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.     

Perception of Hand Force in Power Grip for Females

Borg's rating of perceived exertion (RPE) and category ratio (CR‐10) scales are commonly used to quantify perceived muscular exertion for body segments. Twenty females participated in an experiment to study the power grip force at four perceived exertion levels using either dominant or nondominant hand under two posture conditions. It was found that the subjects tended to apply a higher power grip force (100% of perceived maximum voluntary contraction) than the levels they were requested to apply. The power grip forces between dominant and nondominant hands at low hand exertion levels were negligible. The grip forces between the two hands were significantly different when the exertion level was nearly maximal. Linear regression models were established for the subjects to link the relationship between the perceived hand exertion and measured grip force, hand used, and hand/arm posture. All the models were statistically significant (p < 0.0001) with R² values 0.97 or higher. These models provided better estimates in perceived hand exertion for dominant hand than for nondominant hand. A follow‐up experiment was conducted to measure the subjective rating of both the CR‐10 and RPE when a 98 N grip force was applied. It was found that the subjects reported higher exertion levels when they were using the CR‐10 scale than when they were using the RPE scale. © 2011 Wiley Periodicals, Inc.     

Measuring elemental time and duty cycle using automated video processing

A marker-less 2D video algorithm measured hand kinematics (location, velocity and acceleration) in a paced repetitive laboratory task for varying hand activity levels (HAL). The decision tree (DT) algorithm identified the trajectory of the hand using spatiotemporal relationships during the exertion and rest states. The feature vector training (FVT) method utilised the k-nearest neighbourhood classifier, trained using a set of samples or the first cycle. The average duty cycle (DC) error using the DT algorithm was 2.7%. The FVT algorithm had an average 3.3% error when trained using the first cycle sample of each repetitive task, and had a 2.8% average error when trained using several representative repetitive cycles. Error for HAL was 0.1 for both algorithms, which was considered negligible. Elemental time, stratified by task and subject, were not statistically different from ground truth (p < 0.05). Both algorithms performed well for automatically measuring elapsed time, DC and HAL.

Practitioner Summary: A completely automated approach for measuring elapsed time and DC was developed using marker-less video tracking and the tracked kinematic record. Such an approach is automatic, repeatable, objective and unobtrusive, and is suitable for evaluating repetitive exertions, muscle fatigue and manual tasks.     

The Revised Strain Index: an improved upper extremity exposure assessment model

Abstract

The Revised Strain Index (RSI) is a distal upper extremity (DUE) physical exposure assessment model based on: intensity of exertion, frequency of exertion, duration per exertion, hand/wrist posture and duration of task per day. The RSI improves upon the 1995 Strain Index (SI) by using continuous rather than categorical multipliers, and replacing duty cycle with duration per exertion. In a simulation of 13,944 tasks, the RSI and 1995 SI showed good agreement in risk predictions for 1995 SI scores ≤3 (safe) and >13.5 (hazardous). For tasks with 1995 SI scores of >3 and ≤13.5, the two models showed marked disagreement, with the RSI providing much greater discriminations between ‘safe’ and ‘hazardous’ tasks for various combinations of force, repetition and duty cycle. We believe the RSI is a substantially improved model that will be useful for DUE task analysis, intervention and design.

Practitioner Summary: RSI is a substantial improvement over the 1995 SI. It should be a valuable tool for designing and analysing tasks to determine risk of musculoskeletal injuries. RSI is applicable to a wide variety of tasks including very low force and very high repetition tasks such as keyboard use.     

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.     

A Review of :“Leu Accidents du Travail.” By JACQUES LEPLAT et XAVIER CUNY. (Paris: Presses Universities de France, 1974.( [Pp.128. £0·00.

Police officers spend large amounts of time performing duties within a police cruiser and report a high prevalence of musculoskeletal problems. This study evaluated the effects of driver seat and duty belt design on posture, pressure and discomfort. Ten male and 10 female university students attended two sessions involving simulated driving in a standard police seat (CV) and an active lumbar support (ALS) seat. Participants wore a full duty belt (FDB) or reduced duty belt (RDB) in each seat. Lumbar postures, driver-seat and driver-duty belt pressures and perceived discomfort were measured. Gender × Seat interactions were found for pelvic (p = 0.0001) and lumbar postures (p = 0.003). Females had more lumbar flexion than males and were more extended in the ALS seat (?9.8 ± 11.3°) than CV seat (?19.8 ± 9.6°). The FDB had greater seat pressure than the RDB (p < 0.0001), which corresponded to increased pelvis discomfort. This study supports the use of an ALS seat and RDB to reduce injury risk associated with prolonged sitting in police officers.

