Muscular fatigue and maximum endurance time assessment for male and female industrial workers

A single arm pushing experiment was conducted in an electronic factory in Yantai, China to assess muscular fatigue using the theoretical models of muscular strength and maximum endurance time (MET) developed by Ma et al. (2009). Seventy seven workers, including 38 males and 39 females, participated in the study. The muscular strength of pushing was measured after the subject pushed a stick, with a force of 2.5 kgf, for 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0 min. Fatigue rate was determined based on a regression approach. In addition to the theoretical model, the MET for such a pushing task was also determined using four empirical models in the literature. The results indicated that females were more resistant to muscular fatigue than males in the pushing task. The results of the muscular strength prediction show that the predictability of the muscular strength model is acceptable. The prediction errors for muscular strength for female subjects were significantly lower than those of the male subjects. The predicted MET using the theoretical model, with a group constant k, was highly correlated with those using the empirical models compared in the current study.     

Physiology-based dynamic muscle fatigue model for upper limbs during construction tasks

Due to physically demanding construction tasks, workers frequently suffer from significant levels of muscle fatigue that can cause diverse detrimental effects on safety, health, and productivity. In this regard, evaluating the level of muscle fatigue prior to work is essential to take proper preventive actions before severe fatigue takes place. Although previous research efforts have quantified muscle fatigue using surveys, instruments, and mathematical models, most of them do not take into account irregularly varying muscle activation and fatigue recovery during a task. They are thus limited, especially for construction tasks that have varying forces and intermittent idling/resting periods. This study thus proposes a physiology-based modeling approach to computationally model and empirically validate dynamic muscle fatigue generation and recovery for construction workers through laboratory testing. Specifically, a muscle fatigue estimation model for upper limbs based on System Dynamics, which is a differential equation-based continuous simulation, is developed based on fundamental physiological mechanisms of the accumulation and clearance of intramuscular metabolites during muscle exertion and their effects on muscle contractile processes. Then the model is refined and validated through laboratory experiments. The results demonstrated the immense potential for the developed elbow and shoulder models to evaluate workers' muscle fatigue in upper limbs under varying workloads. The contribution of this study is to provide an analytic tool for understanding the physiological mechanisms of muscle fatigue and estimating workers' muscle fatigue levels during construction tasks, which can help to design appropriate interventions prior to work, thereby reducing undesirable results from muscle fatigue.     

The effects of joint angle variability and different driving load scenarios on maximum muscle activity – A driving posture simulation study

To date, no studies have been conducted on the main and interaction effects of joint angles on maximum muscle activity in different driving load scenarios. To investigate the influence of joint angle variability on the muscular system, this study calculated maximum muscle activity during three static driving load scenarios through the use of musculoskeletal inverse dynamic simulation. Six joint angles in sagittal plane were varied with reference to reported driving posture angles in the literature. A digital manikin with a height of 180 cm and weight of 70 kg was used with simple muscles and a minimum fatigue criterion for muscle activation optimization. Three static driving load scenarios were simulated: sitting with no external forces except gravity, steering, and pedaling operation. Prediction models were developed for each driving load scenario using Least Squares Support Vector Machine. Finally, the Pareto optimization method was applied for multi-objective optimization combining the three developed models.The results indicate that the developed models can be used for the prediction of simulated maximum muscle activity. The six joint angles explain a higher percentage of maximum muscle activity variance in the steering and pedaling operation scenarios compared to the sitting scenario. The six joint angles differ in their main and interaction effects on maximum muscle activity depending on the driving load scenario. The optimum joint angle values of the driving posture depend on the driving load scenarios. The different driving postures based on minimum maximum muscle activity are presented for the three driving load scenarios.Relevance to industryThe results of this study can be utilized in establishing driving posture simulation models to improve vehicle interiors during the early development stage. Furthermore, the results of this study can provide base data for the development of a tool for real driving posture evaluation of maximum muscle activity.     

