Evidence of long term muscle fatigue following prolonged intermittent contractions based on mechano- and electromyograms

A neuromuscular human body model for lumbar injury risk analysis in a vibration loading environment

BACKGROUND AND OBJECTIVE

Long-term intensive exposure to whole-body vibration substantially increases the risk of low back pain and degenerative diseases in special occupational groups, like motor vehicle drivers, military vehicle occupants, aircraft pilots, etc. This study aims to establish and validate a neuromuscular human body model focusing on improvement of the detailed description of anatomic structures and neural reflex control, for lumbar injury analysis in vibration loading environments.

METHODS

A whole-body musculoskeletal in Opensim codes was first improved by including a detailed anatomic description of spinal ligaments, non-linear intervertebral disc, and lumbar facet joints, and coupling a proprioceptive feedback closed-loop control strategy with GTOs and muscle spindles modeling in Python codes. Then, the established neuromuscular model was multi-levelly validated from sub-segments to the whole model, from regular movements to dynamic responses to vibration loadings. Finally, the neuromuscular model was combined with a dynamic model of an armored vehicle to analyze occupant lumbar injury risk in vibration loadings due to different road conditions and traveling velocities.

RESULT

Based on a series of biomechanical indexes, including lumbar joint rotation angles, the lumbar intervertebral pressures, the displacement of the lumbar segments, and the lumbar muscle activities, the validation results show that the present neuromuscular model is available and feasible in predicting lumbar biomechanical responses in normal daily movement and vibration loading environments. Furthermore, the combined analysis with the armored vehicle model predicted similar lumbar injury risk to the experimental or epidemiologic studies. The preliminary analysis results also showed that road types and travelling velocities have substantial combined effects on lumbar muscle activities, and indicated that intervertebral joint pressure and muscle activity indexes can need to be jointly considered for lumbar injury risk evaluation.

CONCLUSION

In conclusion, the established neuromuscular model is an effective tool to evaluate vibration loading effects on injury risk of the human body and assist vehicle design vibration comfort by directly concerning the human body injury itself.

A non-volitional skeletal muscle endurance test measures functional changes associated with impaired blood flow

Introduction: An electrically stimulated intermittent fatigue test using mechanomyography was recently proposed as a possible tool for detecting clinically relevant changes in muscle function. This study was designed to determine whether the proposed test can detect additional fatigue when it should be present. Methods: Subjects (n = 10) underwent two trials each (occluded and normal blood flow) with a standardized fatigue protocol on the Ankle Dorsiflexors (AD) and Wrist Extensors (WE) using a clinical electrical stimulator. Results: Mean normalized twitch acceleration was strongly predictive of mean normalized torque (R ² = 0.828). The WE experienced lower twitch magnitudes throughout the tourniquet trial (10.81 ± 1.25 m/s²) compared to normal blood flow (18.05 ± 1.06 m/s²). The AD twitches were overall reduced in the tourniquet trial (3.87 ± 0.48 m/s²) compared with the control trial (8.57 ± 0.91 m/s²). Conclusion: Occluding blood flow to a muscle should cause greater muscle fatigue. The ability to detect reduced contraction magnitudes during an electrically stimulated fatigue protocol resulting from low blood flow suggests the proposed test may be capable of detecting clinically relevant muscle deficits.

The Influence of Treadmill Training on the Bioelectrical Activity of the Lower Limb Muscles in Patients with Intermittent Claudication

Aim: Intermittent claudication is the most common symptomatic manifestation of peripheral arterial disease (PAD), presenting as ischemic leg muscle pain and gait dysfunction. The aim of this study was to evaluate the changes in bioelectrical activity of the lower limb muscles activity in claudicating patients over a 12-week period of supervised treadmill training and to verify the hypothesis as to which muscles of lower limbs are activated by training treatment—the proximal, as compensatory mechanism, or the distal, which are the most ischemic. Methods: The study comprised 45 patients aged 60–70 years (height 168.8 ± 6.8 cm, weight 78.9 ± 9.2 kg) with PAD and unilateral intermittent claudication (Fontaine stage IIa/IIb), who participated in a 12-week supervised treadmill training program. Surface electromyography (sEMG) of the gastrocnemius lateralis (GaL), gastrocnemius medialis (GaM), tibialis anterior (TA), biceps femoris (BF), rectus femoris (RF) and gluteus medius (GM) muscles in the claudicated leg were continuously measured during the treadmill test. The average mean amplitude and mean amplitude range of the sEMG signal were analyzed. Results: During the treadmill test, after 12 weeks of training, the average mean amplitude of the GM (105 ± 43 vs. 74 ± 38%, p = 0.000008, ES = 0.76), BF (41 ± 22 vs. 33 ± 12%, p = 0.006, ES = 0.45) and GaM (134 ± 50 vs. 114 ± 30%, p = 0.007, ES = 0.48) muscles was significantly lower compared with baseline. The mean amplitude range was significantly decreased after 12 weeks of training in the GM (229 ± 64 vs. 181 ± 62%, p = 0.008, ES = 0.77) and BF (110 ± 69 vs. 84 ± 31%, p = 0.0002, ES = 0.48) muscles. After 12 weeks of training, the mean amplitude range of the TA muscle was significantly higher compared with baseline (104 ± 46 vs. 131 ± 53%, p = 0.001, ES = 0.54), but without significant changes in the average mean amplitude value. The most favorable changes, suggesting the lowest muscle fatigue and the highest walking capacity, were found in patients with the longest walking time. Conclusions: The obtained results may suggest that after 12 weeks of treadmill training, beneficial changes occurred in both the proximal and distal muscles. Therefore, greater foot plantar flexion and stronger push-off as well as greater hip extension may be considered the main mechanisms of observed gait pattern improvement. It may also be suggested that the therapy of gait alterations in patients with PAD should be focused not only on calf muscle pump improvement, but also on proximal hip extensor strengthening.

The Association between Symmetrical or Asymmetrical High-Arched Feet and Muscle Fatigue in Young Women

