Cycle time influences the development of muscle fatigue at low to moderate levels of intermittent muscle contraction

Why multi-tier surgical instrument table matters? An ergonomic analysis from mento-physical demand perspectives

Using traditional back tables (BT) in operating rooms (OR) can lead to high physical/cognitive demand on nurses due to repetitive manual material handling activities. A multi-tier table (MTT) has been developed to relieve such stressors by providing extra working surfaces to avoid stacking the instrument trays and facilitate access to surgical tools. In this study, sixteen participants performed lifting/lowering and instrument findings tasks on each table, where kinematics, kinetics, subjective, and performance-related measures were recorded. Results indicated that MTT required lesser shoulder flexion (p-value<0.001), ∼14% lower shoulder loads (0.012), task completion time (<0.001), and cognitive/physical workloads (<0.004). Although peak low-back demands were ∼15% higher using MTT, the number of lifts to complete the same task was 60% lower, leading to lower cumulative demand on the low-back musculature. Utilizing MTT in OR could reduce demand and increase nurses' efficiency, leading to reduced risk of WMSDs and the total time of surgery.

Effect of Mental Task on Sex Differences in Muscle Fatigability: A Review

Previous research demonstrated that there are observable sex differences in developing muscle fatigue when mental task during fatiguing activity is present; however, there is no available review on this matter. Therefore, this review aimed to summarize the findings of previous studies investigating the effect of mental task on muscle fatigue in men and women. To conduct the review, we utilized searches using the electronic databases Web of Science, PubMed, Scopus, and EBSCO Cinahl Ultimate. The studies included had no limited publication date and examined the effects of mental task on muscle fatigue in a healthy adult population of any age. The evaluation was performed using the following criteria: time to failure, or subjective scale in various modifications (visual analog scale-VAS, rate of perceived effort-RPE, rate of perceived fatigue-RPF, rate of perceived discomfort-RPD). A total of seven studies met the set criteria, which were subsequently analyzed. Heavy mental task (more demanding math tasks) can reduce the time to failure for both men and women, with the reduction being more pronounced for women than for men. For light mental task (simple math tasks), no reduction in time to failure was observed to a great extent. The mental task in any of the included studies did not affect the subjective perception of fatigue, effort, discomfort, or pain. Although the studies investigating the effect of mental task on sex differences in muscle fatigability are limited, based on our findings we can assume that in jobs requiring heavier mental task, women may be more prone to the faster development of muscle fatigue; thus, employers might consider paying attention to the possibility of adequate rest.

Exploring Localized Muscle Fatigue Responses at Current Upper-Extremity Ergonomics Threshold Limit Values

OBJECTIVE

The purpose of this study was to evaluate localized muscle fatigue responses at three upper-extremity ergonomics threshold limit value (TLV) duty cycles.

BACKGROUND

Recently, a TLV equation was published to help mitigate excessive development of localized muscle fatigue in repetitive upper limb tasks. This equation predicts acceptable levels of maximal voluntary contraction (% MVC) for a given duty cycle (DC). Experimental validation of this TLV curve has not yet been reported, which can help guide utilization by practitioners.

METHOD

Eighteen participants performed intermittent isometric elbow flexion efforts, in three separate counter-balanced sessions, at workloads defined by the American Conference of Governmental Industrial Hygenists' (ACGIH) TLV equation: low DC (20% DC, 29.6% MVC), medium DC (40% DC, 19.7% MVC), and high DC (60% DC, 13.9% MVC). Targeted localized muscle fatigue (LMF) of the biceps brachii was tracked across numerous response variables, including decline in strength (MVC), electromyography (EMG) amplitude and mean power frequency (MnPF), and several psychophysical ratings.

RESULTS

At task completion, biceps MnPF and MVC (strength) were significantly different between each TLV workload, with the high DC condition eliciting the largest declines in MnPF and MVC.

CONCLUSION

Findings demonstrate that working at different DCs along the ACGIH TLV curve may not be equivalent in preventing excessive LMF. Higher DC workloads elicited a greater LMF response across several response variables.

APPLICATION

High DC work of the upper extremity should be avoided to mitigate excess LMF development. Current TLVs for repetitive upper-extremity work may overestimate acceptable relative contraction thresholds, particularly at higher duty cycles.

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.

