Recruitment order of quadriceps motor units: femoral nerve vs. direct quadriceps stimulation

Differential effects of stimulation frequency on isometric and concentric isotonic contractile function in human quadriceps

Electrically evoked contractions are used to assess the relationship between frequency input and contractile output to characterize inherent muscle function, and these have been done mostly with isometric contractions (i.e., no joint rotation). The purpose was to compare the electrically stimulated frequency and contractile function relationship during isometric (i.e., torque) with isotonic (i.e., concentric torque, angular velocity, and mechanical power) contractions. The knee extensors of 16 (5 female) young recreationally active participants were stimulated (∼1-2.5 s) at 14 frequencies from 1 to 100 Hz. This was done during four conditions, which were isometric and isotonic at loads of 0 (unloaded), 7.5%, and 15% isometric maximal voluntary contraction (MVC), and repeated on separate days. Comparisons across contractile parameters were made as a % of 100 Hz. Independent of the load, the mechanical power-frequency relationship was rightward shifted compared with isometric torque-frequency, concentric torque-frequency, and velocity-frequency relationships (all P ≤ 0.04). With increasing load (0%-15% MVC), the isotonic concentric torque-frequency relationship was shifted leftward systematically from 15 to 30 Hz (all P ≤ 0.04). Conversely, the same changes in load caused a rightward shift in the velocity-frequency relationship from 1 to 40 Hz (all P ≤ 0.03). Velocity was leftward shifted of concentric torque in the unloaded isotonic condition from 10 to 25 Hz (all P ≤ 0.03), but concentric torque was leftward shifted of velocity at 15% MVC isotonic condition from 10 to 50 Hz (all P ≤ 0.03). Therefore, isometric torque is not a surrogate to evaluate dynamic contractile function. Interpretations of evoked contractile function differ depending on contraction type, load, and frequency, which should be considered relative to the specific task.NEW & NOTEWORTHY In whole human muscle, we showed that the electrically stimulated power-frequency relationship was rightward shifted of the stimulated isometric torque-frequency relationship independent of isotonic load, indicating that higher stimulation frequencies are needed to achieve tetanus. Therefore, interpretations of evoked contractile function differ depending on contraction type (isometric vs. dynamic), load, and frequency. And thus, isometric measures may not be appropriate as a surrogate assessment when evaluating dynamic isotonic contractile function.

Diagnostic accuracy of Tensiomyography parameters for monitoring peripheral neuromuscular fatigue

The diagnostic accuracy of tensiomyography (TMG) parameters compared to the gold standard in neuromuscular fatigue evaluation using voluntary and electrically induced muscle activation is unclear. This study aimed to investigate the diagnostic accuracy of TMG parameters to detect individual changes after interventions that were designed to induce central or peripheral fatigue. Nineteen males (age: 32.2 ± 9.3 years) performed two interventions, consisting of maintaining 25% of maximal voluntary contraction (MViC_25%) and a 30 s all-out cycling test (Wingate), respectively. TMG parameters, maximum voluntary contraction (PtMViC), voluntary activation (VA%) and electrically elicited double twitches (Dtw) were assessed on the knee extensors before (PRE), one minute (POST) and seven minutes after (POST7) the intervention. The diagnostic accuracy (AUC) of TMG parameters were evaluated in comparison to two criteria measures (PtMViC and Dtw). RM ANOVA revealed a significant interaction between the effects of intervention and time on VA% (p = 0.001) and Dtw (p < 0.001) but not for PtMViC (p = 0.420). AUC showed that TMG parameters had a good ability in detecting muscular fatigue assessed by Dtw but not by PtMViC. The results of the current study suggest that TMG parameters can be used to monitor peripheral neuromuscular fatigue.