The foot arches are responsible for proper foot loading, optimal force distribution, and transmission throughout the soft tissues. Since the foot arch is an elastic structure, able to adapt to forces transmitted by the foot, it was reported that low arch is related to excessive foot pronation, while high arched foot is more rigid and inflexible. Therefore, it is also probable, that foot arch alterations may change the force transmission via myofascial chains. The objective of this study was to evaluate the effect of symmetrical and asymmetrical excessive feet arching on muscle fatigue in the distal body parts such as the lower limbs, trunk, and head. Seventy-seven women (25.15 ± 5.97 years old, 62 ± 10 kg, 167 ± 4 cm) were assigned to three groups according to the foot arch index (Group 1—both feet with normal arch, Group 2—one foot with normal arch and the other high-arched, Group 3—both feet with high-arch). The bioelectrical activity of the right and left hamstrings muscles, erector spine, masseter, and temporalis muscle was recorded by sEMG during the isometric contraction lasting for 60 s. The stable intensity of the muscle isometric contraction was kept for all the time during the measurement. Mean frequency difference (%), slope (Hz), and intercept (Hz) values were calculated for muscle fatigue evaluation. No differences were observed in fatigue variables for all evaluated muscles between the right and left side in women with symmetrical foot arches, but in the group with asymmetric foot arches, the higher muscle fatigue on the normal-arched side compared to the high-arched side was noted. Significantly greater values of the semitendinosus—semimembranosus muscle frequency difference was observed on the normal-arched side compared to the high-arched side (p = 0.04; ES = 0.52; −29.5 ± 9.1% vs. −24.9 ± 8.4%). In the group with asymmetric foot arches, a significantly higher value of lumbar erector spinae muscle frequency slope (p = 0.01; ES = 1.32; −0.20 ± 0.04 Hz vs. −0.14 ± 0.05 Hz) and frequency difference (p = 0.04; ES = 0.92; −7.8 ± 3.1% vs. −4.8 ± 3.4%) were observed on the high-arched foot side compared to the side with normal foot arching. The thoracic erector spine muscle frequency slope was significantly larger in women with asymmetrical arches than in those with both feet high-arched (right side: p = 0.01; ES = 1.25; −0.20 ± 0.08 Hz vs. −0.10 ± 0.08 Hz); (left side: p = 0.005; ES = 1,17; −0.19 ± 0.04 Hz vs. −0.13 ± 0.06 Hz) and compared to those with normal feet arches (right side: p = 0.02; ES = 0.58; −0.20 ± 0.08 Hz vs. −0.15 ± 0.09 Hz); (left side: p = 0.005; ES = 0.87; −0.19 ± 0.04 Hz vs. −0.14 ± 0.07 Hz). In the group with asymmetric foot arches, the frequency difference was significantly higher compared to those with both feet high-arched (right side: p = 0.01; ES = 0.87; −15.4 ± 6.8% vs. 10.4 ± 4.3%); (left side: p = 0.01; ES = 0.96; 16.1 ± 6.5% vs. 11.1 ± 3.4%). In the group with asymmetric foot arches, a significantly higher value of the masseter muscle frequency difference was observed on the high-arched side compared to the normal-arched side (p = 0.01; ES = 0.95; 6.91 ± 4.1% vs. 3.62 ± 2.8%). A little increase in the longitudinal arch of the foot, even though such is often not considered as pathological, may cause visible changes in muscle function, demonstrated as elevated signs of muscles fatigue. This study suggests that the consequences of foot high-arching may be present in distal body parts. Any alterations of the foot arch should be considered as a potential foot defect, and due to preventing muscle overloading, some corrective exercises or/and corrective insoles for shoes should be used. It can potentially reduce both foot overload and distant structures overload, which may diminish musculoskeletal system pain and dysfunctions.

An exploratory study comparing three work/rest schedules during simulated repetitive precision work

The pattern of work and rest can influence both physical fatigue and task performance in manual operations. However, there is relatively limited evidence regarding the influences of specific work/rest schedules in tasks requiring high repetitiveness and precision demands, along with relatively low exertion levels. Eighteen participants completed an exploratory study that simulated such tasks, to compare the effects of three distinct work/rest schedules (i.e. short frequent [short] and long infrequent breaks [long], and a self-selected schedule) on muscle fatigue, task performance (in terms of accuracy and speed), and preference. Schedules with long or self-selected breaks generally induced less muscle fatigue, compared with the short break condition. Participants preferred the self-selected condition the most and the long-break condition the least. The different schedules tested did not influence task performance. A self-selected schedule may be beneficial for repetitive precision task, to achieve a balance across muscle fatigue, task performance, and individual preference. Practitioner summary: Influences of three work/rest schedules (i.e. short and long breaks, and a self-selected schedule) on fatigue, performance, and preference were explored during repetitive precision tasks. Schedules with long or self-selected breaks induced less muscle fatigue and none of the three schedules influenced performance. A self-selected schedule was the most preferred.

Development of a Computer Vision-Based Muscle Stimulation Method for Measuring Muscle Fatigue during Prolonged Low-Load Exposure

Measuring muscle fatigue is one essential and standard method to quantify the ergonomic risks associated with prolonged low-load exposure. However, measuring muscle fatigue using EMG-based methods has shown conflicting results under low-load but sustained work conditions, e.g., prolonged sitting. Muscle stimulation technology provides an alternative way to estimate muscle fatigue development during such work conditions by monitoring the stimulation-evoked muscle responses, which, however, could be restricted by the accessibility and measurability of targeted muscles. This study proposes a computer vision-based method to overcome such potential restrictions by visually quantifying the muscle belly displacement caused by muscle stimulation. The results demonstrate the ability of the developed computer vision-based stimulation method to detect muscle fatigue from prolonged low-load tasks. Current results can be used as a foundation to develop a sensitive and reliable method to quantify the adverse effects of the daily low-load sustained condition in occupational and nonoccupational settings.

Effect of warm-up and muscle fatiguing exercise on knee joint sounds in motion by vibroarthrography: A randomized crossover trial

Vibroarthrography measures joint sounds caused by sliding of the joint surfaces over each other. and can be affected by joint health, load and type of movement. Since both warm-up and muscle fatigue lead to local changes in the knee joint (e.g., temperature increase, lubrication of the joint, muscle activation), these may impact knee joint sounds. Therefore, this study investigates the effects of warm-up and muscle fatiguing exercise on knee joint sounds during an activity of daily living. Seventeen healthy, physically active volunteers (25.7 ± 2 years, 7 males) performed a control and an intervention session with a wash-out phase of one week. The control session consisted of sitting on a chair, while the intervention session contained a warm-up (walking on a treadmill) followed by a fatiguing exercise (modified sit-to-stand) protocol. Knee sounds were recorded by vibroarthrography (at the medial tibia plateau and at the patella) at three time points in each session during a sit-to-stand movement. The primary outcome was the mean signal amplitude (MSA, dB). Differences between sessions were determined by repeated measures ANOVA with intra-individual pre-post differences for the warm-up and for the muscle fatigue effect. We found a significant difference for MSA at the medial tibia plateau (intervention: mean 1.51 dB, standard deviation 2.51 dB; control: mean -1.28 dB, SD 2.61 dB; F = 9.5; p = .007; η² = .37) during extension (from sit to stand) after the warm-up. There was no significant difference for any parameter after the muscle fatiguing exercise (p > .05). The increase in MSA may mostly be explained by an increase in internal knee load and joint friction. However, neuromuscular changes may also have played a role. It appears that the muscle fatiguing exercise has no impact on knee joint sounds in young, active, symptom-free participants during sit to stand.

sEMG: A Window Into Muscle Work, but Not Easy to Teach and Delicate to Practice-A Perspective on the Difficult Path to a Clinical Tool

Surface electromyography (sEMG) may not be a simple 1,2,3 (muscle, electrodes, signal)-step operation. Lists of sEMG characteristics and applications have been extensively published. All point out the noise mimicking perniciousness of the sEMG signal. This has resulted in ever more complex manipulations to interpret muscle functioning and sometimes gobbledygook. Hence, as for all delicate but powerful tools, sEMG presents challenges in terms of precision, knowledge, and training. The theory is usually reviewed in courses concerning sensorimotor systems, motor control, biomechanics, ergonomics, etc., but application requires creativity, training, and practice. Software has been developed to navigate the essence extraction (step 4); however, each software requires some parametrization, which returns back to the theory of sEMG and signal processing. Students majoring in Ergonomics or Biomedical Engineering briefly learn about the sEMG method but may not necessarily receive extensive training in the laboratory. Ergonomics applications range from a simple estimation of the muscle load to understanding the sense of effort and sensorimotor asymmetries. In other words, it requires time and the basics of multiple disciplines to acquire the necessary knowledge and skills to perform these studies. As an example, sEMG measurements of left/right limb asymmetries in muscle responses to vibration-induced activity of proprioceptive receptors, which vary with gender, provide insight into the functioning of sensorimotor systems. Beyond its potential clinical benefits, this example also shows that lack of testing time and lack of practitioner's sufficient knowledge are barriers to the utilization of sEMG as a clinical tool.

