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.     

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.     

Task variation during simulated,repetitive, low-intensity work – influence on manifestation of shoulder muscle fatigue,perceived discomfort and upper-body postures

Work-related musculoskeletal disorders are increasing due to industrialisation of work processes. Task variation has been suggested as potential intervention. The objectives of this study were to investigate, first, the influence of task variation on electromyographic (EMG) manifestations of shoulder muscle fatigue and discomfort; second, noticeable postural shoulder changes over time; third, if the association between task variation and EMG might be biased by postural changes. Outcome parameters were recorded using multichannel EMG, Optotrak and the Borg scale. Fourteen participants performed a one-hour repetitive Pegboard task in one continuous and two interrupted conditions with rest and a pick-and-place task, respectively. Manifestations of shoulder muscle fatigue and discomfort feelings were observed throughout the conditions but these were not significantly influenced by task variation. After correction for joint angles, the relation between task variation and EMG was significantly biased but significant effects of task variation remained absent.

Practitioner Summary: Comparing a one-hour continuous, repetitive Pegboard task with two interrupted conditions revealed no significant influences of task variation. We did observe that the relation between task variation and EMG was biased by posture and therefore advise taking account for posture when investigating manifestations of muscle fatigue in assembly tasks.     

Shoulder muscular activity in individuals with low back pain and spinal cord injury during seated manual load transfer tasks*

This study aimed to compare the activity of four shoulder muscles in individuals with low back pain (LBP), spinal cord injuries (SCI) and a control group, during one-handed load transfer trials. Nine individuals with minimum one-year of LBP, eleven with thoracic/lumbar SCI and nine healthy controls participated in this study. The activations of anterior deltoid, upper trapezius, infraspinatus and pectoralis major were recorded by surface EMG during one-handed transferring of a cylinder from a home shelve to six spatially distributed target shelves. The integrated EMG values were compared using repeated measure ANOVA. Both LBPs and SCIs had higher anterior deltoid activation and LBPs required more upper trapezius activation than controls (p < 0.05). The spatial position of the targets also significantly influenced demands for these two muscles. The anterior deltoid and upper trapezius in LBP and SCI individuals are under higher demand during occupational load transfer tasks.

Practitioner Summary: This study aimed to compare the activation of four shoulder muscles in individuals with low back pain, spinal cord injuries and healthy condition. EMG analysis showed that the injured groups required more upper trapezius and anterior deltoid activation during load transfer tasks, which may predispose them to muscle overexertion.     

Work load and fatigue in repetitive arm elevations

Six females performed continuous series of concentric and eccentric flexions in the shoulder between 0 and 90 degrees with 0 to 3.1 kg weights held in a powergrip. Heart rate (H R), perceived exertion (RPE) and myoelectric activity (EMG) from the descending part of the trapezius muscle, the anterior part of the deltoid muscle, and the biceps brachii were measured during the tasks. The increase of RPE was faster than the increase of HR with work load indicating an increased importance of local factors (i.e. strain on muscles and tendons) with load in the perceived exertion. The local muscular load determined by EMG on the trapezius muscle was closely correlated with the external torque produced in the glenohumeral joint. The time constants of EMG amplitude increase were correlated with work load, endurance time and with slope coefficients of RPE-HR linear regression. Symptoms and complaints 24 hours after the task were often localized to the descending part of the trapezius muscle. It is suggested that exertion of the descending part of the trapezius muscle in tasks involving repetitive shoulder flexion may promote discomfort and complaints referred to the neck.     

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.     

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).     

The ironies of vehicle feedback in car design

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).     

Net shoulder joint moment and muscular activity during light weight-handling at different displacements and frequencies.

The purpose of this study was to calculate net shoulder (gleno-humeral) joint moments from inverse dynamics and to measure muscular activity from six shoulder muscles (supraspinatus, infraspinatus, middle deltoid, anterior deltoid, trapezius, and pectoralis major) during light weight-handling at two different displacements (horizontal and vertical) and frequencies (40 and 60 cycles/min), to simulate an occupational cervicobrachial working task (light weight displacement). Ten normal adult male subjects were asked to move a known weight, representing 15% of the maximal lifted weight, in both horizontal and vertical conditions at frequencies of 40 cycles/min and 60 cycles/min. Raw EMG signals from six shoulder muscles were recorded and synchronized with the cinematographic data during three trials of 6 s each. The raw EMG signals of each muscle were full wave rectified and filtered at 3 Hz. The linear envelope (LE EMG) signals were normalized by time (% cycle) and by amplitude (% MVC), and for the analysis of variance, the normalized LE EMG signals were integrated (IN LE EMG). The average shoulder angular velocities, joint moments, and moment powers were computed from cinematographical data. No significant differences were observed between both tasks for the supraspinatus, infraspinatus, and pectoralis major IN LE EMG data as well as for integrated normalized shoulder joint moment for the whole cycle of movement. IN LE EMG data from middle deltoid, anterior deltoid, and trapezius muscles were significantly higher (p less than 0.05) when performing the vertical displacement task for the whole cycle of movement. This muscular activity difference between vertical and horizontal tasks indicated that the vertical displacement conditions induced higher muscular loads on the shoulder than the horizontal weight displacement conditions, although the vertical displacements were approximately 15% longer than the horizontal displacements. The non-significant difference of IN LE EMG between frequencies obtained for all muscles indicated that neither frequencies induced more muscular activity.     

