Estimating upper limb discomfort level due to intermittent isometric pronation torque with various combinations of elbow angles, forearm rotation angles, force and frequency with upper arm at 90 degrees abduction

peer-reviewedIndustrial jobs involving upper arm abduction have a strong association with musculoskeletal disorders and injury. But there is still paucity of data on the different risk factors that are responsible for the genesis of such disorders or injuries. The current laboratory study is an attempt in that direction. Thirty-six right-handed male university students participated in a full factorial model of three forearm rotation angles (60% prone and supine and neutral range of motion), three elbow angles (45 degrees, 90 degrees and 135 degrees), two exertion frequencies (10 and 20/min) and two levels of pronation torque (10% and 20% MVC). Discomfort rating after each five-minute treatment was recorded on a visual analogue scale. Repeated measures ANCOVA with grip endurance time as a covariate indicated that forearm rotation angle (p = 0.001), elbow flexion angle (p = 0.016), MVC torque (p = 0.001) and frequency (p = 0.049) were significant. Grip endurance time was not significant (p = 0.74). EMG activity of the Pronator Teres (PT) and the Extensor Carpi Radialis Brevis (ECRB) revealed that both muscles were affected by forearm rotation and level of MVC torque. A supplementary experiment in which MVC pronation torque at different articulations was measured showed that some of the increased discomfort appeared to be due to increased relative NIVC at some of the extreme articulations. The findings indicated that, with the upper arm in abduction, an elbow angle of 45 degrees and forearm prone, are a posture vulnerable to injury and should be avoided. Grip endurance time as a covariate warrants further investigation. Relevance to industry There is still a paucity of data on risk factors for musculoskeletal disorders for upper arm articulations typical of industrial jobs, especially postures involving upper arm abduction. Industrial jobs involving upper arm abduction have a strong association with injury as operators must often maintain static upper arm abduction while performing tasks for long durations. This study presents discomfort and pronation torque MVC data at different upper arm articulations to identify and control high-risk tasks in industry well before they develop into Musculoskeletal Disorders, especially at the design stage when using biomechanical models. (c) 2007 Elsevier B.V. All rights reserved.ACCEPTEDpeer-reviewe

The influence of gender and individual differences on, risks of injury due to layout in assembly work

This paper examines variation in elbow and shoulder joint angles between genders for a simulated light assembly task within normal reach at three bin. distances. Joint angle ranges were estimated for 5th, 50th and 95th percentile values pooled from males and females for each of the 17 task elements. Large differences in risk levels were found between the 5th and 95th percentile estimates. Male elbow angles were smaller than the females', but the male shoulder angles were greater than the females', on average. These results have important implications for workplace design

Upper-limb surface electro-myography at maximum supination and pronation torques: the effect of elbow and forearm angle

Forearm pronation and supination, and increased muscular activity in the wrist extensors have been both linked separately to work-related injuries of the upper limb, especially humeral epicondylitis. However, there is a lack of information on forearm torque strength at ranges of elbow and forearm angles typical of industrial tasks. There is a need for strength data on forearm torques at different upper limb angles need to be investigated. Such study should also include the measurement of muscular activity for the prime torque muscles and also other muscles at possible risk of injury due to high exertion levels during tasks requiring forearm torques. Twenty-four male subjects participated in the study that involved maximum forearm torque exertions for the right arm, in the pronation and supination directions, and at four elbow and three forearm rotation angles. Surface EMG (SEMG) was used to evaluate the muscular activity of the pronator teres (PT), pronator quadratus (PQ), biceps brachi (BB), brachioradialis (BR), mid deltoid (DT) and the extensor carpi radialis brevis (ECRB) during maximum torque exertions. Repeated measures ANOVA indicated that both direction and forearm angle had a significant effect on the maximum torques (p<0.05) while elbow angle and the interactions were highly significant (p<0.001). The results revealed that supination torques were stronger overall with a mean maximum value of 16.2 Nm recorded for the forearm 75% prone. Mean maximum pronation torque was recorded as 13.1 Nm for a neutral forearm with the elbow flexed 45°. The data also indicated that forearm angle had a greater effect on supination torque than pronation torque. Supination torques were stronger for the mid range of elbow flexion, but pronation torques increased with increasing elbow extension. The strength profiles for the maximum torque exertions were reflected in the EMG changes in the prime supinators and pronators. In addition, the EMG data expressed as the percentage of Maximum Voluntary Electrical activity (MVE), revealed high muscular activity in the ECRB for both supination (26-43% MVE) and pronation torques (17-55% MVE). The results suggest that the ECRB acts as a stabiliser to the forearm flexors for gripping during pronation torques depending on forearm angle, but acts as a prime mover in wrist extension for supination torques with little effect of elbow and forearm angle. This indicates a direct link between forearm rotations against resistance and high muscular activity in the wrist extensors, thereby increasing stress on the forearm musculo-skeletal system, especially the lateral epicondyle