Effects of Kilohertz Frequency, Burst Duty Cycle, and Burst Duration on Evoked Torque, Perceived Discomfort and Muscle Fatigue: A Systematic Review

ABSTRACT

Kilohertz-frequency alternating current is used to minimize muscle atrophy and muscle weakness and improve muscle performance. However, no systematic reviews have evaluated the best Kilohertz-frequency alternating current parameters for this purpose. We investigated the effects of the carrier frequency, burst duty cycles, and burst durations on evoked torque, perceived discomfort, and muscle fatigue. A search of eight data sources by two independent reviewers resulted in 13 peer-reviewed studies being selected, following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines, and rated using the PEDro scale to evaluate the methodological quality of the studies. Most studies showed that carrier frequencies up to 1 kHz evoked higher torque, while carrier frequencies between 2.5 and 5 kHz resulted in lower perceived discomfort. In addition, most studies showed that shorter burst duty cycles (10%-50%) induced higher evoked torque and lower perceived discomfort. Methodological quality scores ranged from 5 to 8 on the PEDro scale. We conclude that Kilohertz-frequency alternating current develops greater evoked torque for carrier frequencies between 1 and 2.5 kHz and burst duty cycles less than 50%. Lower perceived discomfort was generated using Kilohertz-frequency alternating currents between 2.5 and 5 kHz and burst duty cycles less than 50%.

Quadriceps muscle stimulation evokes heteronymous inhibition onto soleus with limited Ia activation compared to femoral nerve stimulation

Heteronymous excitatory feedback from muscle spindles and inhibitory feedback from Golgi tendon organs and recurrent inhibitory circuits can influence motor coordination. The functional role of inhibitory feedback is difficult to determine, because nerve stimulation, the primary method used in humans, cannot evoke inhibition without first activating the largest diameter muscle spindle axons. Here, we tested the hypothesis that quadriceps muscle stimulation could be used to examine heteronymous inhibition more selectively when compared to femoral nerve stimulation by comparing the effects of nerve and muscle stimulation onto ongoing soleus EMG held at 20% of maximal effort. Motor threshold and two higher femoral nerve and quadriceps stimulus intensities matched by twitch evoked torque magnitudes were examined. We found that significantly fewer participants exhibited excitation during quadriceps muscle stimulation when compared to nerve stimulation (14-29% vs. 64-71% of participants across stimulation intensities) and the magnitude of heteronymous excitation from muscle stimulation, when present, was much reduced compared to nerve stimulation. Muscle and nerve stimulation resulted in heteronymous inhibition that significantly increased with increasing stimulation evoked torque magnitudes. This study provides novel evidence that muscle stimulation may be used to more selectively examine inhibitory heteronymous feedback between muscles in the human lower limb when compared to nerve stimulation.

Intrinsic Neuromuscular Fatigability in Humans: The Critical Role of Stimulus Frequency

Electrically evoked contractions provide insight into intrinsic neuromuscular fatigability and also represent a valuable technique to maintain muscle mass in a clinical setting. To appropriately investigate intrinsic fatigability and design optimal stimulation protocols, it would seem to be crucial to stimulate the muscle at a frequency equivalent to the mean motor unit discharge rate expected at the target force level.

Pressure on the electrode to reduce discomfort during neuromuscular electrical stimulation in individuals with different subcutaneous-fat thickness: is the procedure effective and reliable?

The addition of manual pressure on the electrode during neuromuscular electrical stimulation (NMES) has been used to reduce current intensity and perceived discomfort. In this study we aimed to test i) whether this approach affect the reliability of commonly made torque output measurements and ii) whether subcutaneous-fat thickness influence the efficacy of this approach in reducing current intensity and perceived discomfort. Twenty-one men (24±3.1 years) performed knee extension maximal voluntary isometric contractions with and without manual pressure on the NMES femoral nerve electrode (superimposed and resting doublets, 2 pulses at 100 Hz) during two separate sessions. Torque output was measured in an isokinetic dynamometer and thigh subcutaneous-fat thickness assessed with ultrasonography. A scale of perceived discomfort was presented after contractions. Reductions in current intensity (p<0.001) and discomfort during superimposed doublet (p=0.002) and resting doublet (p=0.002) were confirmed for the condition in which pressure was applied to the electrode. Fat thickness was correlated to changes in current intensity (r=0.63; p=0.002) and changes in discomfort (r=0.45; p=0.04) and no differences between pressure conditions and testing sessions were observed for torque output (p>0.05; ICC 0.95). Adding manual pressure during NMES on femoral nerve reduces discomfort and the maximal NMES intensity required to reach maximum torque without affecting torque output magnitude and reliability. Greater reduction in intensity and discomfort were observed in participants with higher subcutaneous-fat thickness levels after adding pressure on the electrode.

