Functional force stimulation alters motor neuron discharge patterns

Introduction

Beneficial effects have been observed for mechanical vibration stimulation (MVS), which are mainly attributed to tonic vibration reflex (TVR). TVR is reported to elicit synchronized motor unit activation during locally applied vibration. Similar effects are also observed in a novel vibration system referred to as functional force stimulation (FFS). However, the manifestation of TVR in FFS is doubted due to the use of global electromyography (EMG) features in previous analysis. Our study aims to investigate the effects of FFS on motor unit discharge patterns of the human biceps brachii by analyzing the motor unit spike trains decoded from the high-density surface EMG.

Methods

Eighteen healthy subjects volunteered in FFS training with different amplitudes and frequencies. One hundred and twenty-eight channel surface EMG was recorded from the biceps brachii and then decoded after motion-artifact removal. The discharge timings were extracted and the coherence between different motor unit spike trains was calculated to quantify synchronized activation.

Results and discussion

Significant synchronization within the vibration cycle and/or its integer multiples is observed for all FFS trials, which increases with increased FFS amplitude. Our results reveal the basic physiological mechanism involved in FFS, providing a theoretical foundation for analyzing and introducing FFS into clinical rehabilitation programs.

The Effects of Vibratory and Acoustic Stimulations on Postural Control in Healthy People: A Systematic Review

Research on human posture and balance control has grown in recent years, leading to continued advances in their understanding. The ability to maintain balance is attributed to the interplay of the visual, vestibular, and somatosensory systems, although an important role is also played by the auditory system. The lack or deficit in any of these systems leads to a reduced stability that may be counterbalanced by the integration of all the remaining sensory information. Auditory and vibratory stimulation have been found to be useful to enhance balance alongside daily activities either in healthy or pathological subjects; nevertheless, while widely investigated, the literature relating to these approaches is still fragmented. This review aims at addressing this by collecting, organising, and discussing all the literature to date on the effects of the various acoustic and vibratory stimulation techniques available on static upright posture in healthy subjects. In addition, this review intends to provide a solid and comprehensive starting point for all the researchers interested in these research areas. A systematic search of the literature was performed and a total of 33 articles (24 on vibratory stimulation and 9 on acoustic stimulation) were included in our analysis. For all articles, several elements were highlighted including: the study sample, the characteristics of the stimulations, the recording instruments, the experimental protocols, and outcomes. Overall, both stimulations analysed were found to have a positive effect on balance but more research is needed to align those alternative approaches to the traditional ones.

Sensorimotor recalibration of postural control strategies occurs after whole body vibration

Efficient postural control results from an effective interplay between sensory feedbacks integration and muscle modulation and can be affected by ageing and neuromuscular injuries. With this study, we investigated the effect of whole-body vibratory stimulation on postural control strategies employed to maintain an upright posture. We explored both physiological and posturography metrics, through corticomuscular and intramuscular coherence, and muscle networks analyses. The stimulation disrupts balance in the short term, but leads to a greater contribution of cortical activity, necessary to modulate muscle activation via the formation of (new) synergies. We also observed a reconfiguration of muscle recruitment patterns that returned to pre-stimulation levels after few minutes, accompanied by a slight improvement of balance in the anterior-posterior direction. Our results suggest that, in the context of postural control, appropriate mechanical stimulation is capable of triggering a recalibration of the sensorimotor set and might offer new perspectives for motor re-education.

Electromyographic traces of motor unit synchronization of fatigued lower limb muscles during gait

BACKGROUND

The study of the signal in the frequency domain has shown to be a good tool to identify muscular fatigue. Previous research has shown that the low frequency band and 40 Hz frequency band increase their relative intensity with the onset of fatigue. These findings were obtained in rectus femoris, but the behaviours of other muscles of the lower limb are unknown. In this article we explored the changes in the low frequency and 40 Hz frequency band of lower limb muscles with respect to fatigue.

METHODS

Thirty healthy subjects were recruited to analyse the electromyography (EMG) of biceps femoris, tibialis anterior and gastrocnemius medialis and lateralis of both legs during gait. Four two-minutes walks at a self-selected speed were recorded, the first two walks with a normal muscular function and the last two walks after a fatigue protocol. All the signals were decomposed using wavelet transformations. The signals were normalized in time and spectral intensities normalized to the sum of intensities in the frequency domain. Two frequency bands were studied in each walk: the 40-Hz (34-53 Hz) and the low frequency (< 25 Hz) bands. A ratio of the spectral intensities of those frequency bands at each walk was obtained by dividing the 40-Hz frequency band spectral intensity by the low frequency band spectral intensity. Statistical parametric mapping techniques were used to compare the ratios of the prefatigue walks against the postfatigue walks.

