Muscle fatigue associated with repetitive arm lifts: effects of height,weight and reach

A simulated assembly-line task was constructed to investigate the metabolic cost and the pattern of muscular fatigue associated with selected arm lifts. Five healthy males lifted lead-filled cylinders five limes per minute in eight 1 hour sessions. In each session, the effect of varying the weight of the cylinders and the horizontal and vertical location or the point to which they were lifted was studied. Muscle fatigue was assessed from a frequency shift and rectified amplitude analysts of surface electromyographic records (EMG) of the deltoid, biceps, brachioradialis and upper trapezius muscles. Metabolic costs of the work were estimated from an analysis of heart rate and oxygen uptake. Additional measures included tests of strength decrement and ratings of perceived fatigue.

The results indicated that the most fatiguing task variable was weight. At the heavier of the two levels, weight significantly increased the EMG amplitude of the deltoid, biceps and brachioradialis muscles. Significant increases in the EMG amplitude of the biceps were also found when the variable of height was adjusted to the higher of two levels. There was a similar effect on the EMG level of the deltoid muscle when reach was at a maximum. Although not as dramatic as the increases in EMG activity, both heart rate and oxygen uptake were significantly increased when the variables of height and weight were at their maximum levels, i.e. eye level and 80% maximum voluntary contraction ( MVC) The results provided a physiological basis for evaluating the effects of three task variables in lifting tasks involving upper extremity muscles     

Working height and parts bin position effects on upper limb muscular strain for repetitive hand transfer

The study examines the influence of workplace layout on localized muscle fatigue in a simulated repetitive seated hand transfer task. Nine task conditions were used consisting of three working heights and three parts bin positions. Six muscles in the right upper limb were monitored bilaterally using surface electromyography. Motion range data on the upper segmental joints were collected along with subjective perception data. Results showed that awkward postures due to workstation setting and high repetitiveness were the major factors affecting upper limb muscle fatigue, especially for the trapezius and anterior deltoid. The percentage of fatigued subjects generally increased when the mean working height became greater. This study suggests that working height should be organized at or below the elbow level with the parts bin position not above the working height when a repetitive hand transfer task is performed.Relevance to industryThis experimental result should be useful in job analysis for the evaluation of muscle activities and workplace layout design for the reduction of muscular load on the upper limbs during repetitive handling of light weight in industries characterized as hand transfer.     

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.     

Examining kinematics and muscle activity of the upper extremity while performing cleaning tasks: A pre-post shift evaluation

Custodians engage in strenuous manual labour throughout their workday. Uncertainty exists on whether kinematics or muscle activation changes as workers progress through their shift. The purpose of this study was to examine muscle activation and upper extremity kinematics during typical custodial tasks performed at the start and end of the work shift. Electromyography (EMG) of 8 torso and upper extremity muscles and motion capture of the corresponding region were collected for 10 custodial participants while they completed garbage removal, dry mopping, and vacuuming tasks at the start and end of a work shift. The start of shift assessment demonstrated larger anterior deltoid, posterior deltoid and extensor digitorum activation by up to 12.6 %maximal voluntary contraction (%MVC) (p < 0.001–0.023). Task affected mean and peak EMG in all muscles except flexor digitorum (p < 0.001–0.0293), and the greatest activation was found during vacuuming (peak <55%MVC in anterior and middle deltoid) and the heaviest garbage removal task (84% MVC peak in upper trapezius). Mopping created the lowest amounts of activation for almost all muscles. Joint range of motion (p < 0.001–0.031) in the shoulder was highest in vacuuming, while trunk flexion was largest in garbage removal tasks. This work replicated common custodial tasks in a laboratory, using actual custodial workers at times relevant to their typical work shifts. The information presented is useful for ergonomists, work tasks designers and custodial administration staff to develop guidelines for injury prevention.     

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.     

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.     

Effect of a novel teat preparation system on upper extremity muscle activity among U.S. large-herd dairy parlor workers

BackgroundDairy production in the U.S. is moving towards large-herd milking operations resulting in an increase in task specialization and work demands. Large-herd dairy parlor workers experience a high prevalence of musculoskeletal symptoms in the upper extremity. The purpose of this study is to evaluate the effects of an alternative teat scrubber (TS) cow preparation method on upper extremity muscle activity among large-herd parlor workers, as well compare to muscle activity associated with conventional manual milking tasks.MethodsUpper extremity muscle activity was recorded among U.S. large-herd parlor workers (n = 15) using surface electromyography. Participants performed multiple task cycles, using both conventional and TS methods. Muscle activity levels were then compared across conventional manual and TS milking tasks.ResultsConventional manual milking tasks of dip, strip and wipe were associated higher muscle activity levels of the upper trapezius and anterior deltoid. Biceps muscle activity was greatest during teat dip and wipe. Forearm flexor and extensor muscle activity was greatest during teat wipe and dip. The TS system resulted in more desirable anterior deltoid EMG profiles, and less desirable biceps, forearm flexor and extensor profiles.ConclusionsResults suggest that the TS system is effective in reducing anterior deltoid muscle activation levels. The TS system also appears to result in increased biceps, forearm flexor and extensor muscle activation levels. Increases in muscle activation levels could be offset by reduced repetitiveness resulting from three conventional manual milking tasks being replaced with one TS task.Relevance to industryIf parlor production requirements (e.g., quality and onsistency) are maintained while simultaneously reducing cumulative muscle loading and worker fatigue, then TS use should be considered in milking parlor operations.     

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.     