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.     

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 less than 0.05) than for a group with the same work, but without neck/shoulder disorders (n = 11), and shorter than for a control group (n = 11). 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.     

The strain on the shoulder and neck muscles during letter sorting

Shoulder muscle strain was tudied during letter sorting in 10 healthy, experienced male mail carriers. Muscular load on the deltoid, trapezius and infraspinatus muscles was evaluated during a normal working day at the post office by vocational electromyography. To asses electromyographic indications of muscle fatigue the letter sorting was interrupted by a series of test contractions in six of the subjects. The study revealed an unacceptable static work load on the muscles during letter sorting. Electromyographic indications of muscle fatigue were present in the trapezius and infraspinatus muscles in three of the six subjects probably caused by sustained, elevated arm work. Recommendations for an improvement of the work place design are finally given.     

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.     

Sex differences in glenohumeral muscle activation and coactivation during a box lifting task

Abstract

Manual material handling is associated with shoulder musculoskeletal disorders, especially for women. Sex differences in glenohumeral muscle activity may contribute to women’s higher injury risk by affecting shoulder load and stability. We assessed the effects of sex (25 women vs 26 men) and lifting load (6?kg vs 12?kg) on muscle activation during box lifting from hip to eye level. Surface and intramuscular electromyography were recorded from 10 glenohumeral muscles. Most muscles were more activated for the heavier box and for women. These effects were larger for ‘prime movers’ than for stabilisers and antagonists. Despite their apparently heterogeneous effects on muscle activity, sex and mass did not affect Muscle Focus, a metric of coactivation. This may be partly related to the limited sensitivity of the Muscle Focus. Nevertheless, sex differences in strength, more than in coactivation patterns, may contribute to the sex imbalance in the prevalence of musculoskeletal disorders.

Practitioner summary: We studied sex differences in glenohumeral muscle activity in a lifting task to eye level. Women lifting a 6-kg box activated their muscles similarly to men lifting a 12-kg box, i.e. up to 48% of their maximum capacity. Interventions minimising shoulder load should be implemented, especially for women.

Abbreviations: BB: biceps brachii; DeltA: anterior deltoid; DeltL: lateral deltoid; DeltP: posterior deltoid; DoF: degrees of freedom; ED: effect duration; EMG: electromyography; ES: effect size; Infra: infraspinatus; Lat: latissimus dorsi; MF: muscle focus; MMH: manual material handling; MVA: maximal voluntary activation; Pect: pectoralis major; Subscap: subscapularis; Supra: supraspinatus; TB: triceps brachii     

The effect of shoulder posture on performance,discomfort and muscle fatigue whilst working on a visual display unit

An investigation was conducted to determine the relationship between posture and performance while working on a VDU. Twenty-one female subjects performed a 20 min choice reaction time task in a within-subject design with shoulder flexion (0° versus 30°) and noise (55 dB versus 75 dB) as factors. Discomfort measures, electromyography (EMG) measures and measures of performance including reaction time and percentage of errors were collected. The results suggested poorer performance in the 30° shoulder flexion posture compared to the 0° shoulder flexion posture (performance index F_1,20 = 3.95, p = 0.061). There was also significantly greater discomfort and fatigue, as indicated by EMG, in the 30° shoulder flexion compared to 0° shoulder flexion (discomfort: F_1,20 = 99.30; p = 0.0001 EMG: F_1,20 = 4.25, p = 0.052). The results of this study provide the most direct evidence that the posture of VDU users can affect their performance at a task. A number of possible mechanisms for posture to affect performance are explored using the data. The utility of data showing the effect of posture on performance data is discussed in terms of financial modelling to justify the cost of ergonomic intervention. Relevance to industryPoor posture whilst working at a VDU has been previously linked with the development of poor health but has not been directly linked with poor performance. This paper provides evidence to show posture can effect VDU performance. This enables organisations to use estimates of performance changes to model the costs of investment in ergonomic interventions.     

