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.     

Fatigue during prolonged intermittent overhead work: reliability of measures and effects of working height

Shoulder pain is prevalent among industrial workers and existing evidence supports that overhead work is an important specific risk factor. Existing guidelines are limited, with overhead work typically recommended to be avoided, and research on overhead work has been mixed in terms of the effects of increasing arm reach. A laboratory-based simulation of overhead work was conducted, at three working heights, in order to facilitate improved guidelines and to identify potential non-linear effects of overhead work height. Several indicators of shoulder fatigue served as outcome measures and a preliminary study was performed to assess the reliability of several of these measures. Fatigue measures based on electromyography (EMG) generally had low reliability, whereas excellent reliability was exhibited for ratings of perceived discomfort (RPD). Consistent with this, no effects of overhead work height were found on EMG-based measures, yet clear non-linear effects were found on RPD and task performance. The source of the effects of work height appeared to be related to a combination of muscle activation levels and demands on precision/control at the highest location. These results support the utility of subjective measures for relatively low-level intermittent exertions and demonstrate increasingly detrimental fatigue and performance effects at extremes in reach during overhead work.     

Electromyographic fatigue in neck/shoulder muscles and endurance in women with repetitive work

EMG was recorded with surface electrodes from the trapezius and deltoid muscles during a static endurance test at approximately 20% of maximal voluntary contraction. Objective parameters for localized muscular fatigue were derived from the time course of the root mean square (RMS) and mean power frequency (MPF) of the EMG recordings. Isotonic regression is introduced as a tool for assessment of such parameters. The most pronounced sign of fatigue for trapezius was an increase in the RMS values, while for deltoid it was a decrease in the MPF values. This could be explained by the different functions of the two muscles. The endurance time for a group of 11 women in industrial work with repetitive short-cycled work tasks who were diagnosed with neck/shoulder disorders (tension neck) was significantly shorter (p<0·05) than for a group with the same work, but without neck/shoulder disorders (n=ll), and shorter than for a control group (n=ll). Regarding the EMG fatigue measures, there were no significant differences between the three groups. We did not find any relationships between endurance time and the EMG parameters. The results indicate that neck/shoulder disorders were not associated with divergent mechanisms for developing fatigue in the muscles, as recorded with surface EMG.     

Muscular fatigue and endurance during intermittent static efforts: effects of contraction level, duty cycle, and cycle time

OBJECTIVE: To determine the effects of intermittent task parameters on muscle fatigue and endurance time during static shoulder abductions, with a long-term goal of establishing relationships between intermittent task parameters and short-term performance. BACKGROUND: Effects of intermittent work on endurance and fatigue development have been reported, and certain combinations of contraction levels and duty cycles have been proposed as design guidelines. These findings, however, were not derived from systematic manipulations of the task parameters. METHOD: Prolonged (1-hr maximum) intermittent shoulder abductions were performed at different combinations of contraction level (12% or 28% of muscle strength), duty cycle (.25 or .75), and cycle time (34 or 166 s). Fatigue was measured based on reductions in muscle strength and indirectly by changes in ratings of discomfort, electromyographic (EMG) amplitude, and EMG spectral distribution. RESULTS: Contraction level and duty cycle significantly affected endurance time and muscle fatigue, and interactive effects between these parameters were observed for some of the measures. Significant effects of cycle time were found only for EMG spectral measures. CONCLUSION: Endurance time and local fatigue were dependent on the comprehensive effects of the different task parameters. APPLICATION: Design changes to reduce the occurrence of localized fatigue during intermittent work need to take into account all the task parameters simultaneously.     

The effect of a resistance-training program on muscle strength, physical workload, muscle fatigue and musculoskeletal discomfort: An experiment

The aim of the study was to investigate the effectiveness of a resistance-training program on muscle strength of the back and neck/shoulder muscles, relative physical workload, muscle fatigue and musculoskeletal discomfort during a simulated assembly and lifting task. Twenty-two workers were randomized over an 8-week resistance-training group, and a control group. Isokinetic muscle strength was assessed using the Cybex dynamometer, muscle fatigue was measured using EMG, and perceived discomfort was measured using a 10-point scale. At the follow-up, we found no effect of the resistance-training program on isokinetic muscle strength of the back and shoulder muscles. Furthermore, we did not find any effect on EMG data, nor on musculoskeletal discomfort during the simulated work tasks. However, trained workers performed the lifting tasks for a longer time before reporting considerable discomfort than those in the control group.     