Practitioner Summary: Police officers report a high prevalence of musculoskeletal problems to the lower back, associated with prolonged driving and further investigation is needed to reduce injury risk. This simulated driving study investigated seat and duty belt configuration on biomechanical measures and discomfort. Seat design had the greatest impact, regardless of gender and males benefited more from a reduced belt configuration.     

Shoulder strength of females while sitting and standing as a function of hand location and force direction

This study evaluated single-handed isometric push strength capabilities of females working at or above-shoulder level. We examined the influence of force exertion direction (vertical, horizontal and lateral), angle of shoulder flexion from horizontal (0°, 30°, 60° and 90°) and gross body posture (standing and sitting), on maximal volitional shoulder strength. Force exertion direction had the greatest affect on shoulder strength (p < 0.0001). Strength was greatest in the vertical axis pushing downwards and weakest in the horizontal plane pushing forwards. Angle influenced shoulder strength when considered together with direction (p < 0.0001). However, these effects were dominated by direction results. Marginal differences in strength existed between sitting and standing (p > 0.05). These results can be used to design workspaces that consider individual strength limitations and their dependence on force direction, work orientation, and gross body posture.     

Differences in typing forces,muscle activity,comfort, and typing performance among virtual,notebook, and desktop keyboards

The present study investigated whether there were physical exposure and typing productivity differences between a virtual keyboard with no tactile feedback and two conventional keyboards where key travel and tactile feedback are provided by mechanical switches under the keys. The key size and layout were same across all the keyboards. Typing forces; finger and shoulder muscle activity; self-reported comfort; and typing productivity were measured from 19 subjects while typing on a virtual (0 mm key travel), notebook (1.8 mm key travel), and desktop keyboard (4 mm key travel). When typing on the virtual keyboard, subjects typed with less force (p's < 0.0001) and had lower finger flexor/extensor muscle activity (p's < 0.05). However, the lower typing forces and finger muscle activity came at the expense of a 60% reduction in typing productivity (p < 0.0001), decreased self-reported comfort (p's < 0.0001), and a trend indicating an increase in shoulder muscle activity (p's < 0.10). Therefore, for long typing sessions or when typing productivity is at a premium, conventional keyboards with tactile feedback may be more suitable interface.     

The effects of hyperoxia on performance during simulated firefighting work

This study evaluated the effects of hyperoxia (inspired oxygen fraction=40%) on performance during a simulated firefighting work circuit (SFWC) consisting of five events. On separate days, 17 subjects completed at least three orientation trials followed by two experimental trials while breathing either normoxic (NOX) and hyperoxic (HOX) gas mixtures that were randomly assigned in double-blind, cross-over design. Previously, ventilatory threshold (Tvent) and VO₂max had been determined during graded exercise (GXT) on a cycle ergometer. Lactate concentration in venous blood was assessed at exactly 5 min after both the experimental trials and after the GXT. Total time to complete the SFWC was decreased by 4% (p< 0.05) with HOX. No differences were observed in individual event times early in the circuit, however HOX resulted in a 12% improvement (p< 0.05) on the final event. A significantly decreased rating of perceived exertion (RPE) was also recorded immediately prior to the final event. No differences were observed in mean heart rate or post-exercise blood lactate when comparing NOX to HOX. Heart rates during the SFWC (both conditions) were higher than HR at Tvent, but lower than HR at VO₂max (p< 0.05). Post-SFWC lactate values were higher (p< 0.05) than post-VO₂max. These results demonstrate that hyperoxia provided a small but significant increase in performance during short duration, high intensity simulated firefighting work.     

Self-selected duty cycle times for grip force,wrist flexion postures and three grip types

Performance and health issues are common in industry. On-the-job productivity gains related to good design, which could help justify ergonomics intervention, are often not considered. More quantitative data are needed to model the discomfort/productivity relationship for upper limb activity in simulated repetitive assembly type work. Eighteen participants completed an experiment, simulating a repetitive upper limb task with force, posture and grip type recorded as independent variables. Duty cycle time and discomfort were recorded as dependent variables. Participants performed 18 experiment combinations (block designed around force); each treatment lasted 35 min, including breaks. Analysis indicated a significant two-way interaction between posture and grip type. Results from this experiment were used to model the effect of these variables on operator discomfort and performance.     

