Date Published: January 26, 2017
Publisher: Public Library of Science
Author(s): Ing-Shiou Hwang, Yen-Ting Lin, Wei-Min Huang, Zong-Ru Yang, Chia-Ling Hu, Yi-Ching Chen, Yih-Kuen Jan.
Discharge patterns from a population of motor units (MUs) were estimated with multi-channel surface electromyogram and signal processing techniques to investigate parametric differences in low-frequency force fluctuations, MU discharges, and force-discharge relation during static force-tracking with varying sizes of execution error presented via visual feedback. Fourteen healthy adults produced isometric force at 10% of maximal voluntary contraction through index abduction under three visual conditions that scaled execution errors with different amplification factors. Error-augmentation feedback that used a high amplification factor (HAF) to potentiate visualized error size resulted in higher sample entropy, mean frequency, ratio of high-frequency components, and spectral dispersion of force fluctuations than those of error-reducing feedback using a low amplification factor (LAF). In the HAF condition, MUs with relatively high recruitment thresholds in the dorsal interosseous muscle exhibited a larger coefficient of variation for inter-spike intervals and a greater spectral peak of the pooled MU coherence at 13–35 Hz than did those in the LAF condition. Manipulation of the size of error feedback altered the force-discharge relation, which was characterized with non-linear approaches such as mutual information and cross sample entropy. The association of force fluctuations and global discharge trace decreased with increasing error amplification factor. Our findings provide direct neurophysiological evidence that favors motor training using error-augmentation feedback. Amplification of the visualized error size of visual feedback could enrich force gradation strategies during static force-tracking, pertaining to selective increases in the discharge variability of higher-threshold MUs that receive greater common oscillatory inputs in the β-band.
Force steadiness is a useful paradigm for investigating fine motor control and perceptuo-motor variability [1, 2]. During constant isometric contraction, the smoothness of a force trajectory is undermined by numerous intermittent drifts of the force output away from the target force, known as force fluctuations. Accumulating evidence has convincingly shown that force variability, particularly for those low-frequency force fluctuations under 4 Hz, are related to visuomotor processes [3, 4, 5] and movement corrections [6, 7]. The amount of visual spatial information can influence the properties of force fluctuations. For static isometric contraction at very low exertion levels (2%-10% maximal voluntary contraction), the majority of previous studies reported that a high spatial resolution of the visual display (or a smaller change in force occupying a greater number of pixels) produced superior force steadiness for the young adults, supported by reduction in the size of force fluctuations [8, 9, 10]. Although motor unit discharge is a major determinant of force fluctuations [11, 12], the mean and coefficient of variation of the inter-spike interval of motor units of young adults are unexpectedly not tuned to visual spatial information [3, 13], except that Laine et al  reported that all spectral bands of the common oscillatory input to motor units varied with the spatial resolution of a visual display. In addition to experimental contexts, some of the inconsistency in the findings on the visual impact on force fluctuations and motor unit discharge was due to the failure of previous studies to specify the roles of high-threshold motor units. On account of lower and more variant discharge rates, high-threshold motor units with a larger twitch force are liable to produce a greater size of force fluctuations [11, 15].
The participants were 14 healthy adults (6 males and 8 females; mean age: 25.87 ± 1.19 years, range: 21–35 years old) from a university campus or the local community. All were self-reported as being right-handed, and none had symptoms or signs of neuromuscular diseases. The research project was approved by an authorized institutional human research review board (IRB) at the University Hospital of the National Cheng Kung University, Taiwan. All of the participants signed an informed consent before the experiment, conforming to the Declaration of Helsinki.
The novel findings of this study were that 1) error-augmentation feedback led to more high-frequency components, power dispersion, and irregularity of low-frequency force fluctuations than did error-reducing feedback, 2) error-augmentation feedback increased the standardized variability and pooled motor unit coherence at 13–35 Hz of the high-threshold MUs relative to those of error-reducing feedback, and 3) non-linear approaches were better able to characterize the size effect on the association of force fluctuations and global MU discharge. The force-discharge relation waned with increasing size of error feedback.
This study presents a preliminary investigation of behavioral and neurophysiological mechanisms of force regulation when the size of error feedback varies. Capitalizing on characteristic changes in force fluctuations, error-augmentation feedback provides a more frequent and intricate force gradation process, which could increase the effectiveness of motor training by increasing exploration variability. Force control associated with error-augmentation feedback is pertinent to increases in the neurological degree of freedom for the firing patterns of high-threshold MUs that receive β-band common oscillatory inputs and unidentified neural drives.