Mechanomyogram and electromyogram responses of upper limb during sustained isometric fatigue with varying shoulder and elbow postures

Variability of Time- and Frequency-Domain Surface Electromyographic Measures in Non-Fatigued Shoulder Muscles

OCCUPATIONAL APPLICATIONSLocalized Muscle Fatigue (LMF) can be monitored or predicted based on the relative change in the values of surface electromyography (sEMG) measures with respect to the "fresh" or no-fatigue condition. Quantification of LMF based on relative change, though, relies on the assumption that the sEMG measures recorded in a no-fatigue condition can serve as an appropriate reference. Results of this study indicate that sEMG measures in a no-fatigue condition are affected by various work-related factors and provide further guidance on the variability of commonly used time- and frequency-domain sEMG measures to assist the ergonomist in improving the accuracy of LMF assessment.

Changes of agonist and synergist muscles activity during a sustained submaximal brake-pulling gesture

We analyzed the time course of changes in muscle activity of the prime mover and synergist muscles during a sustained brake-pulling action and investigated the relationship between muscle activity and braking force fluctuation (FF). Thirty-two participants performed a continuous fatiguing protocol (CFP) at 30% of maximal voluntary contraction (MVC) until failure. Surface electromyography was used to analyze root mean square (RMS) values in the flexor digitorum superficialis (FD), flexor carpi radialis (FC), extensor digitorum communis (ED), extensor carpi radialis (EC), brachioradialis (BR), biceps brachii (BB), and triceps brachii (TB). The FF and RMS in all muscles increased progressively (P<0.01) during the CFP, with sharp increments at time limit particularly in FD and FC (P<0.001). The RMS of the FD and FC were comparable to the baseline MVC values at time limit, in comparison to the other muscles that did not reach such levels of activity (P<0.003). The three flexor/extensor ratios used to measure coactivation levels decreased significantly (P<0.001). In contrast to RMS, MVC was still depressed at the minute 10 of recovery. The results suggest that the time limit was mainly constrained by fatigue-related mechanisms of the FD and FC but not by those of other synergist and antagonist muscles.

Electrode Size and Placement for Surface EMG Bipolar Detection from the Brachioradialis Muscle: A Scoping Review

The brachioradialis muscle (BRD) is one of the main elbow flexors and is often assessed by surface electromyography (sEMG) in physiology, clinical, sports, ergonomics, and bioengineering applications. The reliability of the sEMG measurement strongly relies on the characteristics of the detection system used, because of possible crosstalk from the surrounding forearm muscles. We conducted a scoping review of the main databases to explore available guidelines of electrode placement on BRD and to map the electrode configurations used and authors’ awareness on the issues of crosstalk. One hundred and thirty-four studies were included in the review. The crosstalk was mentioned in 29 studies, although two studies only were specifically designed to assess it. One hundred and six studies (79%) did not even address the issue by generically placing the sensors above BRD, usually choosing large disposable ECG electrodes. The analysis of the literature highlights a general lack of awareness on the issues of crosstalk and the need for adequate training in the sEMG field. Three guidelines were found, whose recommendations have been compared and summarized to promote reliability in further studies. In particular, it is crucial to use miniaturized electrodes placed on a specific area over the muscle, especially when BRD activity is recorded for clinical applications.

Recovery and Fatigue Behavior of Forearm Muscles during a Repetitive Power Grip Gesture in Racing Motorcycle Riders