The effect of overhead drilling position on shoulder moment and electromyography

The effect of overhead drilling tasks on electromyographic (EMG) activity and shoulder joint moment was examined in this study. Twenty subjects simulated an overhead drilling task using a close, middle and far reach position while standing on either a lower or a higher step of a stepladder. Root mean square amplitude (AMP) of EMG activity from the dominant side anterior deltoid, biceps brachii and triceps brachii muscles was used to determine muscular load. Digital video was used to determine shoulder joint moment using 2-dimensional static link segment modelling in the sagittal plane. The results demonstrated that, compared to the far reach position, using the close reach position significantly decreased anterior deltoid AMP and biceps brachii AMP and moment, but increased triceps brachii AMP. Compared to the lower step, using the higher step significantly decreased anterior deltoid AMP and triceps AMP and moment, while increasing biceps AMP in the close position. There was no significant change noted in EMG median frequency indicating that fatigue was minimized. Moment increased monotonically with AMP. The findings indicated that workers performing overhead tasks should work close to their body in order to minimize shoulder forces. The implications of this recommendation are discussed.     

User discomfort,work posture and muscle activity while using a touchscreen in a desktop PC setting

An experimental study was conducted to evaluate physical risk factors associated with the use of touchscreen in a desktop personal computer (PC) setting. Subjective rating of visual/body discomfort, shoulder and neck muscle activity, elbow movement and user-preferred positions of the workstation were quantified from 24 participants during a standardised computer use task with a standard keyboard and a mouse (traditional setting), with a touchscreen and the standard keyboard (mixed-use condition) and with the touchscreen only. The use of a touchscreen was associated with a significant increase of subjective discomfort on the shoulder, neck and fingers, myoelectric activity of shoulder and neck muscles and percentage of task duration that arms were in the air. Participants placed the touchscreen closer and lower when using touch interfaces compared with the traditional setting. Results suggest that users would need more frequent breaks and proper armrests to reduce physical risks associated with the use of a touchscreen in desktop PC settings.

Statement of Relevance: In this study, subjective discomfort, work posture and muscle activity of touchscreendesktop PC users were quantitatively evaluated. The findings of this study can be used to understand potential risksfrom the use of a touchscreen desktop PC and to suggest design recommendations for computer workstations with the touchscreen.     

The effect of overhead drilling position on shoulder moment and electromyography

The effect of overhead drilling tasks on electromyographic (EMG) activity and shoulder joint moment was examined in this study. Twenty subjects simulated an overhead drilling task using a close, middle and far reach position while standing on either a lower or a higher step of a stepladder. Root mean square amplitude (AMP) of EMG activity from the dominant side anterior deltoid, biceps brachii and triceps brachii muscles was used to determine muscular load. Digital video was used to determine shoulder joint moment using 2-dimensional static link segment modelling in the sagittal plane. The results demonstrated that, compared to the far reach position, using the close reach position significantly decreased anterior deltoid AMP and biceps brachii AMP and moment, but increased triceps brachii AMP. Compared to the lower step, using the higher step significantly decreased anterior deltoid AMP and triceps AMP and moment, while increasing biceps AMP in the close position. There was no significant change noted in EMG median frequency indicating that fatigue was minimized. Moment increased monotonically with AMP. The findings indicated that workers performing overhead tasks should work close to their body in order to minimize shoulder forces. The implications of this recommendation are discussed.     

User discomfort, work posture and muscle activity while using a touchscreen in a desktop PC setting

An experimental study was conducted to evaluate physical risk factors associated with the use of touchscreen in a desktop personal computer (PC) setting. Subjective rating of visual/body discomfort, shoulder and neck muscle activity, elbow movement and user-preferred positions of the workstation were quantified from 24 participants during a standardised computer use task with a standard keyboard and a mouse (traditional setting), with a touchscreen and the standard keyboard (mixed-use condition) and with the touchscreen only. The use of a touchscreen was associated with a significant increase of subjective discomfort on the shoulder, neck and fingers, myoelectric activity of shoulder and neck muscles and percentage of task duration that arms were in the air. Participants placed the touchscreen closer and lower when using touch interfaces compared with the traditional setting. Results suggest that users would need more frequent breaks and proper armrests to reduce physical risks associated with the use of a touchscreen in desktop PC settings. Statement of Relevance: In this study, subjective discomfort, work posture and muscle activity of touchscreen desktop PC users were quantitatively evaluated. The findings of this study can be used to understand potential risks from the use of a touchscreen desktop PC and to suggest design recommendations for computer workstations with the touchscreen.