Muscle fatigue during intermittent isokinetic shoulder abduction: Age effects and utility of electromyographic measures

Most existing evidence regarding the effects of age on muscular fatigue has focused on prolonged isometric contractions, repeated maximum dynamic contractions and individuals beyond traditional retirement age (>65 years). In the present study, age-related differences in muscle fatigue during submaximal dynamic efforts were examined. There were 24 younger (18–25 years) and 24 older (55–65 years) participants, all of whom were healthy and active, with equal numbers of each gender within each age group. Participants performed repetitive, intermittent shoulder abductions until exhaustion, at peak moments of 30% and 40% of individual maximum voluntary isokinetic contraction (MVIC) and with cycle durations of 20 and 40 s. Fatigue development was determined based on changes in MVIC, electromyographic (EMG) signals and ratings of perceived discomfort (RPD). Following the exhaustive exercises, strength recovery was monitored using a series of MVICs over a 15-min period. Results indicated the existence of an age-related fatigue resistance, with the older group demonstrating significantly slower rates of MVIC decline and RPD increase and smaller modifications in EMG-based fatigue measures. These age effects were generally more pronounced at the higher effort level. Main effects of effort level and cycle duration were also significant, while gender effects appeared to be marginal. Rates of strength recovery were not significantly influenced by age. In addition, the utility of standard EMG-based fatigue measures was assessed. Findings indicated that time-dependent changes in static and dynamic EMG-based measures were roughly comparable in terms of sensitivity and variability, supporting the use of standard EMG analyses for fatigue monitoring during intermittent dynamic contractions.     

A study stressing the need for a static postural force model for work analysis

The maximum endurance time (MET) in static force exertions was used as a parameter for the assessment of five working postures. By applying the methodology of Rohmert to the construction of a general model for static muscular work and evaluating the measured MET results, the need for a new static posture model has been shown.

The aim of the present pilot study was to test MET in load situations that would indicate when the general model can be used or when a new static postural force model is needed.

Subjects exerted static postural forces at different load levels until exhaustion. In the first two postures, the strain was concentrated on the upper limbs, where active forces (muscular) play a key-role and justify the use of the model. In the remaining postures, the strain affected mainly the back/trunk, where the mechanical equilibrium of the body is brought about by active (muscles) and passive (skeleton and ligaments) structures.

During the tests electromyographic (EMG) measurements of selected muscles (objective measurements) as well as rated perceived exertion (RPE; subjective measurements on Borg's CR-10 scale) were recorded.

The results show that the maximum endurance times in upper extremity postures are predicted by the general model whilst in the back/trunk postures the measured MET was longer than predicted by the model. New models are presented for static postural force on the back.

The EMG measurements supported the conclusion that the muscles studied play a key role in the chosen upper-extremity postures but gave no clear indication in the back-oriented postures. Ratings of perceived exertion coincided with the EMG measurements in upper extremity postures and proved to be a good substitute for measurement and calculation of the load levels studied. The initial RPE can therefore be used in models for predicting maximal endurance times in complex cases.

For the range of relative postural loads tested, an exponential function for predicting MET in static posture exertions produced the best fit curve.     

Real-time indicators and influence factors of muscle fatigue in push-type work

Muscle fatigue is a significant cause of musculoskeletal injury and can easily induce unsafe behaviour. Push-type work is a common type of physical work, and if not designed appropriately, may lead to muscle fatigue. Previous studies on muscle fatigue mainly focus on investigating continuous force exertion, and in most of them, a constant muscle force is assumed, thereby ignoring the fluctuations present in exertion. In this study, bolt hole drilling was chosen to represent typical push-type work, and the muscle fatigue from this work was examined. The experimental system designed in this study monitored the muscle force in real time. In the experiments, different thrust angles (15°, 45°, and 75°), different relative force values (20% MVC, 40% MVC, and 60% MVC; MVC: maximum voluntary contraction) and different working time intervals (0 s, 30 s, 60 s, and 90 s) were considered. The results demonstrate that there is a significant positive correlation between the rating of perceived exertion (RPE) and muscle force attenuation (r = 0.786, p = 0). The cubic regression model (Y = − 0.00071x³ − 0.024x² − 0.334x + 1.146, R² = 0.639) fits the data most closely. Therefore, force attenuation can be used as a real-time indicator of muscle fatigue. In addition, the relative force value and thrust angle have a significant impact on the RPE score, whereas the working time interval has no major effect on it. This study provides a new method for evaluating muscle fatigue and a basis for the design of push-type work to reduce fatigue-induced accidents and musculoskeletal injuries.     