Effects of Prolonged Sitting with Slumped Posture on Trunk Muscular Fatigue in Adolescents with and without Chronic Lower Back Pain

Background and Objectives: This study investigated the effects of prolonged sitting on trunk muscular fatigue and discomfort in participants with and without chronic lower back pain (LBP). Material and Methods: This study included 15 patients with LBP and 15 healthy controls. All participants were instructed to sit on a height-adjustable chair with their knee and hip joints bent at 90° for 30 min, in slumped sitting postures. Surface electromyography was used to assess the median frequency of the internal obliques (IO)/transversus abdominis (TrA) and multifidus (MF) muscles. Perceived discomfort was measured using a Borg category ratio-scale. Median frequency of the trunk muscles and perceived discomfort after 30 min of sitting were compared with baseline. Result: There were no significant differences within the group and between both groups in the median frequency of bilateral IO and MF muscles. The LBP group showed significantly greater perceived discomfort after prolonged sitting, as compared to the control group. Conclusions: Prolonged sitting with slumped posture could increase the risk of experiencing lower back discomfort.

Modelling performance during repetitive precision tasks using wearable sensors: a data-driven approach

In modern manufacturing systems, especially assembly lines, human input is a critical resource to provide dexterity and flexibility. However, the repetitive precision tasks common in assembly lines can have adverse effects on workers and overall system performance. We present a data-driven approach to evaluating task performance using wearable sensor data (kinematics, electromyography and heart rate). Eighteen participants (gender-balanced) completed repeated cycles of maze tracking and assembly/disassembly. Various combinations of input data types and classification algorithms were used to model task performance. The use of the linear discriminant analysis (LDA) algorithm and kinematic data provided the most promising classification performance. The highest model accuracy was found using the LDA algorithm and all data types, with respective levels of 62.4, 88.6, 85.8 and 94.1% for predicting maze errors, maze speed, assembly/disassembly errors and assembly/disassembly speed. The presented approach provides the possibility for real-time, on-line and comprehensive monitoring of system performance in assembly-lines or similar industries. Practitioner summary: This paper proposed models the repetitive precision task performance using data collected from wearable sensors. The use of the LDA algorithm and kinematic data provided the most promising classification performance. The presented approach provides the possibility for real-time, on-line and comprehensive monitoring of system performance in assembly lines or similar industries. Abbreviations: AD: anterior deltoid; BB: biceps brachii; ECR: extensor carpi radialis; ECU: extensor carpi ulnaris; FCR: flexor carpi radialis; FCU: flexor carpi ulnaris; FN: false negatives; FP: false positives; HR: heart rate; HRR: heart rate reserve; IMUs: inertial measurement units; kNN: k-nearest neighbors; LDA: linear discriminant analysis; MD: medial deltoid; MF: median power frequency; MNF: mean power frequency; MVIC: maximum voluntary isometric contraction; nRMS: normalized root-mean-square amplitudes; PD: posterior deltoid; RandFor: random forests; RHR: resting heart rate; RMS: root-mean-square amplitudes; sEMG: surface electromyographic; SVM: support vector machines; TB: triceps brachii medial; TN: true negatives; TP: true positives; t-SNE: t-distributed Stochastic Neighbor Embedding; UT: upper trapezius.

Quadriceps mechanomyography reflects muscle fatigue during electrical stimulus-sustained standing in adults with spinal cord injury - a proof of concept

This study investigates whether mechanomyography (MMG) produced from contracting muscles as a measure of their performance could be a proxy of muscle fatigue during a sustained functional electrical stimulation (FES)-supported standing-to-failure task. Bilateral FES-evoked contractions of quadriceps and glutei muscles, of four adults with motor-complete spinal cord injury (SCI), were used to maintain upright stance using two different FES frequencies: high frequency (HF - 35 Hz) and low frequency (LF - 20 Hz). The time at 30° knee angle reduction was taken as the point of critical "fatigue failure", while the generated MMG characteristics were used to track the pattern of force development during stance. Quadriceps fatigue, which was primarily responsible for the knee buckle, was characterized using MMG-root mean square (RMS) amplitude. A double exponential decay model fitted the MMG fatigue data with good accuracy [R2 = 0.85-0.99; root mean square error (RMSE) = 2.12-8.10] implying changes in the mechanical activity performance of the muscle's motor units. Although the standing duration was generally longer for the LF strategy (31-246 s), except in one participant, when compared to the HF strategy, such differences were not significant (p > 0.05) but suggested a faster muscle fatigue onset during HF stimulation. As MMG could discriminate between different stimulation frequencies, we speculate that this signal can quantify muscle fatigue characteristics during prolonged FES applications.

Does Physically Demanding Work Hinder a Physically Active Lifestyle in Low Socioeconomic Workers? A Compositional Data Analysis Based on Accelerometer Data

Leisure time physical activity (LTPA) is strongly associated with socioeconomic position (SEP). Few studies have investigated if demanding occupational physical activity (OPA) could impede a physically active lifestyle in low SEP groups. The aim of this study was to investigate the association between OPA and LTPA among low SEP men and women. We used cross-sectional data from 895 low SEP workers who wore accelerometers for 1–5 consecutive workdays. The associations between the relative importance of activities performed during work and leisure time were assessed using compositional regression models stratified on sex. Compositional isotemporal substitution models were used to assess the implication of increasing occupational walking, standing, or sitting on LTPA. We found dissimilarity in LTPA between the sexes, with men spending more waking leisure time sedentary than women (men ~67%, women ~61%), suggesting women performed more household tasks. In men, the associations between OPA and LTPA were weak. In women, the strongest association was observed between the relative importance of occupational walking and leisure time standing (β^ = −0.16; p = 0.01), where reallocating 15 min work time to occupational walking showed an expected decrease in leisure time standing of 7 min. If this time was spent on additional sedentary leisure time, it could have adverse health consequences.

Exploring the Relationship of Task Performance and Physical and Cognitive Fatigue During a Daylong Light Precision Task

OBJECTIVE

Our aim was to explore the relationship between fatigue and operation system performance during a simulated light precision task over an 8-hr period using a battery of physical (central and peripheral) and cognitive measures.