Despite a reduction in the maximal voluntary isometric contraction (MVC_isom) observed systematically in intermittent fatigue protocols (IFP), decrements of the median frequency, assessed by surface electromyography (sEMG), has not been consistently verified. This study aimed to determine whether recovery periods of 60 s were too long to induce a reduction in the normalized median frequency (MF_EMG) of the flexor digitorum superficialis and carpi radialis muscles. Twenty-one road racing motorcycle riders performed an IFP that simulated the posture and braking gesture on a motorcycle. The MVC_isom was reduced by 53% (p < 0.001). A positive and significant relationship (p < 0.005) was found between MF_EMG and duration of the fatiguing task when 5 s contractions at 30% MVC_isom were interspersed by 5 s recovery in both muscles. In contrast, no relationship was found (p > 0.133) when 10 s contractions at 50% MVC were interspersed by 1 min recovery. Comparative analysis of variance (ANOVA) confirmed a decrement of MF_EMG in the IFP at 30% MVC_isom including short recovery periods with a duty cycle of 100% (5 s/5 s = 1), whereas no differences were observed in the IFP at 50% MVC_isom and longer recovery periods, with a duty cycle of 16%. These findings show that recovery periods during IFP are more relevant than the intensity of MVC_isom. Thus, we recommend the use of short recovery periods between 5 and 10 s after submaximal muscle contractions for specific forearm muscle training and testing purposes in motorcycle riders.

Forearm muscles fatigue induced by repetitive braking on a motorcycle is best discriminated by specific kinetic parameters

Maneuvering a motorcycle in racing conditions or for prolonged time is sufficiently demanding that on many occasions forearm muscles reach a state of functional failure when riders cannot properly brake or operate the throttle. This study intends to discriminate which ones of the several dynamometric parameters used in the literature to characterize the Force-time (F-t) curve during voluntary contractions are more sensitive to neuromuscular fatigue in simulated motorcycle-riding conditions. Thirty-three adults performed an intermittent fatiguing protocol (IFP) that simulated the brake-pulling and throttle-twisting actions, by using a hydraulic system equipped with a pressure sensor. Sixty pressure-time (P-t) curve parameters, including the rate of pressure development (RPD) and area under the curve were measured to characterize the time course of the braking maximal voluntary contraction (MVC). Two types of variables were used to analyze the P-t curve: 1) Times interval (from 0 to 30-50-100-500-1000 and 2000 ms); 2) Percentages of MVC (10-30-60-90%MVC). Overall significant (p ≤ 0.05) fatigue-related declines were observed only at time intervals longer than 100 ms and contraction intensities higher than 30%MVC. Strong and significant linear declines (p < 0.001) were observed at 500 ms and 1 s for normalized pressures, as well as for the ratio RPD_60%MVC/MVC (p < 0.003) throughout the IFP. Our results suggest considering RPD at time windows of 0–500 ms and 0–1 s, and contraction intensities comprised between 30% and 60% of MVC, as more suitable criteria to study fatigue-related decrements in performance rather than the classical MVC force.

Functional connectivity between core and shoulder muscles increases during isometric endurance contractions in judo competitors

PURPOSE

The aim of this study was to assess the surface electromyogram (SEMG) changes within and between muscles of the torso and shoulder region during static endurance contraction in elite judokas. We hypothesized an increased functional connectivity of muscles from the shoulder and torso regions during sustained isometric contraction.

METHODS

Twelve healthy, right-handed judo competitors participated in the study. The SEMG signals from the dominant trapezius (upper, middle and lower part), deltoideus anterior, serratus anterior, and pectoralis major muscles were recorded during isometric endurance contraction consisting of bilateral arm abduction at 90°. The normalized mutual information (NMI) was computed between muscle pairs as an index indicating functional connectivity.

RESULTS

The NMIs increased significantly during endurance test for 10 of the 15 muscle pairs (P < 0.001).

CONCLUSION

We concluded that the increases in NMIs highlighted functional changes in the interplay between core and shoulder muscles during an endurance contraction in elite judokas.

