Documents measurement of load on muscles 10 results

Objectives The purpose of this study was to develop and validate a sampling strategy for characterizing the finger force exposures associated with computer mouse use. Methods Mouse forces were measured from 16 subjects (8 men, 8 women), on 3 separate days, at their actual workstations while they performed (i) their regular work, (ii) a battery of standardized tasks, and (iii) simulated mouse use. Results The forces applied to the mouse did not vary between hours or days. During regular work, the mouse was used 78.0 (SD 40.7) times per hour, accounting for 23.7 (SD 9.5)% of the worktime. The mean forces applied to the sides and button of the mouse were low, averaging 0.6 % of the maximal voluntary contraction (%MVC) (0.43 N) and 0.8 % MVC (0.35 N), respectively. The forces applied to the mouse during the standardized tasks differed from the regular work forces; however, there were moderate-to-strong correlations between the 2 measures. Conclusions With respect to performing exposure assessment studies, the 3 major findings were (i) mouse force measurements should be made while subjects perform their actual work in order to characterize the absolute applied force accurately, (ii) the forces applied to the mouse during the performance of a short battery of standardized tasks can be used to characterize relative exposure and identify computer operators or work situations for which higher forces are applied to the mouse, and (iii) subjects cannot accurately simulate mouse forces.

"The electromyographic (EMG) activity of shoulder and forearm muscles was recorded during a standardized computer task with different combinations of time pressure, precision demands, and mental demands to study the interaction of these factors and their effect on muscular response during simulated computer work. The interaction between work pace and other exposure factors must be taken into account when the effects of changes in exposure demands on muscular response are predicted. Only then can it be predicted whether changing demands will constitute a risk of developing musculoskeletal disorders."

"La manutention manuelle de mobiles (chariots, containers, transpalettes...) engendre des efforts physiques importants et peut, sous certaines conditions, présenter des risques d'accidents du travail et de maladies professionnelles. Des normes recommandent des valeurs maximales de forces de tirer-pousser. La mesure de ces forces requière toutefois un équipement spécifique et une méthodologie adaptée. La description de trois interventions menées dans différents secteurs permet d'apprécier le protocole de mesure à mettre en place, les résultats pouvant être obtenus en termes de forces mais également de répercussions physiologiques, et l'influence de certains facteurs physiques sur les efforts de tirer-pousser. L'ensemble des données obtenues dans ces interventions montrent par ailleurs que les forces mesurées sont souvent supérieures aux recommandations normatives. Des améliorations techniques du mobile peuvent réduire ces efforts. Toutefois, la problématique des efforts de tirer-pousser devrait être discutée dès la conception du mobile, de l'environnement et de l'organisation du travail."

"The aim of this experiment was to compare lumbar spinal loads during individual and team lifting tasks. Ten healthy male subjects performed individual lifts with a box mass of 15, 20 and 25 kg and two-person team lifts with a box mass of 30, 40 and 50 kg from the floor to standing knuckle height. Boxes instrumented with force transducers were used to measure vertical and horizontal hand forces, whilst sagittal plane segmental kinematics were determined using a video based motion measurement system. Dynamic L4/L5 torques were calculated and used in a single equivalent extensor force model of the lumbar spine to estimate L4/L5 compression and shear forces. A significant reduction in L4/L5 torque and compression force of approximately 20% was found during team lifts compared to individual lifts. Two main reasons for the reduced spinal loads in team lifting compared to individual lifting were identified: (1) the horizontal hand force (i.e. pulling force) was greater in team lifting, and (2) the horizontal position of the hands was closer to the lumbar spine during team lifts. The horizontal hand force and position of the hands had approximately equal contributions in reducing the spinal load during team lifting compared to individual lifting."

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.

Computer mouse use has become an integral part of office work in the past decade. Intensive mouse use has been associated with increased risk of upper extremity musculoskeletal disorders, including carpal tunnel syndrome. Sustained, elevated fluid pressure in the carpal tunnel may play a role in the pathophysiology of carpal tunnel syndrome. Carpal tunnel pressure was measured in 14 healthy individuals while they performed tasks using three different computer mice. Participants performed a multidirectional dragging ('drag and drop') task starting with the hand resting (static posture) on the mouse. With one mouse, an additional pointing ('point-and-click') task was performed. All mice were associated with similar wrist extension postures (p=0.41) and carpal tunnel pressures (p=0.48). Pressures were significantly greater during dragging and pointing tasks than when resting the hand (static posture) on the mouse (p=0.003). The mean pressures during the dragging tasks were 28.8- 33.1 mmHg, ~ 12 mmHg greater than the static postures. Pressures during the dragging task were higher than the pointing task (33.1 versus 28.0 mmHg), although the difference was borderline non-significant (p=0.06). In many participants the carpal tunnel pressures measured during mouse use were greater than pressures known to alter nerve function and structure, indicating that jobs with long periods of intensive mouse use may be at an increased risk of median mononeuropathy. A recommendation is made to minimize wrist extension, minimize prolonged dragging tasks and frequently perform other tasks with the mousing hand.

The development of the RULA (rapid upper limb assessment) survey method is described. The method requires no special equipment and may be used to provide a quick assessment of the postures of the neck, trunk and upper limbs along with muscle function and the external loads experienced by the body. A coding system is used to generate an action list which indicates the level of intervention required to reduce the risks of injury due to physical loading on the operator. The technique is of particular assistance in fulfilling the assessment requirements of the EC Directive 90/270/EEC and UK guidelines.

"The quantitative relationship between exposure to physical risk factors and upper extremity work-related musculoskeletal disorders (UE-WMSDs) is virtually unknown. To explore the variation, objective measurements were derived in 43 types of work (686 individuals), using goniometry for the wrists and electromyography (EMG) for the forearm extensor muscles.The variations due to work were great for wrist movements, wrist positions, muscular rest, as well as peak load, ranging 1.4-54 °/s (flexion velocity; 50th percentile), -30°-3° (flexion angle; 50th percentile), 0.2-23% of time, and 3.4-41% of maximal EMG (90th percentile), respectively. Even within work categories, e.g. "repetitive industrial", there were large variations for all measures. Hence, classification without measurements has limited value.Most low-velocity work was accompanied by much muscular rest; however, the low velocity for mouse-intensive computer-work meant very little rest.Technical measurements are suitable as exposure measures in epidemiological studies, as well as a base for decisions about interventions. The multidimensional character of exposure - wrist movements, wrist postures, muscular recovery, and peak load - has to be considered.Relevance to industryDirect measurements provide objective and quantitative measures of the main physical risk factors for UE-WMSDs, appropriate for estimating the risk, as well as giving priority to, and evaluating, interventions."