BACKGROUND

Fatigue may play an important role in the relationship between poor ergonomics and deficits in quality and productivity. However, well-controlled laboratory studies in this area have several limitations, including the lack of work relevance of fatigue exposures and lack of both physical and cognitive measures. There remains a need to understand the relationship between physical and cognitive fatigue and task performance at exposure levels relevant to realistic production or light precision work.

METHOD

Errors and fatigue measures were tracked over the course of a micropipetting task. Fatigue responses from 10 measures and errors in pipetting technique, precision, and targeting were submitted to principal component analysis to descriptively analyze features and patterns.

RESULTS

Fatigue responses and error rates contributed to three principal components (PCs), accounting for 50.9% of total variance. Fatigue responses grouped within the three PCs reflected central and peripheral upper extremity fatigue, postural sway, and changes in oculomotor behavior.

CONCLUSION

In an 8-hr light precision task, error rates shared similar patterns to both physical and cognitive fatigue responses, and/or increases in arousal level.

APPLICATION

The findings provide insight toward the relationship between fatigue and operation system performance (e.g., errors). This study contributes to a body of literature documenting task errors and fatigue, reflecting physical (both central and peripheral) and cognitive processes.

Responsive upper limb and cognitive fatigue measures during light precision work: an 8-hour simulated micro-pipetting study

Many contemporary occupations are characterised by long periods of low loads. These lower force levels, which are relevant to the development of work-related musculoskeletal disorders, are usually not the focus of fatigue studies. In studies that did measure fatigue in light manual or precision work, within and between measurement responses were inconsistent. The aim of this study was to identify fatigue measures that were responsive at lower force levels (<10% MVC) over the course of an 8-h period. A complementary set of fatigue measures, reflecting both neuromuscular and cognitive mechanisms, was measured during a light precision micro-pipetting task performed by 11 participants. Nine measures were found to be significantly responsive over the 8-h period, including: ratings of perceived fatigue, postural tremor, blink frequency and critical flicker fusion frequency threshold. Common field measures, specifically electromyography RMS amplitude and maximum voluntary contractions, did not lead to extraordinary time effects. Practitioner summary: The findings provide insight towards the responsiveness of a complementary set of field usable fatigue measures at low work intensities Although commonly used measures did not reveal significant increases in fatigue, nine alternative measures were significantly responsive over the 8-h period.

Long-Lasting Changes in Muscle Twitch Force During Simulated Work While Standing or Walking

OBJECTIVE

The aim of this study was to evaluate the long-lasting effects of prolonged standing work on a hard floor or floor mat and slow-pace walking on muscle twitch force (MTF) elicited by electrical stimulation.

BACKGROUND

Prolonged standing work may alter lower-leg muscle function, which can be quantified by changes in the MTF amplitude and duration related to muscle fatigue. Ergonomic interventions have been proposed to mitigate fatigue and discomfort; however, their influences remain controversial.

METHOD

Ten men and eight women simulated standing work in 320-min experiments with three conditions: standing on a hard floor or an antifatigue mat and walking on a treadmill, each including three seated rest breaks. MTF in the gastrocnemius-soleus muscles was evaluated through changes in signal amplitude and duration.

RESULTS

The significant decrease of MTF amplitude and an increase of duration after standing work on a hard floor and on a mat persisted beyond 1 hr postwork. During walking, significant MTF metrics changes appeared 30 min postwork. MTF amplitude decrease was not significant after the first 110 min in any of the conditions; however, MTF duration was significantly higher than baseline in the standing conditions.

CONCLUSION

Similar long-lasting weakening of MTF was induced by standing on a hard floor and on an antifatigue mat. However, walking partially attenuated this phenomenon.

APPLICATION

Mostly static standing is likely to contribute to alterations of MTF in lower-leg muscles and potentially to musculoskeletal disorders regardless of the flooring characteristics. Occupational activities including slow-pace walking may reduce such deterioration in muscle function.

Combining regression and mean comparisons to identify the time course of changes in neuromuscular responses during the process of fatigue

The purposes of the present study were to apply a unique method for the identification of the time course of changes in neuromuscular responses and to infer the motor unit activation strategies used to maintain force during a fatiguing, intermittent isometric workbout. Eleven men performed 50, 6 s intermittent isometric muscle actions of the leg extensors, each separated by 2 s of rest at 60% maximal voluntary isometric contraction (MVIC). Electromyographic (EMG) and mechanomyographic (MMG) amplitude (root mean square; RMS) and frequency (mean power frequency; MPF) were obtained from the vastus lateralis (VL) every 5 of the 50 repetitions and normalized as a percent of the initial repetition. Polynomial regression analyses were used to determine the model of best fit for the normalized EMG RMS, EMG MPF, MMG RMS, and MMG MPF versus repetition relationships and one-way repeated measures ANOVAs with post-hoc Student Newman-Keuls were used to identify when these neuromuscular parameters changed from the initial repetition. The findings of the present study indicated two unique phases of neuromuscular responses (repetitions 1-20 and 20-50) during the fatiguing workbout. The time course of changes in these four neuromuscular responses suggested that the after-hyperpolarization theory could not explain the maintenance of force production, but Muscle Wisdom and the Onion Skin Scheme could. The findings of the current study suggested that the time course of changes in neuromuscular responses can provide insight in to the motor unit activation strategies used to maintain force production and allow for a greater understanding of the fatiguing process by identifying the time-points at which these neuromuscular parameters changed.

Long-Term Muscle Fatigue After Standing Work

OBJECTIVE

The aims of this study were to determine long-term fatigue effects in the lower limbs associated with standing work and to estimate possible age and gender influences.

BACKGROUND

The progressive accumulation of muscle fatigue effects is assumed to lead to musculoskeletal disorders, as fatigue generated by sustained low-level exertions exhibits long-lasting effects. However, these effects have received little attention in the lower limbs.

METHOD

Fourteen men and 12 women from two different age groups simulated standing work for 5 hr including 5-min seated rest breaks and a 30-min lunch. The younger group was also tested in a control day. Muscle fatigue was quantified by electrically induced muscle twitches (muscle twitch force [MTF]), postural stability, and subjective evaluation of discomfort.

RESULTS

MTF showed a significant fatigue effect after standing work that persisted beyond 30 min after the end of the workday. MTF was not affected on the control day. The center of pressure displacement speed increased significantly over time after standing work but was also affected on the control day. Subjective evaluations of discomfort indicated a significant increase in perception of fatigue immediately after the end of standing work; however, this perception did not persist 30 min after. Age and gender did not influence fatigue.

CONCLUSION

Objective measures show the long-term effects of muscle fatigue after 5 hr of standing work; however, this fatigue is no longer perceived after 30 min of rest postwork.

APPLICATION

The present results suggest that occupational activities requiring prolonged standing are likely to contribute to lower-extremity and/or back disorders.