Force-time course parameters and force fatigue model during an intermittent fatigue protocol in motorcycle race riders

Fatigue in forearm muscles may be critical for motorcycle riders in relation to performance and forearm disorders. Force-time course parameters were examined to better characterize the reduction in the maximal force generating capacity (MVC) during an intermittent fatigue protocol (IFP) specifically designed for motorcycle riders. Also, a mathematical force fatigue model is proposed. Forty motorcyclists (aged 27.6 ± 6.8 years) performed an IFP that simulated the braking gesture and posture of a rider. Fatigue was confirmed by a 40% decrement of the normalized MVC in comparison with basal value. Contraction time increased in comparison with basal condition (P ≤ 0.034). Relaxation kinetics presented two phases: (a) a pre-fatigue phase where half relaxation time (HRTraw ) and normalized (HRTnor ) decreased (P ≤ 0.013) while relaxation rate (RRraw ) remained unchanged; and (b) a fatiguing phase where HRTraw , HRTnor increased and RRraw decreased (P ≤ 0.047). Normalized RRraw (RRnor ) declined progressively (P ≤ 0.016). The proposed nonlinear force fatigue model confirmed a satisfactory adjustment (R(2)  = 0.977 ± 0.018). This mathematical expression derived three patterns of force fatigue: three-phase, exponential and linear, representing 70%, 13%, and 17% of the participants, respectively. Overall, these results provided further support to force fatigue theoretical and applied proposals.

Torque and mechanomyogram correlations during muscle relaxation: effects of fatigue and time-course of recovery

To assess the validity and reliability of the mechanomyogram (MMG) as a tool to investigate the fatigue-induced changes in the muscle during relaxation, the torque and MMG signals from the gastrocnemius medialis muscle of 23 participants were recorded during tetanic electrically-elicited contractions before and immediately after fatigue, as well as at min 2 and 7 of recovery. The peak torque (pT), contraction time (CT) and relaxation time (RT), and the acceleration of force development (d2RFD) and relaxation (d2RFR) were calculated. The slope and τ of force relaxation were also determined. MMG peak-to-peak was assessed during contraction (MMG p-p) and relaxation (R-MMG p-p). After fatigue, pT, d2RFD, d2RFR, slope, MMG p-p and R-MMG p-p decreased significantly, while CT, RT and τ increased (P < 0.05 for all comparisons), remaining altered throughout the entire recovery period. R-MMG p-p correlated with pT, MMG p-p, slope, τ and d2RFR both before and after fatigue. Reliability measurements always ranged from high to very high. In conclusion, MMG may represent a valid and reliable index to monitor the fatigue-induced changes in muscle mechanical behavior, and could be therefore considered an effective alternative to the force signal, also during relaxation.

Evaluation of the effectiveness of muscle assistive device using muscle fatigue analysis

There have been many kinds of wearable robots or wearable assistive devices to reduce the burden of workers in several industries. But there is no quantitative and objective method to evaluate the effectiveness of the device. In this study, a new method to evaluate the effectiveness of the muscle assistive device is suggested.

Comparison of an intermittent and continuous forearm muscles fatigue protocol with motorcycle riders and control group

Motorcycle races' long duration justify the study of forearm muscles fatigue, especially knowing the frequently associated forearm discomfort pathology. Moreover, while continuous fatigue protocols yield unequivocal results, EMG outcomes from an intermittent protocol are quite controversial. This study examined the forearm muscle fatigue patterns produced during these two protocols, comparing riders with a control group, and relating maximal voluntary contraction with EMG parameters (amplitude - NRMS and median frequency - NMF) of both protocols to the forearm discomfort among motorcycle riders. Twenty riders and 39 controls performed in separate days both protocols simulating the braking gesture and posture of a rider. EMG of flexor digitorum superficialis (FS) and carpi radialis (CR) were monitored. CR revealed more differences among protocols and groups compared to FS. The greater CR activation in riders could be interpreted as a neuromotor strategy to improve braking precision. When FS fatigue increased, the control group progressively shift toward a bigger CR activation, adopting an intermuscular activation pattern closer to riders. Despite the absence of NMF decrement throughout the intermittent protocol, which suggest that we should have shorten the recovery times from the actual 1 min, the superior number of rounds performed by the riders proved that this protocol discriminates better riders against controls and is more related to forearm discomfort.