Shoulder muscle fatigue development in young and older female adults during a repetitive manual task

Age may modify the association between occupational physical demand and muscle loading, and ultimately increase the risk of musculoskeletal disorders. The goal of this study was to investigate age-related differences in shoulder muscle fatigue development during a repetitive manual task. Twenty participants in two age groups completed an 80-minute simulated low-intensity assembly task. Electromyographic (EMG) manifestation of muscle fatigue was observed in the upper trapezius, deltoid and infraspinatus muscles in both age groups, and coincided with an increase in the subjective ratings of perceived exertions. Compared with the younger group, older group showed a more monotonic decrease in EMG power frequency in the upper trapezius and deltoid muscles. However, the age-related difference in EMG amplitude was less consistent. Relative rest time of the upper trapezius muscle in the older group was less than the young group throughout the task. The observed patterns of EMG measures suggest that older participants may have disadvantages in fatigue resistance in the upper trapezius and posterior deltoid muscles during the simulated repetitive manual task.     

Predicted endurance times during overhead work: influences of duty cycle and tool mass estimated using perceived discomfort

Abstract

A need for overhead work remains in several industries and such work is an important risk factor for shoulder musculoskeletal problems. In this study, we evaluated the effects of duty cycle and tool mass on endurance times during overhead work. A psychophysical approach was used, via a new methodology that was implemented to more efficiently estimate endurance times (rather than through direct measurements). Participants performed a simulated overhead task in specified combinations of tool mass and duty cycle. Both duty cycle and tool mass have substantial effects on the development of fatigue and estimated endurance times, though the former was more substantial and an interactive effect was evident. Gender differences were not substantial, except when using the largest tool mass. We recommend that, for two-hour periods of overhead work, tool masses greater than 1.25 kg should be avoided, as should duty cycles greater than 50%.

Practitioner Summary: The current results may facilitate enhanced design and evaluation of overhead work tasks. In addition, the new estimation approach that was employed may enhance the efficiency of future studies using a psychophysical approach (ie using extrapolation of patterns of reported discomfort to predict longer term outcomes).     

Differences in functional performance of the shoulder musculature with obesity and aging

The workforce includes an increasing number of workers who are obese and/or older, which may lead to higher rates of workplace injuries. We examined the main and interactive effects of obesity and age on strength and functional performance during sustained isometric exertions involving shoulder flexion in two postures. Four groups of eight participants each (non-obese (18.5 < BMI < 25 kg/m²) young (18–25 years), non-obese older (50–65 years), obese (30 < BMI < 40 kg/m²) young, and obese older) completed static endurance tasks in each posture, at fixed target levels of shoulder moment. Shoulder strength was ∼25% higher with obesity and equivalent between age groups. Both obesity and age affected endurance time, with the obese and younger groups both having shorter endurance. Obesity and age did not have an interactive effect on endurance time and the results were inconclusive regarding acute fatigue effects for individuals who are older and obese. Further work is needed under more realistic task conditions, to explore the likely complex effects of these individual differences.     

The influence of age on isometric endurance and fatigue is muscle dependent: a study of shoulder abduction and torso extension

The present study examined differences in isometric muscle capacity between older (55–65 years) and younger (18 – 25 years) individuals. A total of 24 younger and 24 older participants (gender balanced within each group) performed sustained shoulder abductions and torso extensions to exhaustion at 30%, 50% and 70% of individual maximal voluntary contraction (MVC). Along with endurance time, manifestations of localized fatigue were determined based on changes in surface electromyographic signals obtained from the shoulder (middle deltoid) and the torso (multifidus and longissimus thoracis) muscles. Strength recovery was monitored using post-fatigue MVCs over a 15-min period. Compared to the younger group, older individuals exhibited lower muscular strength, longer endurance time and slower development of local fatigue. Age effects on fatigue were typically moderated by effort level, while effects of gender appeared to be marginal. Non-linear relationships between target joint torque and endurance time were observed, with effects of age differing between shoulder abduction and torso extension. Overall, the effects of age on endurance and fatigue were more substantial and more consistent for the shoulder muscle than for the torso muscles and were likely related to differences in muscle fibre type composition. For strength recovery rates, no significant age or gender effects were found in either experiment. In summary, this study suggests that differences in isometric work capacity do exist between older and younger individuals, but that this effect is influenced by effort level and the muscle tested.     