Electromyographic, mechanomyographic, and metabolic responses during cycle ergometry at a constant rating of perceived exertion

Ten subjects performed four 8-min rides (65%-80% peak oxygen consumption) to determine the physical working capacity at the OMNI rating of perceived exertion (RPE) threshold (PWCOMNI). Polynomial regression analyses were used to examine the patterns of responses for surface electromyographic (EMG) amplitude (EMG AMP), EMG mean power frequency (EMG MPF), mechanomyographic (MMG) AMP, and MMG MPF of the vastus lateralis as well as oxygen consumption rate, respiratory exchange ratio (RER), and power output (PO) were examined during a 1-h ride on a cycle ergometer at a constant RPE that corresponded to the PWCOMNI. EMG AMP and MMG MPF tracked the decreases in oxygen consumption rate, RER, and PO, while EMG MPF and MMG AMP tracked RPE. The decreases in EMG AMP and MMG MPF were likely attributable to decreases in motor unit (MU) recruitment and firing rate, while the lack of change in MMG AMP may have resulted from a balance between MU de-recruitment as PO decreased, and an increase in the ability of activated fibers to oscillate. The current findings suggested that during submaximal cycle ergometry at a constant RPE, MU de-recruitment and mechanical changes within the muscle may influence the perception of effort via feedback from group III and IV afferents.

Gravitational demand on the neck musculature during tablet computer use

Tablet computer use requires substantial head and neck flexion, which is a risk factor for neck pain. The goal of this study was to evaluate the biomechanics of the head-neck system during seated tablet computer use under a variety of conditions. A physiologically relevant variable, gravitational demand (the ratio of gravitational moment due to the weight of the head to maximal muscle moment capacity), was estimated using a musculoskeletal model incorporating subject-specific size and intervertebral postures from radiographs. Gravitational demand in postures adopted during tablet computer use was 3-5 times that of the neutral posture, with the lowest demand when the tablet was in a high propped position. Moreover, the estimated gravitational demand could be correlated to head and neck postural measures (0.48 < R(2) < 0.64, p < 0.001). These findings provide quantitative data about mechanical requirements on the neck musculature during tablet computer use and are important for developing ergonomics guidelines. Practitioner Summary: Flexed head and neck postures occur during tablet computer use and are implicated in neck pain. The mechanical demand on the neck muscles was estimated to increase 3-5 times during seated tablet computer use versus seated neutral posture, with the lowest demand in a high propped tablet position but few differences in other conditions.

Effects of visual feedback absence on force control during isometric contraction

PURPOSE

The aim of the study was to evaluate the force control in the complete absence of visual feedback and the effect of repeated contractions without visual feedback.

METHODS

Twelve physically active males (age 23 ± 1 years; stature 1.74 ± 0.07 m; body mass 71 ± 6 kg) performed isometric tasks at 20, 40 and 60% maximal voluntary contraction (MVC) for 20 s. For each intensity, a trial with force visual feedback (FB) was followed by 3 trials without FB (noFB-1, noFB-2, noFB-3). During contraction, force and surface electromyogram (EMG) from the vastus lateralis muscle were recorded. From force signal, the coefficient of variation (CV, force stability index), the distance of force from target (ΔF, force accuracy index) and the time within the target (t-target) were determined. From EMG signal, the root mean square (RMS) and mean frequency (MF) were calculated.

RESULTS

MVC was 679.14 ± 38.22 N. In noFB-1, CV was similar to FB, ΔF was higher and t-target lower (P < 0.05) than in FB. EMG-RMS in noFB-1 was lower than in FB at 40 and 60%MVC (P < 0.05). A decrease in ΔF between noFB-1 and noFB-3 (P < 0.05) and an increase in t-target from noFB-1 to noFB-3 (P < 0.05) occurred at 20% MVC. A difference in EMG-RMS among noFB conditions was retrieved only at 60% MVC (P < 0.05).

CONCLUSIONS

These findings suggest that the complete absence of visual feedback decreased force accuracy but did not affect force stability. Moreover, the repetition of noFB trials improved force accuracy at low exercise intensity, suggesting that real-time visual information could be obviated by other feedbacks for force control.

Detecting within- and between-day manifestations of neuromuscular fatigue at work: an exploratory study

Cumulative neuromuscular fatigue may result from exposure to physically demanding work, such as repetitive and/or sustained work with insufficient recovery. The aims of this exploratory study were to develop a battery of field usable fatigue measures and to document hand/arm fatigue in physically demanding work over multiple workdays and after a weekend break. Sixteen plumbers were observed for five days and measures of handgrip force, variability, tremor and discomfort were obtained pre-, mid- and post-shift. This exploratory study demonstrated increasing fatigue of the hand/arm over the day and persistent fatigue from Tuesday to Friday, and that a number of the measures did not return to baseline values following a weekend break. The findings provide preliminary evidence of cumulative fatigue in residential plumbing and insight into neuromuscular fatigue measurement. However, further work is needed to develop and refine a set of fatigue measures to detect neuromuscular fatigue at the workplace.

PRACTITIONER SUMMARY

Cumulative fatigue has been linked to long-term health outcomes, including work-related musculoskeletal disorders. This paper presents findings from a physically demanding job (i.e. plumbing) revealing persistent fatigue over the work shift(s) and insufficient recovery after a weekend break, and provides insight into fatigue measurement at the workplace.

Bulk movement included in multi-channel mechanomyography: similarity between mechanomyography of resting muscle and that of contracting muscle

Although mechanomyography (MMG) reflects local vibrations from contracting muscle fibers, it also includes bulk movement: deformation in global soft tissue around measuring points. To distinguish between them, we compared the multi-channel MMG of resting muscle, which dominantly reflected the bulk movement caused by arterial pulsations, to that of the contracting muscle. The MMG signals were measured at five points around the upper arms of 10 male subjects during resting and during isometric ramp contraction from 5% to 85% of maximal voluntary contraction (MVC) of the biceps brachii muscle. The characteristics of bulk movement were defined as the amplitude distribution and phase relation among the five MMG signals. The bulk movement characteristics during the rest state were not necessarily the same among the subjects. However, below 30 Hz, each subject's characteristics remained the same from the rest state (0% MVC) to the contracting state (80% MVC), at which the bulk movement mainly originates from muscle contraction activity. Results show that the MMG of the low frequency domain (<30 Hz) includes bulk movement depending on the mechanical deformation characteristics of each subject's body, for a wide range of muscle contraction intensities.

Variation of force amplitude and its effects on local fatigue

Trends in industry are leaning toward stereotyped jobs with low workloads. Physical variation is an intervention to reduce fatigue and potentially musculoskeletal disorders in such jobs. Controlled laboratory studies have provided some insight into the effectiveness of physical variation, but very few have been devoted to force variation without muscular rest as a component. This study was undertaken to determine multiple physiological responses to five isometric elbow extension protocols with the same mean amplitude (15% maximum voluntary contraction, MVC), cycle time (6 s), and duty cycle (50 %). Sustained (15 %Sus) and intermittent contractions including zero force (0-30 %Int) differed significantly in 19 of 27 response variables. Contractions varying by half the mean force (7.5-22.5 %Int) led to 8 and 7 measured responses that were significantly different from 0-30 %Int and 15 %Sus, respectively. A sinusoidal condition (0-30 %Sine) resulted in 2 variables that were significantly different from 0-30 %Int, and 16 different from 15 %Sus. Finally, ten response variables suggested that varying forces with 1 % as the lower contraction level was significantly less fatiguing than 15 %Sus, while no responses were significantly different from 0-30 %Int. Sustained contractions led to decreased twitch force 24-h post-exercise, whereas recovery was complete within 60 min after intermittent contractions. This suggests that time-varying force may be a useful intervention to reduce local fatigue in workers performing low-load tasks, and also that rest per se did not seem to cause any extraordinary effects beyond those predictable from the force variation amplitude.

