The size of cycle-to-cycle variability in biomechanical exposure among butchers performing a standardised cutting task

The effects of employment duration and pain development on motor variability were investigated during repetitive work. Electromyographic (EMG) and kinematics data from two previous studies were re-analysed. Newly employed butchers were followed prospectively in relation to employment duration and pain development. Healthy butchers with long-term experience were compared with novices. The variability of the cycle time, EMG ratio and arm and trunk movement was expressed as cycle-to-cycle standard deviations. During the first 6 months of employment, cycle time variability decreased, while posture and movement variability increased (p < 0.05). In presence of pain, the variability of the initial arm position decreased while it increased for the trunk (p < 0.05). Experienced butchers showed a larger variability than novices for work cycle and several kinematic variables, but a smaller EMG ratio variability (p < 0.05). These findings indicate that the variability of motor patterns in repetitive work changes with experience and pain. A change towards a more variable motor strategy may protect workers from work-related musculoskeletal disorders.     

Standardized low-load repetitive work: evidence of different motor control strategies between experienced workers and a reference group

The purpose of this laboratory study was to evaluate the possible differences in motor strategies to a new standardized low-load repetitive work task in between healthy experienced workers and a reference group. Work task event duration, i.e. working rhythm, cutting forces, surface electromyographic (EMG) activity from four shoulder muscles, postural activity, and arm and trunk movements in 3D were recorded during low-load repetitive work simulation. The experienced group showed lower EMG activity and frequency contents (P<0.05), more abducted position of the upper arm and forward flexion of the trunk prior to work simulation (P<0.05), and increased arm and trunk range of motion (P<0.05) compared with the reference group. The results highlight that experienced butchers have a different motor strategy compared with a reference group, i.e. more variable form of coordination pattern. Furthermore, the initial implementation of a possible protective motor strategy by experienced workers might be a very important prognostic factor.     

Effects of electromyographic and mechanomyographic biofeedback on upper trapezius muscle activity during standardized computer work

The purpose of this laboratory study was to investigate the effects of surface electromyography (EMG)- and mechanomyography (MMG)-based audio and visual biofeedback during computer work. Standardized computer work was performed for 3 min with/without time constraint and biofeedback in a randomized order. Biofeedback was given on the basis of an individual preset threshold value for the right trapezius EMG and MMG signal and a time factor (repetition of events above the threshold). The duration of muscle activity above the preset threshold, the right trapezius EMG and MMG root mean square (RMS) values as well as the work performance in terms of number of completed graph/mouse clicks/errors, the rating of perceived exertion (RPE) and the usefulness of the biofeedback were assessed. The duration of muscle activity above the threshold was significantly lower with MMG compared with EMG as source of biofeedback (p < 0.05). Biofeedback led to a significant decrease in the right trapezius EMG RMS, lower RPE and decreased number of errors and mouse clicks, but also decreased number of completed graphs (p < 0.05). Audio and visual biofeedbacks were as effective. MMG-based biofeedback is a potential reliable alternative to EMG in ergonomics. A lowering of the trapezius muscle activity may contribute to diminish the risk of work related musculoskeletal disorders development.     

Cycle to cycle variability in a repetitive upper extremity task

The purpose of this study was to examine the variability in muscle activity at rest and work during a repetitive task. A total of 20 participants performed a bimanual push task using three frequencies (4, 8, 16 pushes/min), three loads (1 kg, 2 kg, 4 kg) and two grip conditions (no grip, 30% maximum). The coefficient of variation (CoV) of muscle activity was determined for the anterior deltoid, biceps brachii, extensor digitorum and flexor digitorum superficialis. Faster push frequencies and heavier loads had lower work–rest ratio CoV and higher mean muscle activity (p < 0.01). Sixteen pushes per minute produced the lowest CoV for the anterior deltoid (p < 0.01), while the 1- kg load produced the lowest CoV for the extensor digitorum and flexor digitorum superficialis (p < 0.01). Changes were driven by the rest phase rather than by the work phase, except for grip decreasing forearm muscle CoV. These findings underscore the importance of variability at rest and indicate that low variability of muscle activity is associated with ergonomic risk factors.