A comparison of the physical demands associated with various right-angle direct-current power tools

This work examined the physical demands associated with 4 commercially available direct current right-angle power tools along with their computer controlled fastening strategies. Physical demands were measured via an external instrumented handle that could also control the trigger of each tool. Data revealed that forces recorded by the external handle for each of the 4 tools differed. Independent of Joint-Hardness and Target Torque the Atlas Copco TurboTight® and Cleco Low Torque Reaction fastening strategies, both using a rapid spindle-head rotation strategy, produced the lowest peak and impulse force, respectfully, when compared to the 2 tools that were designed to fasten with a longer duration. This work has shown that short duration fastening strategies provide an ergonomics benefit to the users as it requires less force to operate, such force reduction in previous research has been linked to reduced handle displacement and lower muscle effort.

Effects of Vertical Lifting Distance on Upper-Body Muscle Fatigue

Manual material handling (MMH) is commonly demanded in the manufacturing industry. Occupational muscle fatigue of the arm, shoulder, and back, which arise from MMH tasks, can cause work absences and low efficiency. The available literature presents the lack of the fatigue comparison between targeted muscles, on the same part or on different parts. The main aim of the present study was to evaluate and compare the fatigue of upper-body muscles during repetitive bending tasks, an experiment involving 12 male subjects has been conducted to simulate material handling during furniture board drilling. The vertical lifting distance was chosen to be the single independent variable, and the three levels were 0, 250, and 500 mm. Surface electromyography (sEMG) was used to measure the muscle fatigue of the biceps brachii, upper trapezius, and multifidus, while the sEMG parameters, including the normalized electromyographic amplitude (Normalized EA) and mean power frequency (MPF), of the target muscles were analyzed. The experimental results reveal that during the manual handling tasks, the biceps brachii was the most relaxed muscle, contributing the least muscle tension, while the multifidus was the most easily fatigued muscle. Furthermore, the EMG MPF fatigue threshold (MPFFT) of multifidus muscle tension was tested to estimate its maximum workload in the long-term muscle contraction. In conclusion, bending angle should be maintained to a small range or bending should even be avoided during material-handling tasks.

Smartphone-based human fatigue level detection using machine learning approaches

Human muscle fatigue is the main result of diminishing muscle capability, leading to reduced performance and increased risk of falls and injury. This study provides a classification model to identify the human fatigue level based on the motion signals collected by a smartphone. 24 participants were recruited and performed the fatiguing exercise (i.e. squatting). Upon completing each set of squatting, they walked for a fixed distance while the smartphone attached to their right shank and the gait data were associated with the Borg's Rating of Perceived Exertion (i.e. data label). Our machine-learning model of two (no- vs. strong-fatigue), three (no-, medium-, and strong-fatigue) and four (no-, low-, medium-, and strong-fatigue) levels of fatigue reached the accuracy of 91, 78, and 64%, respectively. The outcomes of this study may facilitate the accessibility of a fatigue-monitoring tool in the workplace, which improves the workers' performance and reduce the risk of falls and injury. Practitioner Summary: This study aimed to develop a machine-learning model to identify human fatigue level using motion data captured by a smartphone attached to the shank. Our results can facilitate the development of an accessible fatigue-monitoring system that may improve the workers' performance and reduce the risk of falls and injury. Abbreviations: WMSD: work-related musculoskeletal disorders; IMU: inertial measurement unit; RPE: rating of perceived exertion; SVM: support vector machine; IRB: institutional review board; SOM: self-organizing map; LDA: linear discriminant analysis; PCA: principal component analysis; FT: fourier transformation; RBF: radial basis function; CUSUM: cumulative sum; ROM: range of motion; MVC: maximum voluntary contractions.