RESULTS

The results of the Statistical Non-Parametric Mapping (SnPM) analysis of all muscles depict a higher relative spectral intensity in the low frequency band in the comparison of fatigue versus prefatigue recordings except for the right gastrocnemius lateralis. The critical thresholds F* were exceeded by multiple suprathreshold clusters with p values <0.05, showing that the low frequency band increased its relative spectral intensity in the case of fatigue.

CONCLUSION

The obtained results suggest that the low frequency band increases its relative spectral intensity in all the studied muscles when fatigue onsets. This increase in relative spectral intensity may be linked to an increase in motor unit synchronization promoted by the central nervous system to ensure good motor control.

Relevance of spectral peaks in electromyographic recordings during force-modulated vibration exercise

Vibration exercise (VE) has been suggested for effective muscle training and conditioning. Surface electromyogram (EMG) is employed as a powerful tool for the analysis of VE. However, sharp peaks are observed in the EMG spectrum. The interpretation of these peaks is controversial, complicating the extraction of EMG parameters for VE analysis. The present study is therefore aiming at quantifying the relevance of these spectral peaks in EMG recording during VE. To this end, surface EMG was recorded on the biceps brachii during VE at different amplitudes and frequencies. The power percentage contained in the two narrow bands (±0.5 Hz) around the vibration frequency and its first harmonic (PPv) was calculated. The root mean square (RMS) value of the EMG was calculated with and without including the spectral peaks and then compared. The results show an average PPv value of 20.7 ± 7.9 % and a relative RMS difference (∆RMS) of 12.2 ± 3.8 %. In PPv and addition, RMS∆ seem to be influenced by vibration amplitude and frequency. Our results provide useful information for the analysis of VE and contribute to better understanding of the EMG spectral peaks.

Groundtruth: A Matlab GUI for Artifact and Feature Identification in Physiological Signals

Groundtruth is a Matlab Graphical User Interface (GUI) developed for the identification of key features and artifacts within physiological signals. The ultimate aim of this GUI is to provide a simple means of assessing the performance of new sensors. Secondary, to this is providing a means of providing marked data, enabling assessment of automated artifact rejection and feature identification algorithms. With the emergence of new wearable sensor technologies, there is an unmet need for convenient assessment of device performance, and a faster means of assessing new algorithms. The proposed GUI allows interactive marking of artifact regions as well as simultaneous interactive identification of key features, e.g., respiration peaks in respiration signals, R-peaks in Electrocardiography signals, etc. In this paper, we present the base structure of the system, together with an example of its use for two simultaneously worn respiration sensors. The respiration rates are computed for both original as well as artifact removed data and validated using Bland–Altman plots. The respiration rates computed based on the proposed GUI (after artifact removal process) demonstrated consistent results for two respiration sensors after artifact removal process. Groundtruth is customizable, and alternative processing modules are easy to add/remove. Groundtruth is intended for open-source use.

Does vibration superimposed on low-level isometric contraction alter motor unit recruitment strategy?

OBJECTIVE

Beneficial effects, including improved muscle strength and power performance, have been observed during vibration exercise (VE) and partially ascribed to a specific reflex mechanism referred to as Tonic vibration reflex (TVR). TVR involves motor unit (MU) activation synchronized and un-synchronized with the vibration cycle; this suggests VE to alter the temporal MU recruitment strategy. However, the effects of VE on MU recruitment remain poorly understood. This study aims to elucidate the influence of VE on MU recruitment indirectly, by investigating the effects of low-intensity VE on muscle activation.

APPROACH

Twenty volunteers performed isometric contractions on the biceps brachii of the right arm at a baseline (low) force equal to 30% of the maximum voluntary contraction without vibration (control) and with vibration at 20, 30, 40, and 55 Hz. Three vibration amplitudes were employed at 12.5%, 25%, and 50% of the baseline. Mean muscle-fiber conduction velocity (mCV), mean frequency (MF), and root mean square (RMS) value were estimated from surface electromyography as indicators of the alteration in MU recruitment strategies.

MAIN RESULTS

The mCV estimates during VE were significantly (p  <  0.05) higher compared to the control condition. Furthermore, six VE conditions produced significantly larger RMS values compared to control condition. The estimated MF did not show any consistent trend.

SIGNIFICANCE

These results suggest that vibration superimposed on low-level isometric contraction alters the MU recruitment strategy, activating larger and faster MUs.