Fatigue induced by static work

Abstract

Despite its low energy cost, isometric contraction can result in the onset of local muscle fatigue. The onset of fatigue occurs more rapidly when the relative force exerted is greater than 15–20% of the maximum voluntary contraction (MVC) of the muscle considered, and when the contraction time is increased. The maximum maintenance time (limit-time) and the corresponding relative force are linked by a hyperbolic relation. Ischaemia promotes accumulation of acid metabolites produced during contraction, and hinders their elimination, thus constituting the main causal factor in the onset of local muscle fatigue. The introduction of rest periods of sufficient duration to ensure restoration of normal blood flow through the muscle is an effective way of delaying, or even preventing, the onset of muscle fatigue. Other factors may also be taken into account, such as the position in which the static work is performed, and the nature and number of muscles used simultaneously, etc. Numerous laboratory and field studies have allowed the development of various models that take into account the conditions relating to isometric contractions during static work.     

Experimental validation of a subject-specific maximum endurance time model

This study aimed at experimentally validating a subject-specific maximum endurance time (MET) model. Thirty health participants (15 males and 15 females; Age: mean = 21.5 years, SD = 1.6 years) volunteered to conduct an isometric elbow flexion task until exhaustion. The endurance times of each participant were measured under relative exertion levels ranging from 30% MVC (Maximum Voluntary Contraction) to 70% MVC at 10% intervals. Assessment of the model showed that the intensity–endurance time relationship for each studied individual could be well fitted by the subject-specific MET model (R² > 0.89). The fatigue rates identified from the model fitting were normally distributed (Mean = 0.96 min^?1, SD = 0.29 min^?1). In addition, the fatigue rates of the male group were significantly higher than the female group. The subject-specific MET model can be used to predict the MET for individual workers, and further support physical task design, based on the fatigability data of a targeted worker population.

Practitioner Summary: Ergonomists have extensively used MET models in physical fatigue assessment and physical task design. A subject-specific MET model could be used to predict the MET at individual levels, and also to support work design for a target worker population, based on the fatigability data distribution obtained from sampled workers.     

Fatigue induced by static work

Despite its low energy cost, isometric contraction can result in the onset of local muscle fatigue. The onset of fatigue occurs more rapidly when the relative force exerted is greater than 15-20% of the maximum voluntary contraction (MVC) of the muscle considered, and when the contraction time is increased. The maximum maintenance time (limit-time) and the corresponding relative force are linked by a hyperbolic relation. Ischaemia promotes accumulation of acid metabolites produced during contraction, and hinders their elimination, thus constituting the main causal factor in the onset of local muscle fatigue. The introduction of rest periods of sufficient duration to ensure restoration of normal blood flow through the muscle is an effective way of delaying, or even preventing, the onset of muscle fatigue. Other factors may also be taken into account, such as the position in which the static work is performed, and the nature and number of muscles used simultaneously, etc. Numerous laboratory and field studies have allowed the development of various models that take into account the conditions relating to isometric contractions during static work.     

Getting the most out of it: Optimal experiments for parameter estimation of microalgae growth models

Mathematical models are expected to play a pivotal role for driving microalgal production towards a profitable process of renewable energy generation. To render models of microalgae growth useful tools for prediction and process optimization, reliable parameters need to be provided. This reliability implies a careful design of experiments that can be exploited for parameter estimation. In this paper, we provide guidelines for the design of experiments with high informative content based on optimal experiment techniques to attain an accurate parameter estimation. We study a real experimental device devoted to evaluate the effect of temperature and light on microalgae growth. On the basis of a mathematical model of the experimental system, the optimal experiment design problem was formulated and solved with both static (constant light and temperature) and dynamic (time varying light and temperature) approaches. Simulation results indicated that the optimal experiment design allows for a more accurate parameter estimation than that provided by the existing experimental protocol. For its efficacy in terms of the maximum likelihood properties and its practical aspects of implementation, the dynamic approach is recommended over the static approach.     

