1 was predominantly associated with force fluctuations from the ankle plantarflexors, and this component was denoted ‘‘ankle noise’’, while ffPC 2 was almost entirely composed of force fluctuations from the knee extensors, and termed ‘‘knee noise’’
Variability during task performance is known to be increased in subjects who have fallen [12,31,39], but the aetiology of variability during static and dynamic functional task performance has until now, remained unclear
Summary
The order of recruitment as well as the variability in the firing of multiple motor units during voluntary isometric submaximal muscular contraction results in an inability of the musculature to generate constant forces, causing oscillations or fluctuations in the resulting force output [1,2,3,4,5,6,7]. The variability of kinetics and kinematics are dependent upon the requirements of the task, such that e.g. final location accuracy is reduced with increasing trajectory speed [10,11] Since this variability within the generated force is known to influence the intended movement trajectory [4,11,12,13], the presence of neuromuscular noise is thought to affect the trial-totrial repetitions of a task [1,10]. The level of force fluctuations during muscular contractions [1,2,4,17] needs to be accounted for in order to optimise kinetics and kinematics during task performance, and this is achieved through feedback mechanisms within the sensori-motor system [1,11,15,16,18,19,20,21]. The quantification of variability during task performance has become a target for evaluating the human sensori-motor system [4,16,22,23,24,25,26,27,28]
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