Thought Bytes...
December 2003

RSI Risk Factors #2: Static postures

How long can you hold your breath? Most people can comfortably hold their breath for about 30 seconds. The more fit you are, or the more you practice, the longer you can hold it. In most cases, the urge to breathe again isn’t actually due to lack of oxygen, but a build up of carbon dioxide in the lungs.

This carbon dioxide is one of the byproducts of metabolism. When we contract a muscle, oxygen is used, and carbon dioxide and lactic acid are produced. The more you contract a muscle, the more oxygen is required and the more byproducts are generated. We breath faster so that the lungs can clear out the CO2 and bring in more O2 to the blood that is rushing past the alveoli and capillaries in the lungs.

In dynamic movements, the contraction of the muscles helps to pump the blood to and from the muscle so that there is a good supply of O2 and the byproducts don’t build up. When the muscle is relaxed there is less demand for O2, so the rhythmic beating of the heart is sufficient to supply what is required.

But what about when you contract a muscle and hold the position? This requires energy and oxygen to create the contraction. The problem is that a contracted muscle restricts it’s own blood supply (in the same way that the contraction and relaxation of a muscle pumps the blood through). So, that contracted muscle is still getting signals from the brain to stay tight, but doesn’t have the oxygen that it needs. When this happens, the muscle switches into anaerobic metabolism. (Don’t worry, that’s as deep into Grade 12 Biology as I am going to go).

The thing about anaerobic metabolism though, is that it produces more lactic acid than aerobic (with oxygen) metabolism. And the problem with lactic acid is that it is an acid. A build up of lactic acid is what creates that burning feeling in your neck and shoulders when you have been sitting unsupported, hunched over the desk, holding a hand tool or carrying a heavy object. Not only do your muscles feel like they are burning, they are truly being irritated by the build-up of lactic acid in them.

Whether it is repeated short exposures or occasional long exposures, the build of lactic acid from holding static postures can damage the tissues. The key is to vary your activities and try to introduce more movement.

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