Investigating the effect of fatigue on muscle microvasculature blood flow during intermittent isometric contraction

Localized muscle fatigue (LMF) decreases muscular strength, while affects the performance and potentially increases the risk of musculoskeletal disorders (MSD). An important mechanism in recovering from muscle fatigue is blood flow (BF). The BF response to muscle contraction and fatigue is highly dynamic and difficult to predict, as it depends on both metabolic demand and intramuscular pressure. The aim of this study was to measure both fatigue and BF during intermittent exertion of the first dorsal interosseous (FDI) muscle, in order to better characterize the relationship between BF and LMF during muscle contraction and rest. This study utilized Diffuse Correlation Spectroscopy (DCS) for BF measurement within the microvasculature of the FDI muscle. Exertion levels (EL) for intermittent fatiguing contraction were set to 20%, 30%, and 40% of an individual's maximum voluntary contraction (MVC). Our results showed that as an individual fatigued, relative BF rates increased, on average, by ~66% during exertion periods and ~330% during rest periods. Differences between exerting and resting BF increased over time for every EL (p<0.04), increasing by up to 11 times the baseline BF. At the same levels of muscle capacity (%MVC), resting BF was also found to increase with EL consistently. Our findings highlight BF dependence on both EL and history of muscle contraction. These results imply a variable recovery rate based on both the current state of contraction, (i.e., exertion vs. rest), and the muscle contraction history. The outcome of our study may facilitate the estimation of BF, thus, the muscle recovery rate, which can be implemented in the fatigue models to improve the prediction of muscle capacity to generate force/power.

Postural and muscular adaptations to repetitive simulated work

Complex repetitive tasks are common in the workplace and have been associated with upper extremity disorders. The purpose of this study was to examine the progressive effects of highly repetitive work on joint kinematics and muscle activity of the trunk and upper extremity. Fifteen healthy men performed 60 one-minute cycles of 4 simulated automotive-related tasks. Electromyography of eight muscles and kinematics of the trunk and right upper extremity were collected. Data were analysed at 12-min intervals and divided into a complete work cycle. The time to complete the work cycle decreased by 6.3 s over the trials. Peak shoulder flexion decreased and peak elbow flexion increased during the work cycle. Muscle activity magnitude and variability was influenced by time during the repetitive tasks. This study found adaptations to highly repetitive but light work in only 1 h; redistributing muscle demands within the shoulder over time may reduce muscle fatigue development. Practitioner Summary: While the work was not strenuous, we were able to demonstrate muscular and postural adaptations in a single hour of simulated work. By evaluating both the whole work cycle and the sub-tasks, we aim to develop new methods for evaluating the risk of complex tasks in prolonged repetitive work.

Simulating the influence of physical overload on assembly line performance: A case study in an automotive electrical component plant

Although the workstations of a Brazilian automotive electrical harness production line are set close to TAKT time (the production rate required to meet demand), factory performance is compromised regarding: (i) sick leaves due to occupational disease (105 employees last year) and (ii) a production rate at only 42% of capacity. Our objective was to simulate the performance of a production line balanced against physical overload by the addition of an extra workstation. Based on ergonomic work analysis, the study applied System Dynamics at the global observation stage to obtain a systemic interpretation of the factors involved in production line performance. According to the indicators, the alternative configuration reduced physical overload by 36%, which would result in a sick leave rate of 50.8 employees/year (51.6% lower than the current configuration), as well as a production rate at 99% of capacity (a 92.7% increase over the current configuration). We found that reducing physical overload allows the "workforce control" loop to govern the system, producing favorable results. We conclude that setting the work cycle overly close to TAKT time leads to overload, due to the shorter recovery times at the end of each cycle. Thus, it is necessary to seek a balance between efficiency gains through downtime reduction and the physiological recovery of workers.

Ratings of perceived fatigue predict fatigue induced declines in muscle strength during tasks with different distributions of effort and recovery

The purpose of this study was to quantify the relationship between ratings of perceived fatigue (RPF), using a modified Borg CR-10 scale, and muscle fatigue accumulation, as defined by maximal voluntary contraction strength (MVC) declines, during two complex MVC-relative tasks (conditions) that cause muscle fatigue and allow recovery. Nine female participants completed the fatiguing tasks, composed of a series of submaximal, isometric efforts (task plateaus) requiring isometric flexion at the distal interphalangeal joint of the thumb. Significant partial correlations between RPF and MVC, while controlling for task plateau intensity (%MVC), were found in 6/9 participants. A significant linear regression model, explaining 86.2% of the variance in mean MVC decline, was obtained with 3 predictor variables: mean RPF (p < 0.001), Task Plateau (p < 0.001), and the interaction between mean RPF and Task Plateau (RPF × Task Plateau; p = 0.014). The observed linear relationship between RPF and MVC declines, both at the participant and group level support, the use of RPF to estimate the instantaneous fatigue status of the muscle in tasks that allow both muscle fatigue and recovery.