Calf massager: Intervention for body muscle discomfort during prolonged standing

Many occupations require workers to stand for long periods of time without proper interventions, which causes discomfort in the back and lower limbs. Therefore, this study aims to assess the effectiveness in alleviating body muscle discomfort during prolonged standing through the use of a calf massager. This study was conducted among male workers at a manufacturer with production line workers and the list was obtained from the HR Department and simple random sampling was done by number categorization. A total of 100 respondents (50 respondents for both the control and the experimental groups) participated in this study. The experiment took place in a room with a similar setup for the production line. Each respondent was requested to perform the simulated task for 2 hr. For the experimental group, the calf massager was turned on every 15 min. At every 15‐min interval after turning on the calf massager, respondents from both groups were required to evaluate their discomfort level on a Borg's scale CR‐10 questionnaire. The results showed that the level of body discomfort among respondents in the experimental group reduced (20–30%) compared with that of the control group. Multivariate analysis results revealed that the discomfort rating for the lower back, knees, thighs, calves, and feet was significantly lower (p < .05) among the experimental group compared with the control group. For lower body parts, the lower back region was statistically significant (p < .05) at the 90th, 105th, and 120th min; the thigh region was statistically significant (p < .05) at the 120th min; the knee region was statistically significant (p < .05) at the 105th and 120th min; the calf region was statistically significant (p < .05) at all minute intervals except the 15th and 45th min, while the feet region, was statistically significant at the 105th and 120th min. Therefore, this study indicates that calf massage treatment is capable of reducing body muscle discomfort during prolonged standing and highlights the significance of calf massage.     

Short-term changes in upper extremity dynamic mechanical response parameters following power hand tool use

Dynamic mechanical response parameters (stiffness, damping and effective mass), physiological properties (strength and swelling) and symptoms of the upper limb were measured before power tool operation, immediately following and 24?h after power tool operation. Tool factors, including peak torque (3 Nm and 9 Nm) and torque build-up time (50?ms and 250?ms), were controlled in a full factorial design. Twenty-nine inexperienced power hand tool users were randomly assigned to one of four conditions and operated a pistol grip nutrunner four times per min for 1?h in the laboratory. Isometric strength decreased immediately following tool use (15%) (p?<?0.01) and 24?h later (9%) (p?<?0.05). Mechanical parameters of stiffness (p?<?0.05) and effective mass (p?<?0.05) were affected by build-up time. An average decrease in stiffness (43%) and effective mass (57%) of the upper limb was observed immediately following pistol grip nutrunner operation for the long (250?ms) build-up time. A previously developed biomechanical model was used to estimate handle force and displacement associated with the tool factors in the experiment. The conditions associated with the greatest predicted handle force and displacement had the greatest decrease in mechanical stiffness and effective mass, and the greatest increase in localized discomfort.     

Metabolic cost and subjective assessment of palletizing and subsequent recovery

Twenty-one male blue collar workers repeatedly lifted (palletized) a box weighing 22 kg six times min^-1 for 5 min to a shelf of fixed height. The experimental conditions included two planes of lifting (symmetries), two shelf clearances, and three headrooms. The metabolic (heart rate, caloric cost and ventilation volume) and psychophysical variables (rate of perceived exertion, RPE; visual analogue score, VAS; and body part discomfort ratings, BPDR) were measured during resting, palletization, and recovery phases. In palletization the heart rate and metabolic cost ranged between 25 to 35% of the maximal aerobic capacity. Of the three factors only headroom had a significant effect on metabolic cost (p < 0.02) and the BPDR for low back (p < 0.05). In the recovery phase only headroom had significantly effect (p < 0.001) on metabolic cost. The metabolic recovery took 10 min; however, recovery measured through psychophysical indices appeared to continue for 20 min.     

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.     

Lean job design and musculoskeletal disorder risk: A two plant comparison

This study examined the relationship between lean job design and work‐related musculoskeletal disorder (WMSD) risk factors. Repetition, force, and posture were assessed for a sample of 56 production jobs across departments at a lean automobile‐manufacturing plant and compared to 56 similar jobs at a traditional automobile‐manufacturing plant. The results showed greater productivity in the lean plant: less waiting (p = .006) and walking (p < .001); and greater repetition exposure (p = .001). The mean rating for repetition was 5.5 in the lean plant, compared to 5.0 in the traditional plant based on the Latko (1997) hand activity level scale. However, the lean plant had significantly lower peak hand force ratings (p = .01). When examining force and repetition combined, the lean plant had a lower percentage of jobs above the American Conference for Governmental Industrial Hygienists (ACGIH)‐recommended Threshold Limit Value (TLV^®). The findings suggest that lean manufacturing does not necessarily increase workers' risk for WMSD injuries. © 2009 Wiley Periodicals, Inc.     