Rapid Communication Effects of posture,weight and frequency on trunk muscular activity and fatigue during repetitive lifting tasks

Abstract

We examined the effects of posture, weight and frequency on trunk muscular activity and fatigue during repetitive dynamic lifting. Electromyographic (EMG) signals from eight primary trunk muscles were collected during 120 min for four different task conditions. The patterns of muscle recruitment and the levels of relative activation were analysed using the normalized EMG data. Median power frequency (MPF) shift patterns were analysed to examine muscular fatigue. The muscles in the dorsal part of the trunk were activated at the symmetric posture, while the muscles on the contralateral side to the workload were more strongly activated at the asymmetric posture. Decreasing trends of MPFs were found in some active muscles, and they were more pronounced for the asymmetric posture than for the symmetric posture. It was also seen that the muscles became fatigued faster for the light load-high frequency task than for the heavy load-low frequency task.     

A novel approach for determining fatigue resistances of different muscle groups in static cases

In ergonomics and biomechanics, muscle fatigue models based on maximum endurance time (MET) models are often used to integrate fatigue effect into ergonomic and biomechanical application. However, due to the empirical principle of those MET models, the disadvantages of this method are: 1) the MET models cannot reveal the muscle physiology background very well; 2) there is no general formation for those MET models to predict MET. In this paper, a theoretical MET model is extended from a simple muscle fatigue model with consideration of the external load and maximum voluntary contraction in passive static exertion cases. The universal availability of the extended MET model is analyzed in comparison to 24 existing empirical MET models. Using mathematical regression method, 21 of the 24 MET models have intraclass correlations over 0.9, which means the extended MET model could replace the existing MET models in a general and computationally efficient way. In addition, an important parameter, fatigability (or fatigue resistance) of different muscle groups, could be calculated via the mathematical regression approach. Its mean value and its standard deviation are useful for predicting MET values of a given population during static operations. The possible reasons influencing the fatigue resistance were classified and discussed, and it is still a very challenging work to find out the quantitative relationship between the fatigue resistance and the influencing factors.

Relevance to industry

MSD risks can be reduced by correct evaluation of static muscular work. Different muscle groups have different properties, and a generalized MET model is useful to simplify the fatigue analysis and fatigue modeling, especially for digital human techniques and virtual human simulation tools.     

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

Abstract

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.     

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.     

Electromyographical study on surgeons in urology. II. Determination of muscular fatigue

Abstract

An electromyographical fatigue analysis was performed in the operating theatre on four surgeons during 14 urological operations in which ‘direct endoscopy’ (cf. Luttmann el al. 1996, Part I) was applied. Surface electromyograms (EMG) were derived from the m. trapezius on both sides of the body, the right m. deltoideus, and the left m. erector spinae. The surgeons’ activities were documented by simultaneously recording an electrical activity code signal parallel to the EMGs. The electrical activity (EA) was formed from the raw EMG by rectification and continuous averaging. For purposes of spectral analysis, the EMGs were digitized and converted to the frequency domain by Fast Fourier Transformation. During the performance of endoscopic surgery, an increase in EA, as well as a spectral shift towards lower frequencies, are observed for at least one of the muscles under test in all of the operations. This indicates the development of muscular fatigue in the course of the operations. The finding is confirmed by applying a newly developed method for the joint analysis of the spectrum and the amplitude of EMGs (JASA) which permits discrimination between fatigue-induced and force-related changes in the EMG. Utilizing this method, the development of fatigue was confirmed for 11 of the 14 operations in the case of the right m. trapezius. The right trapezius muscle therefore constitutes a bottleneck for the performance of the operations. The endurance time was estimated from the increase in EA and compared with the actual duration of the operations. Since both are of the same order of magnitude, it is concluded that the final part of an operation has to be performed when the muscles are already fatigued. Application of the newly available method known as ‘monitor endoscopy’ (cf. Part I) can lead to a reduction in muscular strain and fatigue. This, in turn, will enable operations to be performed at less risk to the patient.     