Muscle activity during patient transfers: a preliminary study on the influence of lift assists and experience

The purpose of this study was to examine muscle activity patterns during patient handling during manual transfers, and transfers using floor and ceiling lifts. EMG patterns during transfers from bed to wheelchair and wheelchair to bed as well as patient repositioning in novices and experienced participants were examined. Surface EMG was recorded from the upper and lower erector spinae, latissimus dorsi and trapezius muscles bilaterally. Overall, normalized mean and peak muscle activity were lowest using the ceiling lift, increasing with the floor lift, which were lower than manual transfers (novices: all p < 0.01). Experienced patient handlers demonstrated approximately two times greater trapezius and latissimus dorsi activity than novices, combined with lower mean erector spinae activity (p < 0.05, for most tasks). Integrated EMG for all muscles was directly proportional to the transfer time and was lowest during the manual transfer followed by the ceiling lift, with the floor lift being highest. The difference between the muscle activity patterns between the experienced and novice patient handlers may suggest a learned behaviour to protect the spine by distributing load to the shoulder. Further examination of the muscle activation patterns differences between experience levels could improve training techniques to develop better patient handling strategies.     

The effect of phone design on upper extremity discomfort and muscle fatigue

OBJECTIVE: To compare a small cellular clamshell phone with a traditional office phone in the development of discomfort and muscle fatigue over time during phone use. BACKGROUND: Phone use involves low-level static exertions that may be influenced by phone design. Phone design and its interactions with anthropometry may change shoulder and hand postures assumed during use, which in turn may modify the length-strength relationship and moment arms of the muscles. METHOD: Ten adults participated in a study that simulated phone use using a small clamshell and a traditional office phone. Discomfort information and electromyographic (EMG) muscle activity were monitored on four upper extremity muscles. Discomfort and fatigue data (EMG median frequency shifts) were analyzed to assess differences between phones as well as differing effects attributable to anthropometry. RESULTS: Median frequency shifts supported discomfort claims and indicated muscle fatigue in the deltoid and thenar muscles. Biomechanical measures demonstrated that participants with short limb lengths developed more severe signs of thenar fatigue. Participants with longer arms developed greater discomfort in the neck, shoulder, and back. The deltoid confirmed this occurrence, showing signs of muscle fatigue. CONCLUSION: Phone design and anthropometry influenced the development of discomfort and fatigue during phone use. Phone design dictated grip style, resulting in differing discomfort and fatigue levels. Anthropometry influenced the severity of the discomfort and fatigue present in the shoulder and hand. APPLICATION: Use of small clamshell phones may contribute to a lack of rest and recovery from typical workday exposures. It should be explored from an ergonomic perspective.     

Task rotation effects on upper extremity and back muscle activity

Job rotation is an intuitive approach to distributing work to minimize muscular fatigue. The purpose of the current study was to evaluate rotation between lifting and gripping on muscle activity and effort. Ten male participants performed all 4 combinations of two 15 min tasks in 30 min trials split between separate days to prevent fatigue. The tasks of lifting a 12 kg box and gripping at 20% of maximum were performed 6 times per minute (5 s work: 5 s rest). Muscle activity (percentiles, gaps) and perceived effort were significantly affected by the task combinations. The forearm and upper erector spinae muscles did not benefit as greatly from rotating between lifting and gripping tasks as the lower erector spinae, deltoid or trapezius. In addition to gross task differences, overlaps in muscle activity between “low back” and “upper extremity” tasks must be considered when creating effective job rotation schemes.     

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.     

Muscle activity during patient transfers: a preliminary study on the influence of lift assists and experience

The purpose of this study was to examine muscle activity patterns during patient handling during manual transfers, and transfers using floor and ceiling lifts. EMG patterns during transfers from bed to wheelchair and wheelchair to bed as well as patient repositioning in novices and experienced participants were examined. Surface EMG was recorded from the upper and lower erector spinae, latissimus dorsi and trapezius muscles bilaterally. Overall, normalized mean and peak muscle activity were lowest using the ceiling lift, increasing with the floor lift, which were lower than manual transfers (novices: all p?<?0.01). Experienced patient handlers demonstrated approximately two times greater trapezius and latissimus dorsi activity than novices, combined with lower mean erector spinae activity (p?<?0.05, for most tasks). Integrated EMG for all muscles was directly proportional to the transfer time and was lowest during the manual transfer followed by the ceiling lift, with the floor lift being highest. The difference between the muscle activity patterns between the experienced and novice patient handlers may suggest a learned behaviour to protect the spine by distributing load to the shoulder. Further examination of the muscle activation patterns differences between experience levels could improve training techniques to develop better patient handling strategies.     

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 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.     

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.     

Attention-related muscle activity in different body regions during VDU work with minimal physical activity

Abstract

The purpose of this study was to characterize regional variation in task-irrelevant muscle activity. Surface EMG was recorded at 20 different muscle sites during a complex choice-reaction task. Up to ten muscle sites were recorded in parallel and always including the upper trapezius muscles. The experimental setting was designed to minimize the physical activity while performing a VDU-based task demanding continuing attention. Consequently, the label ‘attention-related muscle activity’ has been adopted. The presence of this activity varied between subjects and between the different muscle sites. It tended to decrease in a caudal direction and to be low in the muscles of the extremities, with the exception of the extensor muscles of the hand and foot. Attention-related activity was prevalent in the frontalis and upper trapezius muscles. This result is discussed with respect to a potentially harmful health effect of task-irrelevant muscle activity.     

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.