Biofeedback effectiveness to reduce upper limb muscle activity during computer work is muscle specific and time pressure dependent

Continuous electromyographic (EMG) activity level is considered a risk factor in developing muscle disorders. EMG biofeedback is known to be useful in reducing EMG activity in working muscles during computer work. The purpose was to test the following hypotheses: (1) unilateral biofeedback from trapezius (TRA) can reduce bilateral TRA activity but not extensor digitorum communis (EDC) activity; (2) biofeedback from EDC can reduce activity in EDC but not in TRA; (3) biofeedback is more effective in no time constraint than in the time constraint working condition. Eleven healthy women performed computer work during two different working conditions (time constraint/no time constraint) while receiving biofeedback. Biofeedback was given from right TRA or EDC through two modes (visual/auditory) by the use of EMG or mechanomyography as biofeedback source. During control sessions (no biofeedback), EMG activity was (mean ± SD): 2.4 ± 1.1, 2.5 ± 2.1, and 9.1 ± 3.1%max-EMGrms for right and left TRA and EDC, respectively. During biofeedback from TRA, activity was reduced in right TRA (1.7 ± 1.6%max-EMGrms) and left TRA (1.2 ± 2.0%max-EMGrms) compared to control. During biofeedback from EDC, activity in EDC was reduced (8.3 ± 3.3%max-EMGrms) compared with control. During time constraint, activity was reduced in right TRA (1.9 ± 1.3%max-EMGrms), left TRA (1.5 ± 1.5%max-EMGrms), and EDC (8.4 ± 3.2%max-EMGrms), during biofeedback compared to control.

CONCLUSION

biofeedback reduced muscle activity in TRA by ∼ 30-50% and in EDC by ∼ 10% when given from the homologous or bilateral muscle but not from the remote muscle, and was significant in the time constraint condition; while feedback source and presentation mode showed only minor differences in the effect on reducing homologous muscle activity. This implies that biofeedback should be given from the most affected muscle in the occupational setting for targeting relief and prevention of muscle pain most effectively.

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.

Low mean level sustained and intermittent grip exertions: influence of age on fatigue and recovery

The goal of this study was to quantify localised muscle fatigue resulting from low mean levels of exertion in younger (< 40 years) and older (> 50 years) adults. Fatigue, elicited in the finger flexor muscles by intermittent (10% mean maximum voluntary contraction (MVC)) and sustained (8% MVC) handgrip exercises, was quantified by a muscle twitch force response before, immediately after and during 3 h following exercise. Despite greater mean loads, recovery time was shorter following intermittent than sustained contractions, which suggests that recovery from fatigue is more sensitive to rest within the work cycle than mean work. The more pronounced effects for younger than older individuals following the sustained exertion indicate that changes in muscle fibre type composition might predispose older individuals to be more resistant to fatigue resulting from sustained contractions of low level. Performing hand exertion tasks requiring low mean force levels contributes to similar long-lasting fatigue effects regardless of gender and age. Intermittent periods of complete rest reduce muscle fatigue. Since fatigue was not perceived during recovery from the tested sustained and intermittent contractions, subjective evaluations may not be a reliable indicator of localised muscle fatigue.

Manifestations of shoulder fatigue in prolonged activities involving low-force contractions

Shoulder fatigue has been suggested to be a useful risk indicator for shoulder disorders in repetitive, low-force work tasks. In contrast to high-force or purely isometric tasks, it is unclear whether measurable fatigue develops in realistic low-force work. The question addressed in this review was: 'Is there evidence of objective signs of fatigue in the shoulder region in realistic, low-force work tasks?' Studies on objective measures of fatigue applied in realistic low-force work tasks were systematically reviewed, using a task duration of more than 1 h and an intensity level of less than 20% maximum voluntary contraction (MVC) for the median trapezius activation level as inclusion criteria. Thirteen studies were found to fulfil the criteria. All these studies addressed fatigue-related changes in the electromyographic signal in the descending part of the trapezius muscle. Seven did find a combination of frequency decrease and amplitude increase over time, which is generally considered as an objective manifestation of fatigue. Thus, there is evidence of objective signs of fatigue in some of the realistic, low-force tasks. The intensity level appeared to be a main determinant here. In the studies demonstrating signs of fatigue an intensity level of 15%MVC or more was used, while the intensity level in the studies with a negative result was generally lower.

Neuromuscular recovery of the biceps brachii muscle after resistance exercise

The aim of this study was to determine the time to restore the biceps brachii (BB) electromyographic (EMG) activity after the biceps curl (BC) exercise, at different intensities. Ten males performed initially maximal voluntary isometric contractions (MVC) of the elbow flexors, followed by one isometric submaximal contraction at 50% MVC (reference contraction). After this, four bouts of the BC at 25%, 30%, 35%, and 40% 1 RM during 1 minute (randomly assigned, with 10 minutes rest between them) were performed. During the rest intervals at preestablished moments (15 seconds, 1, 3, 5, and 10 min), isometric 50% MVC were performed. The EMG variables (root mean square [RMS], zero crossings [ZC], median frequency, [MF] and peak power [PP]) at rest were compared with reference values. Immediately after the exercise, RMS and PP increased, while ZC and MF decreased, indicating fatigue. After 1 minute most of the variables were similar to the reference. Different load levels did not affect the EMG recovery. In conclusion, the EMG variables recovered after 1 minute rest, indicating the optimal muscular condition for subsequent bouts.

The effect of operating time on surgeons' muscular fatigue

INTRODUCTION

A study was completed to determine if operating has an effect on a surgeon's muscular fatigue.

SUBJECTS AND METHODS

Six head and neck surgery consultants, two ENT registrars, 20 normal controls from two tertiary referral centres in the West Midlands participated in the study. Electromyography (EMG) measurements were taken throughout a day of operating and fatigue indices were compared to controls performing desk work.

RESULTS

The percentage changes in mean frequency of muscular contractions were examined; there was no significant difference in fatigue levels between consultants and registrars. Operating led to an increase in fatigue in all subjects, compared to no increase in controls performing desk work. It was also found that the brachioradialis muscle is used more than the mid-deltoid muscle and, hence, fatigues at a faster rate.

CONCLUSIONS

Surgeons should be aware that their muscular fatigue levels will increase as an operation progresses; therefore, if possible, more complex parts of the operation should be performed as early as possible, or, in the case of a very long operation, a change in surgeon may be necessary.