Practitioner Summary: Decreased motor variability has been associated with pain and injury. A cyclical push task, evaluated in terms of work and rest phases, found that greater workloads increased variability primarily due to changes in the rest phase. Muscle variability, especially for the rest phase, may provide insight into injury risk.     

The interrelationship of the thorax and pelvis under varying task constraints

The purpose of this study was to investigate the interrelationship between the thorax and pelvis during coupled movement patterns. Fifty-seven participants were assessed using an infrared motion analysis system to track trunk movement during maximal pelvis and thorax rotations over four trunk inclinations and two pelvic constraint conditions. A repeated-measures multivariate analysis of variance investigated the effects of forward trunk inclination and pelvic constraint on thorax and pelvic rotation. Forward trunk inclination from neutral to 45° resulted in a 46% (p < 0.001) decrease in axial pelvic rotation and a 15% (p < 0.001) decrease in axial thorax rotation with an unconstrained pelvis. A constrained pelvis resulted in a 15% (p < 0.001) decrease in axial thorax rotation. An externally constrained pelvis allowed the thorax to achieve an average of 18° (SD = 2°) greater rotational range of motion across all angles. This study reinforced the importance of allowing the pelvis to rotate during whole body axial rotation tasks.

Practitioner Summary: Results indicated that maximum axial trunk rotation is best achieved in a neutral posture, when the pelvis is allowed to contribute and flexion at the hips should be minimised. For example, if a recumbent task requires rotation of the torso, then the chair seat should be allowed to swivel.     

The effects of force and exertion duration on duty cycle time: Implications for productivity

Ergonomics has positive effects on both physical health and productivity, but estimating productivity benefits is difficult at the task design/redesign stage. Rest‐allowance prediction models are not suitable for repetitive, short‐cycle dynamic tasks, and methods–time measurement (MTM) techniques are limited in their suitability for considering ergonomics risk factors such as posture and force. The purpose of this study was to investigate the relationship between force and exertion duration on self‐selected duty cycle time and discomfort. Twenty‐one participants completed repetitive, upper‐limb exertion treatments, each of a 10‐minute duration. Five levels of force (10, 20, 40, 65, and 80% maximum voluntary contraction [MVC]) and exertion period (1, 2, 4, 6.5, and 8 seconds) were investigated. The psychophysical adjustment method was used whereby participants self‐selected a work pace for the second half of each treatment. Duty cycle, derived from the self‐paced cycle time, was the measure of productivity effects in the experiment. Analysis of variance revealed a significant effect on duty cycle time for force, exertion period, and their interaction (each p < 0.0001). Friedman's test indicated a significant effect of force (p < 0.0001) and exertion period (p < 0.0001) on discomfort. Spearman's correlation analysis showed a strong correlation between discomfort and duty cycle time (p < 0.05). Multiple regression analysis was used to develop a predictive model for duty cycle time based on force and exertion period, and this was a good fit to the data (R² = 0.98, p < 0.05). Profiles were generated presenting zones of acceptable self‐selected duty cycle times based on force and exertion duration. © 2010 Wiley Periodicals, Inc.     

Biomechanical analysis of upper trapezius,erector spinae and brachioradialis fatigue in repetitive manual packaging tasks: Evidence from Chinese express industry workers

