There are three types of force, the static force required to lift and hold a load, the kinetic force required to move an object (push or pull) and grip force. Force is the exertion on internalized bodily tissues, including vertebral disc compression from lifting a heavy box, or tension within a muscles during a pinch grip. Forceful muscular exertions place high loads on the muscles, tendons, joints, and discs, and are therefore associated with most musculoskeletal disorders. Muscles fatigue with increased muscular exertion, and the time needed to recover increases. If recovery time is limited, soft tissue injury is more likely to occur. Where other risk factors are present, especially frequent repetitions of exertions, awkward postures or static postures, they add to the force required to accomplish the exertion. Generally the larger the force the higher the injury risk, so it is always beneficial to reduce the weight of the object to be handled.
Posture is the position the body is in while completing the movement. Muscles and tendons are strongest in neutral positions, so any deviation reduces stability and strength. With prolonged awkward postures, muscles can be strained and tendons and nerves stretched or pinched. Tasks that involve long reaches require considerably more force to accomplish than tasks that can be performed closer to the body.
Summarized below are postures that accompany higher injury risks:
Common injuries involving repetitive awkward postures include carpal tunnel syndrome in the wrist and impingement syndrome in the shoulder. To avoid injuries caused by awkward postures, avoid lifting objects from overhead, use an ergonomic computer mouse and avoid far reaches with the upper body.
Repetition is described as the number of times the same movement/task is completed within a given time. Highly repetitive tasks are of concern because they work the same muscles, tendons, and ligaments repeatedly, rarely allowing for adequate recovery. With time, the effort to maintain the repetitive movements continually increases. When the work activity is continued, in spite of the developing fatigue, injuries occur.
When the environment is too cold, blood flow is diverted from the extremities, to the core to maintain body temperature. Decreased blood flow results in lower muscle power and early onset of fatigue, as well as reduced sensation in extremities. Consequently, when the work area is too hot, blood flow increases to the skin to dissipate the heat, again reducing blood flow to skeletal muscle. Decreases in strength and early onset of fatigue as well as increased need for recovery time are all consequences of hot work environments.
Vibration causes reduced blood flow to the fingers and hands, resulting in localized numbness. Prolonged and repeated exposure to vibration can permanently damage blood vessels leading to White finger syndrome or even Reynaud’s Disease. Hand Arm Vibration also interferes with sensory receptor feedback, leading to tighter gripping of hand tools.