PHYSICAL TASKS:MANAGEMENT OF MUSCULOSKELETAL DISORDERS

MANAGEMENT OF MUSCULOSKELETAL DISORDERS

Ergonomic Guidelines

Most of the current guidelines for control of musculoskeletal disorders at work aim to reduce the extent of movements at the joints, reduce excessive force levels, and reduce the exposure to highly repetitive and stereotyped movements. For example, some of the common methods to control for wrist posture, which is believed one of the risk factors for carpal tunnel syndrome, are altering the geometry of tool or controls (e.g., bending the tool or handle), changing the location / positioning of the part, and changing the position of the worker in relation to the work object. In order to control for the extent of force required to perform a task, one can reduce the force required through tool and fixture redesign, distribute the application of force, or increase the mechanical advantage of the (muscle) lever system.

It has been shown that in the dynamic tasks involving upper extremities, the posture of the hand itself has very little predictive power for the risk of musculoskeletal disorders. Rather, it is the velocity and acceleration of the joint that significantly differentiate the musculoskeletal disorders risk levels (Schoenmarklin and Marras 1990). This is because the tendon force, which is a risk factor of mus- culoskeletal disorders, is affected by wrist acceleration. The acceleration of the wrist in a dynamic task requires transmission of the forearm forces to the tendons. Some of this force is lost to friction against the ligaments and bones in the carpal tunnel. This frictional force can irritate the tendons’ synovial membranes and cause tenosynovitis or carpal tunnel syndrom (CTS). These new research results clearly demonstrate the importance of dynamic components in assessing CTD risk of highly repetitive jobs.

With respect to task repetitivness, it is believed today that jobs with a cycle time of less than 30 seconds and a fundamental cycle that exceeds 50% of the total cycle (exposure) time lead to increased risk of musculoskeletal disorders. Because of neurophysiological needs of the working muscles, adequate rest pauses (determined based on scientific knowledge on the physiology of muscular fatigue and recovery) should be scheduled to provide relief for the most active muscles used on the job. Furthermore, reduction in task repetition can be achieved by, for example, by task enlargement (increasing variety of tasks to perform), increase in the job cycle time, and work mechanization and automation.

The expected benefits of reduced musculoskeletal disorders problems in industry are improved productivity and quality of work products, enhanced safety and health of employees, higher employee morale, and accommodation of people with alternative physical abilities. Strategies for managing musculoskeletal disorders at work should focus on prevention efforts and should include, at the plant level, employee education, ergonomic job redesign, and other early intervention efforts, including engineering design technologies such as workplace reengineering, active and passive surveillance. At the macro-level, management of musculoskeletal disorders should aim to provide adequate occupa- tional health care provisions, legislation, and industry-wide standardization.

Administrative and Engineering Controls

The recommendations for prevention of musculoskeletal disorders can be classified as either primarily administrative, that is, focusing on personnel solutions, or engineering, that is, focusing on redesign- ing tools, workstations, and jobs (Putz-Anderson 1988). In general, administrative controls are those actions to be taken by the management that limit the potentially harmful effects of a physically stressful job on individual workers. Administrative controls, which are focused on workers, refer to modification of existing personnel functions such as worker training, job rotation, and matching employees to job assignments.

Workplace design to prevent repetitive strain injury should be directed toward fulfilling the fol- lowing recommendations:

1. Permit several different working postures.

2. Place controls, tools, and materials between waist and shoulder height for ease of reach and operation.

3. Use jigs and fixtures for holding purposes.

4. Resequence jobs to reduce the repetition.

5. Automate highly repetitive operations.

6. Allow self-pacing of work whenever feasible.

7. Allow frequent (voluntary and mandatory) rest breaks.

The following guidelines should be followed (for details see Putz-Anderson 1988):

1. Make sure the center of gravity of the tool is located close to the body and the tool is balanced.

2. Use power tools to reduce the force and repetition required.

3. Redesign the straight tool handle; bend it as necessary to preserve the neutral posture of the wrist.

4. Use tools with pistol grips and straight grips, respectively, where the tool axis in use is horizontal and vertical (or when the direction of force is perpendicular to the workplace).

5. Avoid tools that require working with the flexed wrist and extended arm at the same time or call for the flexion of distal phalanges (last joints) of the fingers.

6. Minimize the tool weight; suspend all tools heavier than 20 N (or 2 kg of force) by a counterbalancing harness.

7. Align the tool’s center of gravity with the center of the grasping hand.

8. Use special-purpose tools that facilitate fitting the task to the worker (avoid standard off the- shelf tools for specific repetitive operations).

9. Design tools so that workers can use them with either hand.

10. Use power grip where power is needed and precision grip for precise tasks.

11. The handles and grips should be cylindrical or oval with a diameter of 3.0–4.5 cm (for precise operations the recommended diameter is 0.5–1.2 cm).

12. The minimum handle diameter should be 10.0 cm, and 11.5–12.0 cm is preferable.

13. A handle span of 5.0–6.7 cm can be used by male and female workers.

14. Triggers on power tools should be at least 5.1 cm wide, allowing their activation by two or three fingers.

15. Avoid form-fitting handles that cannot be easily adjusted.

16. Provide handles that are nonporous, nonslip and nonconductive (thermally and electrically).