Enhancing Ergonomic Safety Effectiveness of Repetitive Job Activities: Prediction of Muscle Fatigue in Dominant and Nondominant Arms of Industrial Workers

Work?related musculoskeletal disorders are very common in the workplace, especially with tasks involving repetitive lifting and motions. Repetitive lifting of excessively heavy objects in the workplace could increase the severity and rates of work?related musculoskeletal disorders. In this article, the physiological effect of muscle fatigue on the dominant and nondominant arms of adult industrial workers performing various repetitive tasks was predicted using a muscular endurance model. Twenty?four (n = 24) industrial workers (18–45 years old) were randomly selected for this research. The effects of electromyography (EMG) were observed during incremental loading of 5–40 kg on the muscle of the dominant and nondominant arms of the subjects during static lifting activities. Results of the analysis showed that the endurance time decreased with the application of additional loads on the dominant and nondominant arms of all the subjects. This inverse relationship was used to predict the behavior of muscle fatigue. Additional findings indicated that workers performing repetitive lifting tasks could maintain maximum load capacities ranging from 20 to 30 kg. The acceptable maximum load capacity of 23 kg recommended by the National Institute for Occupational Safety and Health is within this range. The results obtained from this research could be used in the beginning steps of the efforts to reevaluate and reestablish guidelines and limits in the design of industrial jobs involving repetitive motion. © 2014 Wiley Periodicals, Inc.     

Nonlinear modeling for bar bond stress using dynamical self-adjusted harmony search optimization

Design code provisions for reinforced concrete are often based on empirical relations resulting from simple statistical treatments of experimental data. Hence, they may provide inaccurate results for predicting complex structural behavior. In the present study, novel nonlinear regression for prediction of the reinforcing bar development length is developed using dynamical self-adjusted harmony search optimization. The nonlinear mathematical relations are regressed using 534 results of simple pullout tests on short unit bar lengths. A novel bi-nonlinear expression is proposed, and its predictive capability outperformed that of design code formulas such as the ACI 318-14, ACI 408R-03, and Eurocode 2 along with other existing empirical models. A parametric study was conducted to explore the sensitivity of the proposed models to influential input parameters. It was found that the new model offers a powerful predictive tool for reinforcing bar bond strength which differs from that of existing models that assume unrealistic uniform bond stress along the rebar. This flexible and data-intensive model could be further scrutinized for consideration in future design code revisions and enhancements.

     

Log-modified Weibull regression models with censored data: Sensitivity and residual analysis

This paper proposes a regression model considering the modified Weibull distribution. This distribution can be used to model bathtub-shaped failure rate functions. Assuming censored data, we consider maximum likelihood and Jackknife estimators for the parameters of the model. We derive the appropriate matrices for assessing local influence on the parameter estimates under different perturbation schemes and we also present some ways to perform global influence. Besides, for different parameter settings, sample sizes and censoring percentages, various simulations are performed and the empirical distribution of the modified deviance residual is displayed and compared with the standard normal distribution. These studies suggest that the residual analysis usually performed in normal linear regression models can be straightforwardly extended for a martingale-type residual in log-modified Weibull regression models with censored data. Finally, we analyze a real data set under log-modified Weibull regression models. A diagnostic analysis and a model checking based on the modified deviance residual are performed to select appropriate models.     

Effect of fatigue on the stationarity of surface electromyography signals

The estimation of muscle fatigue using surface electromyography (SEMG) is of high relevance to evaluate ergonomic risk factors in the occupational settings. Signal stationarity plays an important role while selecting appropriate SEMG signal processing method for fatigue evaluation. The Fourier algorithm based signal processing methods (mean or median frequency of power spectrum) rely on the assumption that the signal under investigation is stationary. Stationarity of SEMG signals and its association with fatigue is rarely studied in the ergonomics literature. Therefore, this study was aimed at understanding the effect of fatigue on the stationarity of the SEMG data. Ten participants performed 40 min of fatiguing upper extremity exertions and SEMG data were recorded from the right upper trapezius muscle. The SEMG data recorded under static and dynamic conditions at the beginning and at the end of fatiguing exertions were used in the analysis. The stationarity analysis was performed for five window sizes of 128, 256, 512, 768 and 1024 ms using modified reverse arrangement test. The results showed that the muscle fatigue reduced the stationarity of the SEMG signal under static and dynamic conditions. The relationship between the muscle fatigue and the stationarity of the SEMG signal was found to be significant at the window size of 512 ms. A significantly higher fatigue related decrease in the stationarity was observed during dynamic exertions compared to the static exertions.Relevance to industryThe findings from the current study illustrate that the stationarity of SEMG signals could be used to quantify muscle fatigue under static and dynamic task conditions. These findings are useful to the ergonomic practitioners in conducting muscle fatigue estimation using SEMG.     

