1. Tekanan/gaya pada otot yang berlebihan
2. Awkward Posture (postur kerja yang tidak benar)
3. Terjadinya pengulangan-pengulangan pekerjaan pada satu otot
4. Lamanya paparan yang diterima oleh otot
Copy picture from: MSD Prevention ToolBox
Awkward body positions
The key to reducing or eliminating the use of awkward body positions and work postures is to understand why they are being used in the first place. Awkward positions are often the result of the location and orientation of the object being worked on, poor workstation design, product design, tool design, or poor work habits. Several of these causes can be engineered out, eliminating the problem altogether. For example, a worker who bends over to lift objects out of large bins or cardboard boxes must assume an awkward body position. Raising and tilting the bins can easily eliminate the awkward position. Neutral positions are those that the parts of the body naturally assume at rest, placing the least amount of stress on joints and tissues. As muscles, tendons, and ligaments move away from the central portion of their range of motion, they become stretched and vulnerable to injury. As they approach the end of theirrange of motion, they become fully stretched and further motion due to sudden movements or unexpected loads may cause tissue injury. As the angle of a joint increases or decreases past its neutral position, the amount of force that muscles acting on that joint can easilyproduce is reduced because they are no longer in their most favourable positions. To compensate for this mechanicallycaused strength reduction, muscles try todevelop more force and their tendons are placed in even greater tension. This isadded stress that can lead to injury. Less-than-optimal postures such as leaning forward from the waist for extended periods of time, or bending the neck downwards at an exaggerated angle, can load muscles with ‘static work’. Static work involves muscles being tensed in fixed positions and overtime, becoming tired, uncomfortable,and even painful. Production line workers who have to bend their necks and hold them in one position often experience strain in their neck and shoulder muscles. Sedentary work
involving sitting or standing for long periods of time without movement can lead to pain and discomfort in the lower back.
Awkward body positions and their effects can be reduced by:
§ encouraging frequent changes of position. This avoids becoming “locked” into one position for extended periods of time.
§ avoiding forward and downward bending of the head and upper body. This commonly occurs when tasks, work surfaces, or controls are too low relative to the worker’s standing or sitting position.
§ avoiding having the arms held in a raised position, either in front of the body or out to the sides with the elbows bent. Such positions are often the result of work surfaces or controls being too high relative to the worker’s standing or sitting position.
§ avoiding twisted body positions. Arrange the work and workstation so that twisting is avoided.
§ avoiding positions that require a joint to be used for extended periods of time at the limit of its range of motion e.g. constant reaching behind the back can place considerable strain on the shoulder joint.
§ providing adequate back support in all chairs or seats. Back supports, preferably adjustable ones, improve posture, lessen fatigue, and make sitting for long periods of time more comfortable.
§ optimizing the position of arms and legs. Ensure that the arms and legs are positioned within their most favourable range of motion when muscular force needs to be exerted.
Forceful exertions may overload muscles, tendons, and ligaments. Forceful exertions are commonly used when lifting, pushing, pulling, and reaching. A packer on an assembly line
for example, may often use a highly forceful grip to assemble a lightweight item or lift a box or carton, especially if it is slippery or difficult to grasp. Workers who use tools such as handheld grinders for extended periods of time may be at risk of developing MSIs of the hand because of the amount of force needed to use, hold, and trigger the tools. Awkward wrist and arm positions may also contribute to the problem. Research studies have shown that work tasks should not require the worker to exert more than 30% of their maximum force for a particular muscle in a prolonged or repetitive manner. Any tasks that require the worker to exert a force in excess of 50% of a particular muscle’s strength, including occasional tasks, should be avoided. The closer a loaded muscle is to its strength or range of motion limits, the greater the risk of tissue damage and injury. For a given task, decreasing the required effort or load by as little as 10% allows a worker to perform their work at a constant level five to six times longer than if the effort or load
had not been decreased. The load influences worker tiredness and discomfort much more
than the length of time that the work is performed.
High muscular forces can be reduced by:
§ reducing the forces required to perform the task e.g. using mechanical aids when lifting and
handling materials, using jigs, vices, and clamps rather than hands to grip parts, keeping sharp edges of tools and equipment sharp, reducing contact forces on switches and controls, lubricating and maintaining tools and equipment.
§ distributing forces e.g. using a larger body part, such as an arm rather than a finger, to deliver the force.
§ establishing better mechanical advantage e.g. with larger, better positioned tools, with levers, or by involving larger muscle groups.
Repetitive movements eventually wear the body down. Without sufficient time to recover between repetitions, muscles become tired and may cramp. Other muscles try to help but they may also become tired, cramp, and become injured. How quickly this happens depends on how often a repetitive motion is performed, how quickly it is performed, and for how long the repetitive work continues. Repetitive work is more of a problem when it is combined with awkward body positions and forceful exertions. A worker who packages a small product day after day or who uses a stapler or power nailer to assemble wooden frames are examples of workers performing repetitive work.
Worker exposure to repetitive work and its effects can be reduced through:
§ automation of the task or portions of the task . Machines are particularly effective at performing repetitive tasks.
§ job rotation. This breaks up a worker’s exposure to a particular repetitive movement. It is extremely important that the new task involve different movements and muscle groups.
§ job diversity. Training workers to perform a series of properly selected jobs rather than the same, simple one repetitively reduces monotony, boredom, and the potential for injury. Jobs with greater diversity often provide workers with a sense of accomplishment.
§ job enrichment. Workers are given responsibility for a wider range of duties that require a variety of skills and qualifications. As examples, these duties may include work planning, inspection activities, or customer contacts.
§ frequent breaks. Frequent, short breaks from work activities provide workers an opportunity to recover from their activities by stretching, changing body positions, or relaxing hard-working muscles.
Other biomechanical risk factors
Compression and impact stress
Tissues can become compressed when they come into contact with the edges of workbenches, tool handles, machine corners, and poorly designed seating. Forces are concentrated on small areas of tissue, resulting in high localized pressure. This pressure can compress nerves, blood vessels, tendons, and other soft tissues, resulting in damage and injury. Using the hand or knee as a hammer is a form of external tissue compression known as impact stress. Hand hammering can damage one of the arteries that passes through the wrist and palm, eventually affecting the function of the thumb. Using the knee as a hammer can cause “carpet layer’s knee”, an inflammation of the tissues of the knee.
Hand-arm vibration is vibration transmitted to the arms through the hands. It can damage both the small blood vessels and small nerves of the fingers, resulting in two specific injuries: vibration-induced white finger and vibratory neuropathy. Together, these injuries are known as the hand-arm vibration syndrome (HAVS) and result in numbness, loss of finger coordination and dexterity, clumsiness, and an inability to perform fine motor tasks. lanching or loss of colour in the skin usually starts at the tips of the fingers but progresses as exposure time increases. The most important sources of vibration due to tools include grinders, sanders, drills, impact wrenches, jackhammers, riveting and chipping hammers, and chain saws. Sources of vibration having accelerations greater than 2.5 m/s2 are associated with increased rates of injury.While there is substantial evidence that whole body vibration is associated with lower back and neck injuries, identifying practical ways for employers to determine whether vehicles or other equipment produce hazardous exposures is difficult. Operators of off-road vehicles are likely to have the most hazardous exposures, depending on how long they work and the quality of the vehicle’s suspension system, shock absorbers, seats, and tires. Unlike hand tools and hand-arm vibration, there are no specific whole body vibration acceleration values available for vehicle models or other equipment.
Copy paste from: Chapter 3 Musculoskeletal disorder, OSH