Thought Bytes...
February 2004

RSI Risk Factors #4: Force

Strength is a reflection of the amount of force generated when a muscle contracts. So, when we talk about force as a factor in the development of Repetitive Strain Injuries (RSI’s), we are asking how much strength is required to do the action.

Every person has different strength capabilities. Your ability generate force changes depending on what you have been doing with the same muscles over the last few hours, how much recovery time there has been between activities (i.e. the static work and repetition factors discussed previously), what foods you ate to provide nutrients to the muscles, whether there is an injury and you perceive pain, even what your mood is.

When a muscle contracts, elements within the fibres overlap creating tension in the muscle. This tension pulls on the tendons at the ends of the muscles creating pull on the bones. The pull on the bones produces movement at the joint.

Working backwards now, if you need to move a joint forcefully, that means that there must be more pull on the bones, more tension within the muscle and therefore more movement of the elements within fibres of the muscle. If those elements (actually called myosin) are not in an ideal position, you will not be able to generate as much force. And, if the force is more than the muscle can tolerate, the tissues could experience small tears.

The amount of force you can generate is therefore a function of the posture that you use. When you use the “neutral” posture, the muscle fibres are overlapped an ideal amount, so you can generate more force. The more you deviate from the neutral posture, the less force can be generated. Alternatively, if you need to maintain the same force level, then your muscles have to work that much harder.

For example, try squeezing a ball with your hand and arm hanging down at the side of your body. Then try it with your wrist in a flexed (palm closer to the forearm) position. Then try it with the wrist in an extended position. Can you feel how much less pressure you are applying to the ball when the forearm muscles are not working at their ideal length?

Now imagine that it was your job to hold a hand tool – a screwdriver or a drill. If you can’t work with your muscles at an ideal length, then there will be considerably more strain on them over time. As an ergonomist, we try to find ways to reduce the amount of force required to do an activity. It could be as simple as making sure a drawer moves smoothly on it’s track or as complex as recommending lifting assist.

Bear in mind however, that completely eliminating the use of force is not good for us either. We need to use our muscles to maintain strength and flexibility and to burn the energy we consume. As always, it is the combination of reducing the force requirements to a reasonable amount, together with ensuring that the worker uses correct working postures at an optimal level of activity (no static or too repetitive use) that will keep you working well.

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