Electrically induced quadriceps fatigue in the contralateral leg impairs ipsilateral knee extensors performance

Muscle fatigue induced by voluntary exercise, which requires central motor drive, causes central fatigue that impairs endurance performance of a different, nonfatigued muscle. This study investigated the impact of quadriceps fatigue induced by electrically induced (no central motor drive) contractions on single-leg knee-extension (KE) performance of the subsequently exercising ipsilateral quadriceps. On two separate occasions, eight males completed constant-load (85% of maximal power-output) KE exercise to exhaustion. In a counterbalanced manner, subjects performed the KE exercise with no pre-existing quadriceps fatigue in the contralateral leg on one day (No-PreF), whereas on the other day, the same KE exercise was repeated following electrically induced quadriceps fatigue in the contralateral leg (PreF). Quadriceps fatigue was assessed by evaluating pre- to postexercise changes in potentiated twitch force (ΔQtw,pot; peripheral fatigue), and voluntary muscle activation (ΔVA; central fatigue). As reflected by the 57 ± 11% reduction in electrically evoked pulse force, the electrically induced fatigue protocol caused significant knee-extensors fatigue. KE endurance time to exhaustion was shorter during PreF compared with No-PreF (4.6 ± 1.2 vs 7.7 ± 2.4 min; P < 0.01). Although ΔQtw,pot was significantly larger in No-PreF compared with PreF (-60% vs -52%, P < 0.05), ΔVA was greater in PreF (-14% vs -10%, P < 0.05). Taken together, electrically induced quadriceps fatigue in the contralateral leg limits KE endurance performance and the development of peripheral fatigue in the ipsilateral leg. These findings support the hypothesis that the crossover effect of central fatigue is mainly mediated by group III/IV muscle afferent feedback and suggest that impairments associated with central motor drive may only play a minor role in this phenomenon.

Quantification of Neuromuscular Fatigue: What Do We Do Wrong and Why?

Neuromuscular fatigue (NMF) is usually assessed non-invasively in healthy, athletic or clinical populations with the combination of voluntary and evoked contractions. Although it might appear relatively straightforward to magnetically or electrically stimulate at different levels (cortical/spinal/muscle) and to measure mechanical and electromyographic responses to quantify neuromuscular adjustments due to sustained/repeated muscle contractions, there are drawbacks that researchers and clinicians need to bear in mind. The aim of this opinion paper is to highlight the pitfalls inevitably faced when NMF is quantified. The first problem might arise from the definition of fatigue itself and the parameter(s) used to measure it; for instance, measuring power vs. isometric torque may lead to different conclusions. Another potential limitation is the delay between exercise termination and the evaluation of neuromuscular function; the possible underestimation of exercise-induced neural and contractile impairment and misinterpretation of fatigue etiology will be discussed, as well as solutions recently proposed to overcome this problem. Quantification of NMF can also be biased (or not feasible) because of the techniques themselves (e.g. results may depend on stimulation intensity for transcranial magnetic stimulation) or the way data are analyzed (e.g. M wave peak-to-peak vs first phase amplitude). When available, alternatives recently suggested in the literature to overcome these pitfalls are considered and recommendations about the best practices to assess NMF (e.g. paying attention to the delay between exercise and testing, adapting the method to the characteristics of the population to be tested and considering the limitations associated with the techniques) are proposed.

Muscle Fatigue Post-stroke Elicited From Kilohertz-Frequency Subthreshold Nerve Stimulation

Purpose: Rapid muscle fatigue limits clinical applications of functional electrical stimulation (FES) for individuals with motor impairments. This study aimed to characterize the sustainability of muscle force elicited with a novel transcutaneous nerve stimulation technique.

Method: A hemiplegic chronic stroke survivor was recruited in this case study. Clustered subthreshold pulses of 60-μs with kilohertz (12.5 kHz) carrier frequency (high-frequency mode, HF) were delivered transcutaneously to the proximal segment of the median/ulnar nerve bundles to evaluate the finger flexor muscle fatigue on both sides of the stroke survivor. Conventional nerve stimulation technique with 600-μs pulses at 30 Hz (low-frequency mode, LF) served as the control condition. Fatigue was evoked by intermittently delivering 3-s stimulation trains with 1-s resting. For fair comparison, initial contraction forces (approximately 30% of the maximal voluntary contraction) were matched between the HF and LF modes. Muscle fatigue was evaluated through elicited finger flexion forces (amplitude and fluctuation) and muscle activation patterns quantified by high-density electromyography (EMG).