This study aims to assess the effects of repetitive motion-induced fatigue during manual packaging on kinematics and muscle activity of the arm, back and shoulder. Eighteen participants performed a 60-min manual packaging task. Electromyography median frequency of RUT, RES, LES and RB decreased by 13.2% (p < 0.05), 12.8% (p < 0.05), 11.3% (p < 0.05) and 21% (p < 0.001), respectively over time. The head flexion and the right upper arm flexion angles significantly (p < 0.001) differed among different packaging sizes, and similar results were observed for left upper arm flexion and rotation. The trunk and pelvis angles significantly (p < 0.05) differed over time under three kinds of packaging. The results showed there was an increase in the rating of perceived exertion from 8.56 to 16.94 (p<0.01) during the task. The outcomes of this study indicate repetitive movements in the manual packaging task resulted in elevated prevalence of muscle discomfort in packaging workers, especially the right brachioradialis. Interventions such as specific work: rest ratios, workplace redesign should be explored to relieve muscle fatigue and discomfort.Relevance to industryFor workers performing repetitive manual packaging tasks, biomechanical analysis of different muscles groups can help in developing appropriate ergonomic interventions.     

Stand up: comparison of two electrical screed levelling machines to reduce the work demands for the knees and low back among floor layers

Electrical screed levelling machines are developed to reduce kneeling and trunk flexion of sand–cement-bound screed floor layers. An observational intervention study among 10 floor layers was performed to assess the differences between a self-propelled and a manually moved machine. The outcome measures were work demands, production time, perceived load, discomfort and applicability. Compared to the self-propelled machine, the duration of kneeling (?13 min; p = 0.003) and trunk flexion (?12 min; p < 0.001) was shorter using the manually moved machine, and the duration of pushing and pulling increased (?39 min; p < 0.001). No significant or relevant differences were found for production time, perceived load and discomfort. Nine out of ten floor layers found the manually moved machine applicable and three out of ten found the self-propelled machine applicable. When compared with the traditional manner of floor laying, both electrical machines reduced the exposure towards kneeling and trunk flexion.

Practitioner Summary: Electrical machines may help to reduce high physical work demands on floor layers. A manually moved machine is better applicable for the installation of screed floors in residences with smaller floor areas. A self-propelled machine is better applicable on large floor areas with a minimum width of 4 m.     

Effects of posture on dynamic back loading during a cable lifting task

This study evaluated spinal loads associated with lifting and hanging heavy mining cable in a variety of postures. This electrical cable can weigh up to 10 kg per metre and is often lifted in restricted spaces in underground coal mines. Seven male subjects performed eight cable lifting and hanging tasks, while trunk kinematic data and trunk muscle electromyograms (EMGs) were obtained. The eight tasks were combinations of four postures (standing, stooping, kneeling on one knee, or kneeling on both knees) and two levels of cable load (0 N or 100 N load added to the existing cable weight). An EMG-assisted model was used to calculate forces and moments acting on the lumbar spine. A two-way split-plot ANOVA showed that increased load (p<0.05) and changes in lifting posture (p<0.05) independently affected trunk muscle recruitment and spinal loading. The increase in cable load resulted in higher EMG activity of all trunk muscles and increased axial and lateral bending moments on the spine (p<0.05). Changes in posture caused more selective adjustments in muscle recruitment and affected the sagittal plane moment (p<0.05). Despite the more selective nature of trunk EMG changes due to posture, the magnitude of changes in spinal loading was often quite dramatic. However, average compression values exceeded 3400 N for all cable lifting tasks.     

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.     

Changes in trunk muscle activation and lumbar-pelvic position associated with abdominal hollowing and reach during a simulated manual material handling task

The purpose of this study was to investigate the effect of abdominal hollowing (AH) on trunk muscle activation and lumbar-pelvic motion during a controlled lift and replace task. Surface electromyograms were recorded from five abdominal and two back muscle sites. Sagittal lumbar-pelvic motion was recorded by video. Subjects lifted a 3.8 kg load in normal, maximum and extreme reaches, first while performing their preferred lifting style (PLS) and then maintaining an AH technique. The external oblique muscle site activities were significantly higher (p < 0.05) for the AH technique (ranging from 7–20% of maximal voluntary activation (MVIC)) than at any of the abdominal sites for the PLS (ranging from 2–10% MVIC). Differences were found among abdominal sites for the AH, but not for the PLS. The back muscle site activities (ranging from 9–30% MVIC) were significantly higher (p < 0.05) than for any of the abdominal muscles for all conditions, except for the anterior external oblique for AH. The pelvic and lumbar angles changed significantly (p < 0.05) between normal and maximal reaches and between techniques. The AH technique altered abdominal muscle activation amplitudes, with minimal differences in trunk extensors compared to the PLS. The AH resulted in more posterior pelvic tilt.     