Time dependency assessment of muscular fatigue index and hand tremor under operating conditions

The aim of this paper is to determine if operating has an effect on a surgeon's muscular fatigue and hand tremor. Electromyography and tremor measurements were taken throughout a day of operating from the brachioradialis (BR) and mid deltoid (MD) muscles of six head and neck surgery consultants and two Ear, Nose, and Throat (ENT) registrars. The percent change in Mean Frequency (MF) of their EMG signal and tremor amplitudes was compared to twenty normal controls performing desk work. It was found that operating led to an increase in the fatigue level and hand tremor in all subjects, compared to very little increase from within the controls study. It was also determined that the BR is used more than the MD muscle and hence fatigues at a faster rate. Surgeons should be aware that their muscular fatigue levels and consequently their hand tremor will increase as an operation progresses, therefore, if possible, more complex parts of the operation should be performed as early as possible.

Time- and frequency-domain monitoring of the myoelectric signal during a long-duration, cyclic, force-varying, fatiguing hand-grip task

The amplitude and mean power spectral frequency (MNF) of the electromyogram (EMG) of flexor digitorum superficialis and extensor carpi radialis muscles were monitored during cyclic, force-varying, constant-posture, submaximal, grip-force contractions until endurance. These contractions are reminiscent of work tasks associated with the risk of repetitive stress injuries. Based on recommendations from a prior cross-comparison study of these data, the cyclic grip-force contractions were temporally aligned cycle-by-cycle to the achieved grip force profile, then EMG amplitude was computed using signal whitening (500 ms window) and MNF was computed using the short-time Fourier transform (500 ms window). In addition, brief (8s) constant-force (static) contractions were interspersed within the cyclic contractions every 5 min. MNF was tracked during these periods. All subjects reported a marked increase in pain/discomfort/fatigue during the contraction trials, until self-selecting to discontinue contractions after 30-90 min. Discomfort returned to near-baseline levels during the ensuing 45 min recovery (rest) period. No statistical trend was found in either EMG amplitude or MNF during the cyclic contractions or the recovery period. Initial MNF and MNF slope were monitored during the 8s interspersed static contractions. These parameters also did not follow any consistent trend. These results indicate limitations in the use of these EMG descriptors for assessment of fatigue during long-duration, force-varying contractions.

Electromyogram and perceived fatigue changes in the trapezius muscle during typewriting and recovery

The purpose of the present study was to investigate the development and recovery of muscle fatigue in the upper trapezius muscle by analyzing electromyographic signals. Six male subjects performed a simulated typewriting task for four 25-min sessions. During fatigue and the following rest periods, subjective fatigue and surface electromyography (EMG) from the trapezius muscle during isometric contraction at 30% maximum voluntary contraction (MVC) were periodically measured in the interval. We detected a significant decrease in muscle fiber conduction velocity (MFCV) (P = 0.008) and median frequency (MDF) (P = 0.026) as well as an increase in root mean square (RMS) (P = 0.039) and subjective fatigue (P = 0.0004) during the fatigue period. During the recovery period, subjective fatigue decreased drastically and significantly (P = 0.0004), however, the EMG parameters did not recover completely. Thus, physiological muscle fatigue in the trapezius developed in accordance with subjective muscle fatigue during typewriting. On the other hand, differences between the physiological and subjective parameters were found during recovery. Further studies should be necessary to reveal the discrepancy could be a major factor of a transition from temporal phenomena to serious chronic muscle fatigue and to identify the necessity of some guidelines to prevent VDT work-related chronic muscle fatigue in the trapezius.

Development of fatigue and discomfort in the upper trapezius muscle during light manual work

Optimization of the temporal aspects of task design requires a better understanding of the development of muscle fatigue in the neck and shoulder region over time. The objective of the study was to investigate this in two production companies and to determine the relationship between objective and subjective estimates of fatigue. Indicators of fatigue were obtained through electromyography (EMG) during test contractions and ratings of perceived discomfort. EMG amplitude increased during the day in both case studies while mean power frequency decreased only in one case. In both cases, a more detailed frequency analysis of the EMG signals showed an increase in lower frequency power accompanied by a decrease in higher frequency power. Local perceived discomfort in the neck and shoulder increased over the course of the day in both cases. However, no clear relationship between perceived discomfort and objective indicators of fatigue was found. Obtaining sufficient sensitivity to detect effects of temporal aspects of task design probably requires complementary or more refined methods (e.g. EMG arrays, mechanomyography).

Effects of electromyographic and mechanomyographic biofeedback on upper trapezius muscle activity during standardized computer work

The purpose of this laboratory study was to investigate the effects of surface electromyography (EMG)- and mechanomyography (MMG)-based audio and visual biofeedback during computer work. Standardized computer work was performed for 3 min with/without time constraint and biofeedback in a randomized order. Biofeedback was given on the basis of an individual preset threshold value for the right trapezius EMG and MMG signal and a time factor (repetition of events above the threshold). The duration of muscle activity above the preset threshold, the right trapezius EMG and MMG root mean square (RMS) values as well as the work performance in terms of number of completed graph/mouse clicks/errors, the rating of perceived exertion (RPE) and the usefulness of the biofeedback were assessed. The duration of muscle activity above the threshold was significantly lower with MMG compared with EMG as source of biofeedback (p < 0.05). Biofeedback led to a significant decrease in the right trapezius EMG RMS, lower RPE and decreased number of errors and mouse clicks, but also decreased number of completed graphs (p < 0.05). Audio and visual biofeedbacks were as effective. MMG-based biofeedback is a potential reliable alternative to EMG in ergonomics. A lowering of the trapezius muscle activity may contribute to diminish the risk of work related musculoskeletal disorders development.

Mechanomyography for Studying Force Fluctuations and Muscle Fatigue

Measurement of the transverse displacement of the skin over a contracting muscle is known as the surface mechanomyogram. With the concurrent measurement of the electromyogram and twitch force, the mechanomyogram can provide supplementary information about muscle activity and motor performance during such tasks as fatiguing contractions and attempts to perform steady contractions.

Mechanomyographic and electromyographic responses to repeated concentric muscle actions of the quadriceps femoris

In comparison to isometric muscle action models, little is known about the electromyographic (EMG) and mechanomyographic (MMG) amplitude and mean power frequency (MPF) responses to fatiguing dynamic muscle actions. Simultaneous examination of the EMG and MMG amplitude and MPF may provide additional insight with regard to the motor control strategies utilized by the superficial muscles of the quadriceps femoris during a concentric fatiguing task. Thus, the purpose of this study was to examine the EMG and MMG amplitude and MPF responses of the vastus lateralis (VL), rectus femoris (RF), and vastus medialis (VM) during repeated, concentric muscle actions of the dominant leg. Seventeen adults (21.8+/-1.7 yr) performed 50 consecutive, maximal concentric muscle actions of the dominant leg extensors on a Biodex System 3 Dynamometer at velocities of 60 degrees s(-1) and 300 degrees s(-1). Bipolar surface electrode arrangements were placed over the mid portion of the VL, RF, and VM muscles with a MMG contact sensor placed adjacent to the superior EMG electrode on each muscle. Torque, MMG and EMG amplitude and MPF values were calculated for each of the 50 repetitions. All values were normalized to the value recorded during the first repetition and then averaged across all subjects. The cubic decreases in torque at 60 degrees s(-1) (R2 = 0.972) and 300 degrees s(-1) (R2 = 0.931) was associated with a decline in torque of 59+/-24% and 53+/-11%, respectively. The muscle and velocity specific responses for the MMG amplitude and MPF demonstrated that each of the superficial muscles of the quadriceps femoris uniquely contributed to the control of force output across the 50 repetitions. These results suggested that the MMG responses for the VL, RF, VM during a fatiguing task may be influenced by a number of factors such as fiber type differences, alterations in activation strategy including motor unit recruitment and firing rate and possibly muscle wisdom.