Result: Compared with those from the LF stimuli, the elicited forces declined more slowly, and the force plateau was higher under the HF stimulation for both the affected and contralateral sides, resulting in a more sustainable force output at higher levels. Meanwhile, the force fluctuation under the HF stimulation increased more slowly, and, thus, was smaller after successive stimulation trains compared with the LF stimuli, indicating a less synchronized activation of muscle fibers. The efficiency of the muscle activation, measured as the force-EMG ratio, was also higher in the HF stimulation mode.

Conclusion: Our results indicated that the HF nerve stimulation technique can reduce muscle fatigue in stroke survivors by maintaining a higher efficiency of muscle activations compared with the LF stimulation. The technique can help improve the performance of neurorehabilitation methods based on electrical stimulation, and facilitate the utility of FES in clinical populations.

Neural Network-Based Muscle Torque Estimation Using Mechanomyography During Electrically-Evoked Knee Extension and Standing in Spinal Cord Injury

This study sought to design and deploy a torque monitoring system using an artificial neural network (ANN) with mechanomyography (MMG) for situations where muscle torque cannot be independently quantified. The MMG signals from the quadriceps were used to derive knee torque during prolonged functional electrical stimulation (FES)-assisted isometric knee extensions and during standing in spinal cord injured (SCI) individuals. Three individuals with motor-complete SCI performed FES-evoked isometric quadriceps contractions on a Biodex dynamometer at 30° knee angle and at a fixed stimulation current, until the torque had declined to a minimum required for ANN model development. Two ANN models were developed based on different inputs; Root mean square (RMS) MMG and RMS-Zero crossing (ZC) which were derived from MMG. The performance of the ANN was evaluated by comparing model predicted torque against the actual torque derived from the dynamometer. MMG data from 5 other individuals with SCI who performed FES-evoked standing to fatigue-failure were used to validate the RMS and RMS-ZC ANN models. RMS and RMS-ZC of the MMG obtained from the FES standing experiments were then provided as inputs to the developed ANN models to calculate the predicted torque during the FES-evoked standing. The average correlation between the knee extension-predicted torque and the actual torque outputs were 0.87 ± 0.11 for RMS and 0.84 ± 0.13 for RMS-ZC. The average accuracy was 79 ± 14% for RMS and 86 ± 11% for RMS-ZC. The two models revealed significant trends in torque decrease, both suggesting a critical point around 50% torque drop where there were significant changes observed in RMS and RMS-ZC patterns. Based on these findings, both RMS and RMS-ZC ANN models performed similarly well in predicting FES-evoked knee extension torques in this population. However, interference was observed in the RMS-ZC values at a time around knee buckling. The developed ANN models could be used to estimate muscle torque in real-time, thereby providing safer automated FES control of standing in persons with motor-complete SCI.

Utilizing Physiological Principles of Motor Unit Recruitment to Reduce Fatigability of Electrically-Evoked Contractions: A Narrative Review

Neuromuscular electrical stimulation (NMES) is used to produce contractions to restore movement and reduce secondary complications for individuals experiencing motor impairment. NMES is conventionally delivered through a single pair of electrodes over a muscle belly or nerve trunk using short pulse durations and frequencies between 20 and 40Hz (conventional NMES). Unfortunately, the benefits and widespread use of conventional NMES are limited by contraction fatigability, which is in large part because of the nonphysiological way that contractions are generated. This review provides a summary of approaches designed to reduce fatigability during NMES, by using physiological principles that help minimize fatigability of voluntary contractions. First, relevant principles of the recruitment and discharge of motor units (MUs) inherent to voluntary contractions and conventional NMES are introduced, and the main mechanisms of fatigability for each contraction type are briefly discussed. A variety of NMES approaches are then described that were designed to reduce fatigability by generating contractions that more closely mimic voluntary contractions. These approaches include altering stimulation parameters, to recruit MUs in their physiological order, and stimulating through multiple electrodes, to reduce MU discharge rates. Although each approach has unique advantages and disadvantages, approaches that minimize MU discharge rates hold the most promise for imminent translation into rehabilitation practice. The way that NMES is currently delivered limits its utility as a rehabilitative tool. Reducing fatigability by delivering NMES in ways that better mimic voluntary contractions holds promise for optimizing the benefits and widespread use of NMES-based programs.