The size and structure of arm movement variability decreased with work pace in a standardised repetitive precision task

Increased movement variability has been suggested to reduce the risk of developing musculoskeletal disorders caused by repetitive work. This study investigated the effects of work pace on arm movement variability in a standardised repetitive pipetting task performed by 35 healthy women. During pipetting at slow and fast paces differing by 15%, movements of arm, hand and pipette were tracked in 3D, and used to derive shoulder and elbow joint angles. The size of cycle-to-cycle motor variability was quantified using standard deviations of several kinematics properties, while the structure of variability was quantified using indices of sample entropy and recurrence quantification analysis. When pace increased, both the size and structure of motor variability in the shoulder and elbow decreased. These results suggest that motor variability drops when repetitive movements are performed at increased paces, which may in the long run lead to undesirable outcomes such as muscle fatigue or overuse.     

Physical workload,trapezius muscle activity,and neck pain in nurses' night and day shifts: A physiological evaluation

The purpose of this study was to compare physical workload, electromyography (EMG) of the trapezius muscle, neck pain and mental well-being at work between night and day shifts in twenty Swiss nurses. Work pulse (average increase of heart rate over resting heart rate) was lower during night (27 bpm) compared to day shifts (34 bpm; p < 0.01). Relative arm acceleration also indicated less physical activity during night (82% of average) compared to day shifts (110%; p < 0.01). Rest periods were significantly longer during night shifts. Trapezius muscle rest time was longer during night (13% of shift duration) than day shifts (7%; p < 0.01) and the 50th percentile of EMG activity was smaller (p = 0.02), indicating more opportunities for muscle relaxation during night shifts. Neck pain and mental well-being at work were similar between shifts. Subjective perception of burden was similar between shifts despite less physical burden at night, suggesting there are other contributing factors.     

Effects of low back disability status on lower back discomfort during sustained and cyclical trunk flexion

A laboratory study was conducted to determine the effects of back disability status on endurance time and perceived discomfort during trunk flexion. Eighty participants (40 with chronic or recurrent low back pain (CRLBP), 40 pain-free) were tested. The trunk was flexed to 15°, 30°, 45° and 60° under three conditions: 1) continuous static flexion; 2) cyclical flexion with 20% rest; and 3) cyclical flexion with 40% rest. Each condition was performed for up to 600?s or until the participant reached his/her pain tolerance limit. Dependent variables included time to distracting discomfort (TDD), total endurance time (TET) and perceived discomfort. For continuous exertions, CRLBP participants had lower TDD (p?<?0.001), lower TET (p?<?0.001) and greater discomfort (p?<?0.001) compared to pain-free controls. In both groups, TDD and TET decreased and perceived discomfort increased as the flexion angle increased. For intermittent exertions, CRLBP participants reported greater discomfort than pain-free participants (p?<?0.001). Increasing rest from 20 to 40% reduced discomfort in CRLBP participants, but produced no consistent benefit in pain-free participants. To accommodate persons with CRLBP, consideration should be given to reducing both the magnitude (angle) and duration of trunk flexion required by their jobs.     