Monitoring hand flexor fatigue in a 24-h motorcycle endurance race

Motorcycle riders must endure high levels of muscle tension for long periods of time, especially in their arms and forearms, when steering and using handlebar controls. Because the right hand operates the gas handle and front brakes, the present research focuses on fatigue in the right hand flexors. Ten adult riders, aged 32.5±5.5years, volunteered to participate in this study. During the 24h race each rider, on completion of a relay stage, visited the assessment box to do the following handgrip test sequence: (1) 10s of EMG recording at rest, (2) one 3-s maximal voluntary contraction (MVC), (3) 1min rest interval and (4) 50% MVC maintained during 10s. EMG amplitude (MP: μV) and median and mean frequency (MF and MPF: Hz) over the superficial finger flexors were recorded during the whole handgrip test sequence with adhesive surface electrodes. MVC values were maintained during the first two relays (50-60min duration in total) and dropped gradually thereafter (p<0.01). During the monitoring of the 50% MVC, mean amplitude increased (p=0.024) while median and mean frequency tended to decrease. These results suggest fatigue is produced in motorcycle riders in a 24h race. However, the expected reduction of EMG frequency was not confirmed given to a potentially large variability.

On functional motor adaptations: from the quantification of motor strategies to the prevention of musculoskeletal disorders in the neck-shoulder region

BACKGROUND

Occupations characterized by a static low load and by repetitive actions show a high prevalence of work-related musculoskeletal disorders (WMSD) in the neck-shoulder region. Moreover, muscle fatigue and discomfort are reported to play a relevant initiating role in WMSD.

AIMS

To investigate relationships between altered sensory information, i.e. localized muscle fatigue, discomfort and pain and their associations to changes in motor control patterns.

MATERIALS & METHODS

In total 101 subjects participated. Questionnaires, subjective assessments of perceived exertion and pain intensity as well as surface electromyography (SEMG), mechanomyography (MMG), force and kinematics recordings were performed.

RESULTS

Multi-channel SEMG and MMG revealed that the degree of heterogeneity of the trapezius muscle activation increased with fatigue. Further, the spatial organization of trapezius muscle activity changed in a dynamic manner during sustained contraction with acute experimental pain. A graduation of the motor changes in relation to the pain stage (acute, subchronic and chronic) and work experience were also found. The duration of the work task was shorter in presence of acute and chronic pain. Acute pain resulted in decreased activity of the painful muscle while in subchronic and chronic pain, a more static muscle activation was found. Posture and movement changed in the presence of neck-shoulder pain. Larger and smaller sizes of arm and trunk movement variability were respectively found in acute pain and subchronic/chronic pain. The size and structure of kinematics variability decreased also in the region of discomfort. Motor variability was higher in workers with high experience. Moreover, the pattern of activation of the upper trapezius muscle changed when receiving SEMG/MMG biofeedback during computer work.

DISCUSSION

SEMG and MMG changes underlie functional mechanisms for the maintenance of force during fatiguing contraction and acute pain that may lead to the widespread pain seen in WMSD. A lack of harmonious muscle recruitment/derecruitment may play a role in pain transition. Motor behavior changed in shoulder pain conditions underlining that motor variability may play a role in the WMSD development as corroborated by the changes in kinematics variability seen with discomfort. This prognostic hypothesis was further, supported by the increased motor variability among workers with high experience.

CONCLUSION

Quantitative assessments of the functional motor adaptations can be a way to benchmark the pain status and help to indentify signs indicating WMSD development. Motor variability is an important characteristic in ergonomic situations. Future studies will investigate the potential benefit of inducing motor variability in occupational settings.

A mechanomyographic frequency-based fatigue threshold test

Theoretically, the mechanomyographic (MMG) mean power frequency fatigue threshold (MMG MPF(FT)) describes the maximal isometric torque that can be maintained for an extended period of time with no change in the global firing rate of the unfused, activated motor units.

PURPOSE

The purposes of this study were twofold: (1) to determine if the mathematical model for estimating the electromyographic (EMG) MPF(FT) from the frequency of the EMG signal was applicable to the frequency domain of the MMG signal to estimate a new fatigue threshold called the MMG MPF(FT); and (2) to compare the mean torque levels derived from the MMG MPF(FT) test for the vastus lateralis (VL), vastus medialis (VM), and rectus femoris (RF) muscles during isometric leg extension muscle actions.

