Thought Bytes...
November 2003

RSI Risk Factors #1: Non-neutral postures

It doesn’t matter what kind of job you do. Whether you are a postal carrier, a labourer, a dog walker or an office worker, the first risk factor for developing a Repetitive Strain Injury in every ergonomists list is non-neutral postures.

The human body is a well designed machine, provided it is used according to specifications. Each joint has an ideal range of motion, and muscles work best at their ideal fibre length. When these two factors are combined you get the neutral postures.

BACK: When you consider that each of the vertebrae in the back has 3 joints to it (the disc and the 2 facet joints) and there are dozens of muscles controlling movement in the back, it comes as little surprise that we occasionally strain this complex system. Researchers have shown over and over again that maintaining the natural curves of the back – in at the neck, out in the thoracic spine and in again at the lumbar spine – reduces the risk of injury to the discs. When these curves are maintained, the muscles are also at their ideal lengths so that they can generate more force with less effort.

WRIST: Another complex joint, the wrist needs to be used in a flat and straight posture. This differs slightly from what orthopedists refer to as a “neutral posture” (with a slight amount of extension) that is used for healing. Keeping the wrist flat allows the tendons responsible for finger movement to work in a straight line. When the wrist is extended or bent to the side, the tendons have to go around a corner. This adds stress to the tendons, requiring the muscles to work harder which adds even more stress to the tendons.

ARM: The upper arm muscles are strongest when the elbow is used at approximately 90o. In this position, the biceps fibres are at an ideal length and the elbow is the most stable.

NECK: Imagine a 15 lb. bowling ball on top of a pyramid, held in place by a bunch of strings pulling in all directions. Then imagine that the ball is shifted slightly to one side and how much extra tension there would be on the strings on the opposite side to the direction of travel. That is pretty close to the situation the head is in. Keeping the head upright and looking forward reduces the load on the muscles and helps to maintain the curve in the cervical spine.

SHOULDERS: The shoulder has a great range of motion, yet is a highly unstable set of joints. It is actually 3 joints with only one direct connection to the trunk (at the sternum). All the other connections to the torso are via muscles, tendons and ligaments. When the shoulder is in a relaxed position, the upper arm is hanging at the sides of the body and there is no tension in the trapezius or deltoid muscles that bridge from the shoulder to the neck.

Using the muscles at their ideal fibre length reduces the amount of work that they have to do. This is also the safest, most stable, position for the joints. We need to use the whole range of motion regularly to keep good mobility. The next issue will address the age old question of “how much is too much?”.

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