Repetitive strain injury (RSI), also known as occupational overuse syndrome (OOS), is an umbrella term covering all kinds of work-related injuries to the muscles, nerves, and tendons of the upper limbs. RSI is a condition involving pain and swelling in the hands, arms and shoulders suffered by someone working long hours using continuous wrist and hand motions, notably at a computer keyboard and screen.
Occupational overuse syndrome (OOS) occurs due to repeated tensioning of muscles. Tensing a muscle restricts the blood flow to the muscles and tendons. At 50% of full muscle power, the blood flow to that muscle is completely restricted. Without fresh blood, muscles use stored energy until they run out, then they switch to anaerobic (without oxygen) metabolism, which generates by-products like lactic acid, which cause pain. Once one muscle hurts, all its neighbors tense up, perhaps to relieve the load. This makes sense for a normal sort of injury, but it only makes things worse with repetitive motion. More tension means less blood flow, and the cycle continues. Another by-product of the lack of blood flow is tingling and numbness. This arises from lack of blood to the nerves.
OOS problems come in two main types: local conditions and diffuse conditions. Local problems relate to specific muscles, tendons, tendon sheaths, nerves, etc being inflamed or otherwise hurt. Diffuse conditions, often mistaken for local problems, can involve muscle discomfort, pain, burning and/or tingling; with identifiable areas of tenderness in muscles, although they are not necessarily "the problem.
According to the USA Department of Labour, RSI cost American businesses $20 million in 1992, an eightfold increase from 1982. In 1991, approximately 331,000 people needed wrist splints, anti-inflammatory medication or rest to recover from the ailment. About 100,000 people also underwent surgery to cut the ligament and relieve pressure on the nerve (although this causes loss of hand strength in some people). 2000 lawsuits have been filed in the USA against computer manufacturers by individuals claiming keyboard injury. In 1994, one computer manufacturer put warning labels about the risk of RSI on its keyboards. In the UK and Australia, employers have been sued for not protecting their employees adequately.