METHODS

Nine adults (4 men and 5 women; mean+/-S.D. age=21.6+/-1.2 years) performed three or four continuous, fatiguing, isometric muscle actions of the leg extensors at 30, 45, 60, and 75% of maximum voluntary isometric contraction (MVIC) to exhaustion. Surface MMG signals were recorded from the VL, VM, and RF muscles during each fatiguing isometric muscle action. The MMG MPF(FT) was defined as the y-intercept of the isometric torque versus slope coefficient (MMG MPF versus time) plot.

RESULTS

There were no significant differences among the MMG MPF(FT) values for the VL, VM, and RF (34.8+/-23.4, 32.1+/-16.1, and 31.6+/-15.2 Nm, respectively) muscles.

CONCLUSION

The MMG MPF(FT) test may provide a non-invasive method to examine the effects of various interventions on the global motor unit firing rate during isometric muscle actions.

Similar response of agonist and antagonist muscles after eccentric exercise revealed by electromyography and mechanomyography

The purpose of this study was to investigate the influence of eccentric contractions (ECC) on the biceps (BB) and triceps brachii (TB) muscles during maximal voluntary contraction (MVC) of elbow flexors using electrical (EMG) and mechanomyographical activities (MMG). Each of 18 male students performed 25 submaximal contractions (50% MVC) of the elbow flexors. Root mean square amplitude (RMS) and median frequency (MDF) were calculated for the EMG and MMG signals recorded during MVC. All measurements were taken before, immediately after, 24, 48, 72, and 120 h post-ECC from the BB and TB muscles. MVC was reduced by 34% immediately after exercise and did not return to the resting value within 120 h (P0.05). The EMG MDF decreased significantly (P< or =0.05) in both muscles after ECC. The MMG RMS at 24h, 48, 72 and 120 h post-ECC was significantly lower compared to that recorded immediately after ECC in both muscles (P< or =0.05). The present research showed that (i) there were similar changes in electrical and mechanical activities during MVC after submaximal ECC in agonist and antagonist muscles suggesting a common drive controlling the agonist and antagonist motoneuron pool, (ii) the ECC induced different changes in EMG than in MMG immediately after ECC and during 120 h of recovery that suggested an increased tremor and contractile impairments, i.e., reduced rate of calcium release from the sarcoplasmic reticulum (acute effect), and changes in motor control mechanisms of agonist and antagonist muscles, and increased muscle stiffness (chronic effect).

Does the frequency content of the surface mechanomyographic signal reflect motor unit firing rates? A brief review

The purpose of this review is to examine the literature that has investigated the potential relationship between mechanomyographic (MMG) frequency and motor unit firing rates. Several different experimental designs/methodologies have been used to address this issue, including: repetitive electrical stimulation, voluntary muscle actions in muscles with different fiber type compositions, fatiguing and non-fatiguing isometric or dynamic muscle actions, and voluntary muscle actions in young versus elderly subjects and healthy individuals versus subjects with a neuromuscular disease(s). Generally speaking, the results from these investigations have suggested that MMG frequency is related to the rate of motor unit activation and the contractile properties (contraction and relaxation times) of the muscle fibers. Other studies, however, have reported that MMG mean power frequency (MPF) does not always follow the expected pattern of firing rate modulation (e.g. motor unit firing rates generally increase with torque during isometric muscle actions, but MMG MPF may remain stable or even decrease). In addition, there are several factors that may affect the frequency content of the MMG signal during a voluntary muscle action (i.e. muscle stiffness, intramuscular fluid pressure, etc.), independent of changes in motor unit firing rates. Despite the potential influences of these factors, most of the evidence has suggested that the frequency domain of the MMG signal contains some information regarding motor unit firing rates. It is likely, however, that this information is qualitative, rather than quantitative in nature, and reflects the global motor unit firing rate, rather than the firing rates of a particular group of motor units.