Endurance time of grip-force as a function of grip-span, posture and anthropometric variables

The time to volitional exhaustion (endurance time) for sustained contractions is considered as a valid parameter to quantify fatigue and to determine the required rest pauses between two successive contractions. In this study, the effects of grip-span, shoulder posture and anthropometric characteristics on endurance time of grip-force during sustained 30% of maximal voluntary grip-force were investigated. Both subjective and objective measures of fatigue were used in determining the endurance times. Twelve male subjects performed sustained isometric handgrip contractions using a handgrip dynamometer at the combinations of three different grip span settings and two shoulder postures. The investigated three grip spans were the optimal, 2 cm narrower than the optimal, and 2 cm wider than the optimal. The investigated two shoulder postures were neutral and 25° flexion. The outcome measures were: endurance time, surface electromyography of related forearm muscles, heart rate, blood pressure, and ratings of perceived discomfort/pain. The results indicate that the endurance time decreases significantly as the grip span deviates from the optimal in both directions. On the other hand, the considered shoulder postures did not have a significant effect on the endurance time. Further analysis indicated a significant negative correlation between endurance time and rest pause and a marginal positive correlation between maximum voluntary grip-force and rest pause. Body mass index, and volume of forearm and hand had also significant negative correlation with endurance time. The comparisons are made with a number of existing endurance models and the impact of findings are discussed.

Relevance to industry

In accurate establishment of the time standards, muscular fatigue allowances need to be taken into account. The endurance time for sustained isometric contractions is correlated with the required rest allowances (pauses) for intermittent static contractions; and therefore, required muscular fatigue allowances can be estimated from the endurance times.     

Muscle fatigue during intermittent isokinetic shoulder abduction: age effects and utility of electromyographic measures

Most existing evidence regarding the effects of age on muscular fatigue has focused on prolonged isometric contractions, repeated maximum dynamic contractions and individuals beyond traditional retirement age (>65 years). In the present study, age-related differences in muscle fatigue during submaximal dynamic efforts were examined. There were 24 younger (18-25 years) and 24 older (55-65 years) participants, all of whom were healthy and active, with equal numbers of each gender within each age group. Participants performed repetitive, intermittent shoulder abductions until exhaustion, at peak moments of 30% and 40% of individual maximum voluntary isokinetic contraction (MVIC) and with cycle durations of 20 and 40 s. Fatigue development was determined based on changes in MVIC, electromyographic (EMG) signals and ratings of perceived discomfort (RPD). Following the exhaustive exercises, strength recovery was monitored using a series of MVICs over a 15-min period. Results indicated the existence of an age-related fatigue resistance, with the older group demonstrating significantly slower rates of MVIC decline and RPD increase and smaller modifications in EMG-based fatigue measures. These age effects were generally more pronounced at the higher effort level. Main effects of effort level and cycle duration were also significant, while gender effects appeared to be marginal. Rates of strength recovery were not significantly influenced by age. In addition, the utility of standard EMG-based fatigue measures was assessed. Findings indicated that time-dependent changes in static and dynamic EMG-based measures were roughly comparable in terms of sensitivity and variability, supporting the use of standard EMG analyses for fatigue monitoring during intermittent dynamic contractions.     

Muscle fatigue modelling: Solving for fatigue and recovery parameter values using fewer maximum effort assessments

A three-compartment controller model (3CC) predicts muscle fatigue development. Determination of fatigue (F) and recovery (R) model parameters is critical for model accuracy. Numerical methods can be used to determine parameter values using maximum voluntary contractions (MVCs) as input. We tested the effects of using reduced MVC data on parameter solutions using twenty published datasets of intermittent, isometric contractions. The work here examines three sampling variations using approximately half of the MVCs: MVC measurements distributed equally (dMVC), split between the initial and final times (sMVC), and only during the first half (fMVC). Furthermore, solved F and R parameters were used to model fatigue development for three hypothetical task scenarios. Both model parameters and predictions were statistically insensitive to measured data reduction using dMVC, followed closely by sMVC. However, using the fMVC reduction frequently resulted in overestimated parameter values and produced significantly larger prediction errors. We conclude that parameter solutions are robust when using fewer MVCs as long as they are sampled in a manner that captures later fatigue behavior.