Forearm torque strengths and discomfort profiles in pronation and supination

This experiment investigated maximum forearm pronation and supination torques and forearm discomfort, for intermittent torque exertions in supine and prone forearm angles for the right arm. Twenty two subjects participated in the study that comprised two parts, the first of which involved measurement of maximum forearm torque in both twisting directions at five forearm angles including neutral. This was followed by endurance tests at 50% MVC in both directions. The second part of the study involved subjects performing five minute long intermittent isometric torque exercises at 20% MVC in both directions at eleven forearm angles. Regression equations were developed that accurately predict torques as a function of forearm angle expressed as a percentage of maximum motion. Analysis of the discomfort data for the intermittent isometric torque exertions indicated that both forearm angle and twisting direction significantly affected forearm discomfort (p<0.001). A significant two-way interaction (p<0.01) was identified between forearm angle and direction for supine forearm angles only. The results provide important strength and discomfort models for the design of tasks involving static or repetitive forearm twisting. Such tasks have a strong association with forearm injuries including lateral and medial epicondylitis. These results provide needed data on the risk factors associated with these injuries so they can be prevented

Effects of gender and reach distance on risks of musculoskeletal injuries in an assembly task

This study investigated differences in elbow and shoulder flexion angles in an assembly task. The experiment involved ten subjects on a simulated assembly task that consisted of seventeen task elements. The locations of the components were at three distances from the subjects. Confidence intervals (90%) were estimated and large differences in risk levels were found when data were pooled from both males and females. Between gender comparisons of joint angles revealed that the male elbow angles were smaller than the females, but the male shoulder angles were greater than the females on average. A within-gender analysis found greater change in angles for the female group with an increase in task distance from the body. This was not the case for the males. This was explained with reference to a previous study that related anthropometrics to differences in joint angles. The shoulder was identified as a joint sensitive to small physical changes in the workplace layout that may make a task more awkward to reach. This was not the case for the elbow. Finally, it was noted that both directions of movement and initial hand location affected final elbow and shoulder joint angles for task elements

Workplace injury risk prediction and risk reduction tools for electronics assembly work

Ergonomics approaches to risk of musculo skeletal disorders are often reactive rather than proactive, an approach supported by a lack of ergonomics expertise, especially in small to medium size companies. However, the EU Framework Directive on health and safety at work (89/391/EEC) demands a comprehensive risk assessment by the employer. Further, the Machinery Directive (89/392/EEC), demands a comprehensive risk assessment at an early design stage. Presented are the results of field studies that have identified suitable risk evaluation methods for electronics assemlby and rework tasks. To assist non-expert ergonomists, a risk reduction module is also presented. Combined, the risk evaluation and risk reduction approaches presented here provide a framework for reducing musculsoskeletal disorders in line with EU safety directive

Effects of upper arm articulations on shoulder-arm discomfort profile in a pronation task

Twenty right-handed male university students performed a full factorial experiment, consisting of three forearm rotation angles (60% prone and supine, and neutral), two elbow angles (45°and 90°), three humeral rotation angles (45°, 90° and 135°), and two upper arm angles (45° flexion and neutral). The task was a one-second pronation torque of 20% Maximum Voluntary Contraction (MVC) relative to MVC at the standard position of the arm, 15 times per minute for 5 minutes, at each postural combination. Discomfort rating after the end of each five minute exertion was recorded on a visual analogue scale. A repeated measures ANCOVA on discomfort score indicated that endurance time was a significant covariate. Other significant factors were upper arm flexion angle, forearm rotation angle, and the interactions of upper arm*elbow and humeral rotation*forearm*endurance time. A supplementary experiment showed that in some of the deviated postures combinations the subjects required additional muscle force to achieve the 20% MVC from the original testing posture. Such data can be helpful for designing workplaces and developing biomechanical models, especially for assessment of designs in virtual environments

Upper limb discomfort profile due to intermittent isometric pronation torque at different postural combinations of the shoulder-arm system

Twenty-seven right-handed male university students participated in this study, which comprised a full factorial model consisting of three forearm rotation angles (60% prone and supine and neutral range of motion), three elbow angles ( 45°, 90° and 135°), three upper arm angles ( 45° flexion/extension and neutral), one exertion frequency ( 15 per min) and one level of pronation torque (20% maximum voluntary contraction (MVC) relative to MVC at each articulation). Discomfort rating after the end of each 5 min treatment was recorded on a visual analogue scale. Results of a repeated measures analysis of covariance on discomfort score, with torque endurance time as covariate, indicated that none of the factors was significant including torque endurance time (p = 0.153). An initial data collection phase preceded the main experiment in order to ensure that participants exerted exactly 20% MVC of the particular articulation. In this phase MVC pronation torque was measured at each articulation. The data revealed a significant forearm rotation angle effect (p = 0.001) and participant effect (p = 0.001). Of the two-way interactions, elbow*participant (p = 0.004), forearm*participant (p = 0.001) and upper arm* participant (p = 0.005) were the significant factors. Electromyographic activity of the pronator teres and biceps brachii muscles revealed no significant change in muscle activity in most of the articulations. Industrial jobs involving deviated upper arm postures are typical in industry but have a strong association with injury. Data from this study will enable better understanding of the effects of deviated upper arm postures on musculoskeletal disorders and can also be used to identify and control high-risk tasks in industry