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.     

The influence of task design on upper limb muscles fatigue during low-load repetitive work: A systematic review

Ergonomic interventions such as increased scheduled breaks or job rotation have been proposed to reduce upper limb muscle fatigue in repetitive low-load work. This review was performed to summarize and analyze the studies investigating the effect of job rotation and work-rest schemes, as well as, work pace, cycle time and duty cycle, on upper limb muscle fatigue. The effects of these work organization factors on subjective fatigue or discomfort were also analyzed. This review was based on relevant articles published in PubMed, Scopus and Web of Science. The studies included in this review were performed in humans and assessed muscle fatigue in upper limbs. 14 articles were included in the systematic review. Few studies were performed in a real work environment and the most common methods used to assess muscle fatigue were surface electromyography (EMG). No consistent results were found related to the effects of job rotation on muscle activity and subjective measurements of fatigue. Rest breaks had some positive effects, particularly in perceived discomfort. The increase in work pace reveals a higher muscular load in specific muscles. The duration of experiments and characteristics of participants appear to be the factors that most have influenced the results. Future research should be focused on the improvement of the experimental protocols and instrumentation, in order to the outcomes represent adequately the actual working conditions.Relevance to industryIntroducing more physical workload variation in low-load repetitive work is considered an effective ergonomic intervention against muscle fatigue and musculoskeletal disorders in industry. Results will be useful to identify the need of future research, which will eventually lead to the adoption of best industrial work practices according to the workers capabilities.     

Temporal strategy and performance during a fatiguing short-cycle repetitive task

This study investigated temporal changes in movement strategy and performance during fatiguing short-cycle work. Eighteen participants performed six 7-min work blocks with repetitive reaching movements at 0.5 Hz, each followed by a 5.5-min rest break for a total duration of 1 h. Electromyography (EMG) was collected continuously from the upper trapezius muscle, the temporal movement strategy and timing errors were obtained on a cycle-to-cycle basis, and perceived fatigue was rated before and after each work block. Clear signs of fatigue according to subjective ratings and EMG manifestations developed within each work block, as well as during the entire hour. For most participants, timing errors gradually increased, as did the waiting time at the near target. Changes in temporal movement strategy were negatively correlated with changes in the level and variability of EMG, suggesting that an adaptive temporal strategy offset the development of unstable motor solutions in this fatiguing, short-cycle work. PRACTITIONER SUMMARY: Sustained performance of operators is essential to maintain competitiveness. In this study of repetitive work, participants gradually changed their temporal movement strategy, for possibly alleviating the effects of fatigue. This suggests that in order to effectively counteract fatigue and sustain performance, industrial production should allow extensive spatial and temporal flexibility.     

Structural stability and reliability of the Swedish occupational fatigue inventory among Chinese VDT workers

The aim of the present study was to test the structural stability and reliability of the Swedish occupational fatigue inventory (SOFI) for use in a group of Chinese visual display terminal (VDT) workers. A qualified translator was recruited to translate the Chinese version of the SOFI (SOFI-C). The content validity was established with 12 bilingual practitioners and seven professional experts. The translated SOFI was administered to 104 sedentary workers on two occasions with an interval of 60 min. Most of them were female (80.8%) and they had a mean age of 34.5 years. Fifty-one percent of them reported using a VDT for 4h or more at work. Exploratory factor analysis revealed a five-factor solution, which was comparable to the original latent factors. Cronbach's alpha for the five-factor scales was between 0.88 and 0.95. The test-retest reliability was satisfactory with intra-class correlations ranging from 0.69 to 0.83. The workers who used a VDT for 4h or more had significantly higher SOFI scores than those who used one for less than 4 h (p = 0.007 - 0.046). The results indicated that the SOFI-C was valid and reliable for measuring fatigue among Chinese sedentary workers. The satisfactory structural stability suggested that cultural influences on the construct of fatigue were not strong. Its characteristics of discrimination of the sedentary workers who had high VDT exposure suggested that the SOFI-C would be a useful instrument for prevention and intervention programs designed for work-related injuries in the workplace.