The effect of sustained low-intensity contractions on supraspinal fatigue in human elbow flexor muscles

Subjects quickly fatigue when they perform maximal voluntary contractions (MVCs). Much of the loss of force is from processes within muscle (peripheral fatigue) but some occurs because voluntary activation of the muscle declines (central fatigue). The role of central fatigue during submaximal contractions is not clear. This study investigated whether central fatigue developed during prolonged low-force voluntary contractions. Subjects (n=9) held isometric elbow flexions of 15% MVC for 43 min. Voluntary activation was measured during brief MVCs every 3 min. During each MVC, transcranial magnetic stimulation (TMS) was followed by stimulation of either brachial plexus or the motor nerve of biceps brachii. After nerve stimulation, a resting twitch was also evoked before subjects resumed the 15% MVC. Perceived effort, elbow flexion torque and surface EMG from biceps, brachioradialis and triceps were recorded. TMS was also given during the sustained 15% MVC. During the sustained contraction, perceived effort rose from approximately 2 to approximately 8 (out of 10) while ongoing biceps EMG increased from 6.9+/-2.1% to 20.0+/-7.8% of initial maximum. Torque in the brief MVCs and the resting twitch fell to 58.6+/-14.5 and 58.2+/-13.2% of control values, respectively. EMG in the MVCs also fell to 62.2+/-15.3% of initial maximum, and twitches evoked by nerve stimulation and TMS grew progressively. Voluntary activation calculated from these twitches fell from approximately 98% to 71.9+/-38.9 and 76.9+/-18.3%, respectively. The silent period following TMS lengthened both in the brief MVCs (by approximately 40 ms) and in the sustained target contraction (by approximately 18 ms). After the end of the sustained contraction, the silent period recovered immediately, voluntary activation and voluntary EMG recovered over several minutes while MVC torque only returned to approximately 85% baseline. The resting twitch showed no recovery. Thus, as well as fatigue in the muscle, the prolonged low-force contraction produced progressive central fatigue, and some of this impairment of the subjects' ability to drive the muscle maximally was due to suboptimal output from the motor cortex. Although caused by a low-force contraction, both the peripheral and central fatigue impaired the production of maximal voluntary force. While central fatigue can only be demonstrated during MVCs, it may have contributed to the disproportionate increase in perceived effort reported during the prolonged low-force contraction.

Spectral moments of mechanomyographic signals recorded with accelerometer and microphone during sustained fatiguing contractions

The aim of this study was to analyse the trends of the first three power spectral moments of the mechanomyogram (MMG) signal recorded by a microphone (MMG(MIC)) and an accelerometer (MMG(ACC)) during sustained contractions. MMG signals were recorded from the biceps brachii muscle in 14 healthy male subjects during a 3 min isometric elbow flexion at 30% of the maximal voluntary contraction. MMG absolute and normalised root mean square (RMS), mean power frequency (MNF), power spectral variance (Mc2), and skewness (mu3) were computed. For both MMG(MIC) and MMG(ACC), absolute and normalised RMS and Mc2 increased while MNF and mu3 decreased with contraction time (P<0.001). The rates of change of RMS over time were significantly correlated (P<0.001) for MMG(MIC) and MMG(ACC) but not correlated for spectral moments. The coefficient of variation of RMS was higher for MMG(MIC) than for MMG(ACC), while the opposite was observed for mu3 (P<0.05). It was concluded that higher order spectral moments of the MMG signal change during sustained contraction, indicating a complex modification of the shape of the power spectrum and not just scaling of the bandwidth. This is most likely due to the additional motor unit recruitment with fatigue and to the non-linear summation of motor unit contributions to the signal. Moreover, the characteristics of MMG signals recorded with microphones and accelerometers have important differences, which should be taken into account when comparing results from different studies.

Muscle fatigue and endurance during repetitive intermittent static efforts: development of prediction models

Localized muscle fatigue has received growing attention as a potential design variable and exposure metric in research towards prevention of musculoskeletal disorders in the workplace. While fatigue during sustained static work has been investigated extensively, effects during tasks comprising work-rest cycles are less clear. Work-rest models for static intermittent work have been presented in several reports, but the applicability is often limited to specific conditions. A study was conducted that facilitated a description of the relationships between static intermittent efforts and muscle endurance and fatigue. Exercises consisted of 1 h (maximum) of repetitive static arm abductions, involving a range of muscle contraction levels (10-30% maximum voluntary exertion), duty cycles (0.2-0.8) and cycle times (20-180 s). A between-subject central composite experimental design was used and 15 different exercise conditions were examined with six participants (three females and three males) for each. Along with endurance times, temporal changes related to fatigue were monitored using muscle strength, ratings of discomfort and electromyography (EMG) obtained from the middle-deltoid muscle during the contraction phase of the work cycles. The results of this study showed the influence of contraction level and duty cycle on the majority of fatigue measures used, while cycle time tended to affect EMG spectral measures. Using a response surface methodology, several fatigue prediction models and contour plots were developed that can be employed as an aid for design and evaluation of light repetitive static tasks. Good correspondence was generally found between discomfort rating and other measures of fatigue, suggesting the usefulness of this measure for rapid assessments of local fatigue in the workplace.

The effect of postactivation potentiation on the mechanomyogram

To examine whether postactivation potentiation (PAP) was related to changes in the electromyogram (EMG) and in the mechanomyogram (MMG), evoked twitch torque properties were compared before and after a 10 s plantar flexion MVC in ten male subjects. Peak twitch torque (Pt), and the peak acceleration of the twitch torque development (d2T/dt2) were measured from electrical tibial nerve stimulation. In addition, from the medial gastrocnemius muscle, peak to peak MMG (p-p MMG) by means of accelerometer, and peak to peak EMG (M-wave) amplitudes were recorded. Immediately (2 s) following the MVC there were significant increases in Pt (47%), d2T/dt2 (86%), and p-p MMG (70%), but no change in the M-wave. At 2 s, 15 s, 30 s after the MVC, individual percent changes in p-p MMG were significantly and positively correlated with individual percent changes in both Pt and d2T/dt2. These results indicate that evoked MMG reflects acute enhanced force output elicited by PAP, and that the increases in twitch contraction properties represent intrinsic mechanical changes of the muscle, that are not associated with electrical changes. Furthermore, measurements of the evoked p-p MMG signal following PAP support the utility of describing the gross lateral movement phase of the MMG signal.