Effect of different handgrip angles on work distribution during hand cycling at submaximal power levels

The effect of different handle angles on work distribution during hand cycling was determined. Able-bodied subjects performed hand cycling at 20% of maximum power level (mean (SD) power level: 90.0 (25.8) W) at a cadence of 70 rpm using handle angles of ±30°, ±15° and 0°. The handle angle had a significant effect on work during the pull down (p < 0.001) and lift up (p = 0.005) sector, whereby the highest work was performed with handle angles of +30° and ?15° respectively. The cycle sector had a significant effect on work (p < 0.001) and significantly (p = 0.002) higher work was performed in the pull down sector (25% higher than mean work over one cycle) as compared to the lift up sector (30% lower than mean work over one cycle). Therefore, a fixed handle angle of +30° is suggested to be optimal for power generation. The results of this study help to optimise the handbike–user interface. A more pronated handle angle compared to the one conventionally used was found to improve the performance of hand cycling and thereby the mobility of disabled people.     

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.     

Lumbar posture and trunk muscle activation during a typing task when sitting on a novel dynamic ergonomic chair

Low back pain (LBP) is a common musculoskeletal disorder and prolonged sitting often aggravates LBP. A novel dynamic ergonomic chair (‘Back App’), which facilitates less hip flexion while sitting on an unstable base has been developed. This study compared lumbar posture and trunk muscle activation on this novel chair with a standard backless office chair. Twelve painfree participants completed a typing task on both chairs. Lumbar posture and trunk muscle activation were collected simultaneously and were analysed using paired t-tests. Sitting on the novel dynamic chair significantly (p < 0.05) reduced both lumbar flexion and the activation of one back muscle (Iliocostalis Lumborum pars Thoracis). The discomfort experienced was mild and was similar (p > 0.05) between chairs. Maintaining lordosis with less muscle activation during prolonged sitting could reduce the fatigue associated with upright sitting postures. Studies with longer sitting durations, and in people with LBP, are required.

Practitioner Summary: Sitting on a novel dynamic chair resulted in less lumbar flexion and less back muscle activation than sitting on a standard backless office chair during a typing task among pain-free participants. Facilitating lordotic sitting with less muscle activation may reduce the fatigue and discomfort often associated with lordotic sitting postures.     

Maximal and sub-maximal functional lifting performance at different platform heights

Introducing valid physical employment tests requires identifying and developing a small number of practical tests that provide broad coverage of physical performance across the full range of job tasks. This study investigated discrete lifting performance across various platform heights reflective of common military lifting tasks. Sixteen Australian Army personnel performed a discrete lifting assessment to maximal lifting capacity (MLC) and maximal acceptable weight of lift (MAWL) at four platform heights between 1.30 and 1.70 m. There were strong correlations between platform height and normalised lifting performance for MLC (R² = 0.76 ± 0.18, p < 0.05) and MAWL (R² = 0.73 ± 0.21, p < 0.05). The developed relationship allowed prediction of lifting capacity at one platform height based on lifting capacity at any of the three other heights, with a standard error of < 4.5 kg and < 2.0 kg for MLC and MAWL, respectively.     

A history of low back pain affects pelvis and trunk mechanics during a sustained lift/lower task

This study compared three-dimensional trunk and pelvis range of motion (ROM) during a sustained asymmetric box lift/lower task between a group with a history of low back pain (HBP, n = 9) and a group with no history of low back pain (NBP, n = 9). Participants lifted an 11-kg box for 10 min at 12 cycles/min from ankle height in front to shelves 45 deg off-centre at waist height. Kinematic data were collected at the beginning (min1), middle (min5) and end of the bout (min9). Two-way analyses of variance were performed for all variables. Pelvis and trunk transverse ROM were similar at min1. By min9, HBP group did not change (31.9 ± 9 deg); however, ROM decreased in NBP group (21.6 ± 6 deg, p < 0.05). Therefore, despite no current pain, the HBP group demonstrated protective lifting mechanics compared to controls. Also discussed are implications for studying lifting paradigms at sub-maximal effort over longer periods of time.

Practitioner summary: Differences between groups over time demonstrate residual consequences of low back pain (LBP) in a manual materials handling scenario. Individuals with a history of LBP (pain free for 6 months) demonstrated more conservative lifting mechanics towards the end of the bout compared to controls with no history of LBP.