Interleaved neuromuscular electrical stimulation reduces muscle fatigue

INTRODUCTION

Neuromuscular electrical stimulation (NMES) can be delivered over a muscle belly (mNMES) or nerve trunk (nNMES). Both methods generate contractions that fatigue rapidly due, in part, to non-physiologically high motor unit (MU) discharge frequencies. In this study we introduce interleaved NMES (iNMES), whereby stimulus pulses are alternated between mNMES and nNMES. iNMES was developed to recruit different MU populations with every other stimulus pulse, with a goal of reducing discharge frequencies and muscle fatigue.

METHODS

Torque and electromyography were recorded during fatigue protocols (12 min, 240 contractions) delivered using mNMES, nNMES, and iNMES.

RESULTS

Torque declined significantly 3 min into iNMES and 1 min into both mNMES and nNMES. Torque decreased by 39% during iNMES and by 67% and 58% during mNMES and nNMES, respectively.

CONCLUSIONS

iNMES resulted in less muscle fatigue than mNMES and nNMES. Delivering NMES in ways that reduce MU discharge frequencies holds promise for reducing muscle fatigue during NMES-based rehabilitation. Muscle Nerve, 2016 Muscle Nerve 55: 179-189, 2017.

Muscle Fatigue Affects the Interpolated Twitch Technique When Assessed Using Electrically-Induced Contractions in Human and Rat Muscles

The interpolated twitch technique (ITT) is the gold standard to assess voluntary activation and central fatigue. Yet, its validity has been questioned. Here we studied how peripheral fatigue can affect the ITT. Repeated contractions at submaximal frequencies were produced by supramaximal electrical stimulations of the human adductor pollicis muscle in vivo and of isolated rat soleus fiber bundles; an extra stimulation pulse was given during contractions to induce a superimposed twitch. Human muscles fatigued by repeated 30-Hz stimulation trains (3 s on–1 s off) showed an ~80% reduction in the superimposed twitch force accompanied by a severely reduced EMG response (M-wave amplitude), which implies action potential failure. Subsequent experiments combined a less intense stimulation protocol (1.5 s on–3 s off) with ischemia to cause muscle fatigue, but which preserved M-wave amplitude. However, the superimposed twitch force still decreased markedly more than the potentiated twitch force; with ITT this would reflect increased “voluntary activation.” In contrast, the superimposed twitch force was relatively spared when a similar protocol was performed in rat soleus bundles. Force relaxation was slowed by >150% in fatigued human muscles, whereas it was unchanged in rat soleus bundles. Accordingly, results similar to those in the human muscle were obtained when relaxation was slowed by cooling the rat soleus muscles. In conclusion, our data demonstrate that muscle fatigue can confound the quantification of central fatigue using the ITT.

Differences in the recruitment curves obtained with monopolar and bipolar electrode configurations in the quadriceps femoris

INTRODUCTION

We sought to verify whether the stimulation intensity at which M-wave amplitude reaches a plateau actually corresponds to full motor unit activation in monopolar and bipolar configurations.

METHODS

M-waves and twitches were evoked using femoral nerve stimulation of gradually increasing intensity in 21 subjects. Recruitment curves corresponding to the amplitude of the first phase (AmpliFIRST ) and peak-to-peak amplitude (AmpliPP ) of the M-wave were obtained in the vastus lateralis, vastus medialis, and rectus femoris in monopolar and bipolar configurations.

RESULTS

In all muscles, bipolar M-waves and twitches reached plateau at a significantly lower stimulus intensity compared with monopolar M-waves (P < 0.05). The different behavior of monopolar and bipolar M-waves with stimulus intensity was found for both AmpliFIRST and AmpliPP .

CONCLUSIONS

In a bipolar configuration, the stimulus intensity at which M-waves plateau should be increased by at least 10%-20% to achieve complete motor unit recruitment. Muscle Nerve 54: 118-131, 2016.