Modification of a three-compartment muscle fatigue model to predict peak torque decline during intermittent tasks

This study aimed to test whether adding a rest recovery parameter, r, to the analytical three-compartment controller (3CC) fatigue model (Xia and Frey Law, 2008) will improve fatigue estimates during intermittent contractions. The 3CC muscle fatigue model uses differential equations to predict the flow of muscle between three muscle states: Resting (MR), Active (MA), and Fatigued (MF). This model uses a feedback controller to match the active state to target loads and two joint-specific parameters: F, fatigue rate controlling flow from active to fatigued compartments) and R, the recovery rate controlling flow from the fatigued to the resting compartments. This model does well to predict intensity-endurance time curves for sustained isometric tasks. However, previous studies find when rest intervals are present that the model over predicts fatigue. Intermittent rest periods would allow for the occurrence of subsequent reactive vasodilation and post-contraction hyperemia. We hypothesize a modified 3CC-r fatigue model will improve predictions of force decay during intermittent contractions with the addition of a rest recovery parameter, r, to augment recovery during rest intervals, representing muscle re-perfusion. A meta-analysis compiling intermittent fatigue data from 63 publications reporting decline in peak torque (% torque decline) were used for comparison. The original model over-predicted fatigue development from 19 to 29% torque decline; the addition of a rest multiplier significantly improved fatigue estimates to 6-10% torque decline. We conclude the addition of a rest multiplier to the three-compartment controller fatigue model provides a physiologically consistent modification for tasks involving rest intervals, resulting in improved estimates of muscle fatigue.

Assessment of Whole Body and Local Muscle Fatigue Using Electromyography and a Perceived Exertion Scale for Squat Lifting

This research study aims at addressing the paradigm of whole body fatigue and local muscle fatigue detection for squat lifting. For this purpose, a comparison was made between perceived exertion with the heart rate and normalized mean power frequency (NMPF) of eight major muscles. The sample consisted of 25 healthy males (age: 30 ± 2.2 years). Borg’s CR-10 scale was used for perceived exertion for two segments of the body (lower and upper) and the whole body. The lower extremity of the body was observed to be dominant compared to the upper and whole body in perceived response. First mode of principal component analysis (PCA) was obtained through the covariance matrix for the eight muscles for 25 subjects for NMPF of eight muscles. The diagonal entries in the covariance matrix were observed for each muscle. The muscle with the highest absolute magnitude was observed across all the 25 subjects. The medial deltoid and the rectus femoris muscles were observed to have the highest frequency for each PCA across 25 subjects. The rectus femoris, having the highest counts in all subjects, validated that the lower extremity dominates the sense of whole body fatigue during squat lifting. The findings revealed that it is significant to take into account the relation between perceived and measured effort that can help prevent musculoskeletal disorders in repetitive occupational tasks.

Interepisode Sleep Bruxism Intervals and Myofascial Face Pain

Study Objectives

Sleep bruxism (SB) is considered as a possible etiological factor for temporomandibular disorder (TMD) pain. However, polysomnographic (PSG) studies, which are current "gold standard" diagnostic approach to SB, failed to prove an association between SB and TMD. A possible explanation could be that PSG studies have considered only limited characteristics of SB activity: the number of SB events per hour and, sometimes, the total duration of SB per night. According to the sports sciences literature, lack of adequate rest time between muscle activities leads to muscle overloading and pain. Therefore, the aim of this study was to determine whether the intervals between bruxism events differ between patients with and without TMD pain.

Methods

Two groups of female volunteers were recruited: myofascial TMD pain group (n=124) and non-TMD control group (n=46). From these groups, we selected 86 (69%) case participants and 37 (80%) controls who had at least two SB episodes per night based on PSG recordings. A linear mixed model was used to compare case and control groups over the repeated observations of interepisode intervals.

Results

The duration of interepisode intervals was statistically similar in the case (mean [standard deviation {SD}] 1137.7 [1975.8] seconds)] and control (mean [SD] 1192.0 [1972.0] seconds) groups. There were also a similar number of SB episodes per hour and a total duration of SB episodes in both groups.

Conclusions

The current data fail to support the idea that TMD pain can be explained by increasing number of SB episodes per hour of sleep or decreasing the time between SB events.