Effect of cycle time and duty cycle on psychophysically determined acceptable levels in a highly repetitive task

Psychophysical methodology has been used to develop guidelines for lifting and more recently similar methods have been applied to repetitive upper limb movements. While a range of cycle times are usually used, there is often no control for duty cycle. The purpose of this paper is to present psychophysically determined acceptable torques for a common upper limb task, with both cycle time and duty cycle conditions set by the researcher. Eight female participants, sitting at adjustable workstations, performed a simulated in-line screw running task. A computer-controlled torque motor applied a torque every 3, 6, 12 or 20 s with a duty cycle of 25, 50 or 83%. The participants worked with one set of conditions each day and self-selected the highest torque that they felt was acceptable without developing undue pain and discomfort. Duty cycle was found to significantly affect the amount of torque selected. With duty cycle controlled, cycle time was no longer found to have any significant effect on selected torque. Acceptable torques for 25, 50 and 83% duty cycles were 1.09, 0.9 and 0.73 Nm. Discomfort and stiffness were concentrated on the back of the hand and on the thumb web. These findings suggest that increased perception of discomfort with increased frequency (decreased cycle time) may be related to decreased rest/recovery time for muscles.     

Effect of cycle time and duty cycle on psychophysically determined acceptable levels in a highly repetitive task

Psychophysical methodology has been used to develop guidelines for lifting and more recently similar methods have been applied to repetitive upper limb movements. While a range of cycle times are usually used, there is often no control for duty cycle. The purpose of this paper is to present psychophysically determined acceptable torques for a common upper limb task, with both cycle time and duty cycle conditions set by the researcher. Eight female participants, sitting at adjustable workstations, performed a simulated in-line screw running task. A computer-controlled torque motor applied a torque every 3, 6, 12 or 20?s with a duty cycle of 25, 50 or 83%. The participants worked with one set of conditions each day and self-selected the highest torque that they felt was acceptable without developing undue pain and discomfort. Duty cycle was found to significantly affect the amount of torque selected. With duty cycle controlled, cycle time was no longer found to have any significant effect on selected torque. Acceptable torques for 25, 50 and 83% duty cycles were 1.09, 0.9 and 0.73 Nm. Discomfort and stiffness were concentrated on the back of the hand and on the thumb web. These findings suggest that increased perception of discomfort with increased frequency (decreased cycle time) may be related to decreased rest/recovery time for muscles.     

Metabolic demands of law enforcement personal protective equipment during exercise tasks

Many occupations require the use of personal protective equipment (PPE) but the added metabolic demands are unknown for certain professions. The purpose of this study was to quantify metabolic and perceptual differences between activity with and without the PPE ensemble required for police officers. Twelve participants were asked to complete experimental and control exercise sessions consisting of three modes of exercise (walking, jogging and stepping). A significant main effect (p < 0.01) for gear was found for heart rate (beats per minute) and VO₂ (L/min) between conditions. Dependent t-tests revealed significant differences for perceived effort, discomfort and session rating of perceived exertion between trials. Medium to large effect sizes for all variables with significant main effects between modes (p < 0.01, η² = 0.51–0.96, 1–β = 0.98–1.0, d = 0.42–2.7) were observed. These findings help to increase awareness of how PPE affects metabolic demands and perception of discomfort during exercise.     

Vicarious perception of postural discomfort and exertion

Perceived exertion and discomfort have been used extensively in ergonomics practice. Job incumbents typically rate their exertion on scales such as Borg's rated perceived effort (RPE) and their discomfort on scales such as Corlett and Bishop's body part discomfort scales (BPD). This study asks whether exertion and discomfort can be perceived by an external observer, i.e. is vicarious perception possible? Four participants (targets) performed 20 postural holding tasks selected from Ovako Working Posture Analysing System postures and gave RPE and BPD scores for each posture. Video clips of each target in each posture were shown to four expert ergonomists and 23 novices, who also gave RPE and BPD scores. Correlations between targets and observers scores were high, with significance exceeding p = 0.01. Observers were generally conservative, rating easy postures too high and difficult postures too low. All observers rated female targets higher than male targets. Female observers rated all targets higher then male observers. Vicarious perception of discomfort and exertion was possible, but there was not a one-to-one correspondence to ratings given by those experiencing the posture.     

Continuous‐time stable and unstable system modelling with orthonormal basis functions

In this paper, model sets for linear time‐invariant continuous‐time systems which are spanned by fixed‐pole orthonormal bases are investigated. The obtained model sets are shown to be complete in the Lebesque spaces L_p (1<p<∞) and in C, the space of complex‐valued functions that are continuous on the extended imaginary axis. The L_p norm error bounds for estimating systems in L_p by the partial sums of the Fourier series formed by the orthonormal functions are computed for the case 1<p<∞. Some inequalities on the 𝓁_p means of the Fourier coefficients are also derived. These results have application in estimation and model reduction of stable and unstable continuous‐time linear time‐invariant systems. A numerical example illustrates the use of the basis functions for the approximation of unstable infinite‐dimensional dynamics. Copyright © 2000 John Wiley & Sons, Ltd.