EMG amplitude distribution changes over the upper trapezius muscle are similar in sustained and ramp contractions

AIM

To investigate whether global motor unit recruitment to compensate for muscle fatigue during sustained contraction and to regulate force increase during ramp contraction are controlled in similar manners in the upper trapezius muscle.

METHODS

Fourteen subjects performed a 10-s ramp contraction from 0% to 90% of maximal voluntary contraction (MVC) and a 3-min sustained contraction at 25% MVC. Both contractions involved isometric shoulder elevation with a multi-channel surface electromyographical grid placed on the skin above the muscle. To evaluate the global muscle activation pattern, the changes in spatial amplitude distribution of the sustained and the ramp contraction were examined and compared.

RESULTS

In both contraction types, the upper part of the trapezius muscle was spatially non-uniform (inhomogeneous) activated. Throughout the sustained contraction, the amplitude distribution of the upper trapezius muscle became more similar to the amplitude distribution at higher force levels.

CONCLUSION

These findings support the hypothesis that global motor unit recruitment to compensate for muscle fatigue during a sustained contraction, and to regulate force increase during a ramp contraction is controlled in a similar manner. Consequently, they confirm fundamental principles of motor unit activation based on recordings of limited motor unit samples.

Estimation of the muscle fibre semi-length under varying joint positions on the basis of non-invasively extracted motor unit action potentials

Changes in muscle fibre length and surface electrode position with respect to the muscle fibres affect the amplitude and frequency characteristics of surface electromyography (SEMG) in different ways. Knowledge of changes in muscle fibre length would help towards a better interpretation of the signals. The possibility of estimating the length through SEMG during voluntary contractions was checked in this study. The fibres' semi-length was estimated from the product of the conduction velocity and conduction time during which the wave of excitation propagated from the end-plate region to the ends of the fibres. Short (10 s), moderate (30% of maximum voluntary contraction) isometric contractions were performed by 10 subjects at different elbow joint angles (80-140 degrees in steps of 20 degrees ). Monopolar signals were detected non-invasively, using a two-dimensional electrode array. High spatial resolution EMG and a decomposition technique were utilised to extract single motor unit activities for triggered averaging and to estimate conduction velocity. A significant increase with joint angle was found in conduction time and estimated fibre semi-length. Changes in conduction velocity with joint angle were found to be not significant. The methodology described allows the relative changes in fibres' semi-length to be estimated from SEMG data.

Muscular performance modeling of the upper limb in static postures

The purpose of the present paper is to describe and evaluate the polynomial models for predicting the muscular work capacity of the upper limb during sustained holding tasks. This research was concerned with the relationship between indicators of performance, i.e., specific posture or specific level of maximum voluntary contraction (MVC), and then modeling the functional data based on experimental results to estimate factors that may have an effect on task performance. To this end, we designed an experiment using 10 subjects in which each subject performed sustained isometric shoulder and elbow flexion endurance exercise under 27 conditions [3 shoulder angles (SA)x3 elbow angles (EA)x3 levels of %MVC]. Throughout all experiments, subjective perception of effort was assessed using the Borg scale, every 60, 30, and 10 s during the 20%, 40%, and 60% MVC tests, respectively. Proposal models were represented by three approaches: model A: estimation of endurance time (ET), with input variables such as SA, EA, and %MVC; model B: estimation of recommendation time (RT, the time during which the operator was able to maintain a position under the desired condition), with input variables such as SA, EA, %MVC, and required rate on the Borg scale; and model C: estimation of limit strength or %MVC, with input variables such as SA, EA, request limit time for work (LT), and required rate on the Borg scale. Statistical analysis indicated that the three proposal estimation models based on polynomial regression functions showed high significance (p<0.0001). The proposal models suggested and recommended the possibility of finding the best positions entailing the reduction and minimization of total muscular strain from manual material handling tasks in different work situations, with the consequent increase in work efficiency.