Assessment of Neuromuscular Function Using Percutaneous Electrical Nerve Stimulation

Percutaneous electrical nerve stimulation is a non-invasive method commonly used to evaluate neuromuscular function from brain to muscle (supra-spinal, spinal and peripheral levels). The present protocol describes how this method can be used to stimulate the posterior tibial nerve that activates plantar flexor muscles. Percutaneous electrical nerve stimulation consists of inducing an electrical stimulus to a motor nerve to evoke a muscular response. Direct (M-wave) and/or indirect (H-reflex) electrophysiological responses can be recorded at rest using surface electromyography. Mechanical (twitch torque) responses can be quantified with a force/torque ergometer. M-wave and twitch torque reflect neuromuscular transmission and excitation-contraction coupling, whereas H-reflex provides an index of spinal excitability. EMG activity and mechanical (superimposed twitch) responses can also be recorded during maximal voluntary contractions to evaluate voluntary activation level. Percutaneous nerve stimulation provides an assessment of neuromuscular function in humans, and is highly beneficial especially for studies evaluating neuromuscular plasticity following acute (fatigue) or chronic (training/detraining) exercise.

Non-uniform recruitment along human rectus femoris muscle during transcutaneous electrical nerve stimulation

PURPOSE

To test the hypothesis that motor units with different axonal excitability levels are localized in specific portions of the rectus femoris (RF) muscle using transcutaneous electrical nerve stimulation.

METHODS

M-waves were elicited by transcutaneous electrical nerve stimulation and detected from 24 sites along longitudinal line of the muscle. The stimulation was applied to the femoral nerve, and the current level was gradually increased.

RESULTS

The central locus activation, which is calculated from the spatial distribution of M-waves, appeared at the proximal regions at low stimulation level and then moved to the middle site of the muscle with an increase in the stimulation level. The results reveal that groups of motor units activated at different stimulation levels are located in different positions in the proximal-distal muscle direction.

CONCLUSION

Our results suggest that motor unit properties in proximal and other regions are not uniform within the RF muscle.

Torque decrease during submaximal evoked contractions of the quadriceps muscle is linked not only to muscle fatigue

The aim of this study was to analyze the neuromuscular mechanisms involved in the torque decrease induced by submaximal electromyostimulation (EMS) of the quadriceps muscle. It was hypothesized that torque decrease after EMS would reflect the fatigability of the activated motor units (MUs), but also a reduction in the number of MUs recruited as a result of changes in axonal excitability threshold. Two experiments were performed on 20 men to analyze 1) the supramaximal twitch superimposed and evoked at rest during EMS (Experiment 1, n = 9) and 2) the twitch response and torque-frequency relation of the MUs activated by EMS (Experiment 2, n = 11). Torque loss was assessed by 15 EMS-evoked contractions (50 Hz; 6 s on/6 s off), elicited at a constant intensity that evoked 20% of the maximal voluntary contraction (MVC) torque. The same stimulation intensity delivered over the muscles was used to induce the torque-frequency relation and the single electrical pulse evoked after each EMS contraction (Experiment 2). In Experiment 1, supramaximal twitch was induced by femoral nerve stimulation. Torque decreased by ~60% during EMS-evoked contractions and by only ~18% during MVCs. This was accompanied by a rightward shift of the torque-frequency relation of MUs activated and an increase of the ratio between the superimposed and posttetanic maximal twitch evoked during EMS contraction. These findings suggest that the torque decrease observed during submaximal EMS-evoked contractions involved muscular mechanisms but also a reduction in the number of MUs recruited due to changes in axonal excitability.

Mechanomyography and muscle function assessment: a review of current state and prospects

Previous studies have explored to saturation the efficacy of the conventional signal (such as electromyogram) for muscle function assessment and found its clinical impact limited. Increasing demand for reliable muscle function assessment modalities continues to prompt further investigation into other complementary alternatives. Application of mechanomyographic signal to quantify muscle performance has been proposed due to its inherent mechanical nature and ability to assess muscle function non-invasively while preserving muscular neurophysiologic information. Mechanomyogram is gaining accelerated applications in evaluating the properties of muscle under voluntary and evoked muscle contraction with prospects in clinical practices. As a complementary modality and the mechanical counterpart to electromyogram; mechanomyogram has gained significant acceptance in analysis of isometric and dynamic muscle actions. Substantial studies have also documented the effectiveness of mechanomyographic signal to assess muscle performance but none involved comprehensive appraisal of the state of the art applications with highlights on the future prospect and potential integration into the clinical practices. Motivated by the dearth of such critical review, we assessed the literature to investigate its principle of acquisition, current applications, challenges and future directions. Based on our findings, the importance of rigorous scientific and clinical validation of the signal is highlighted. It is also evident that as a robust complement to electromyogram, mechanomyographic signal may possess unprecedented potentials and further investigation will be enlightening.