Champions League:Roy Keane surprisingly recovered sufficiently from a hamstring injury to come on as a second-half substitute for Manchester United last night.

An injury such as Keane had is the rupture of muscle fibres in the back of the leg. This results in internal bleeding and bruising.

The oxygen treatment Keane is believed to have undertaken, called Hyperbaric Oxygen Therapy (HBO), seeks to accelerate the cleaning away of the debris of the injury and promote the formation of new capillary and peripheral blood vessels.

This results in a much faster healing of the injury.

The air we normally breath contains 21 per cent oxygen. With HBO, Keane would have breathed 100 per cent oxygen under much higher pressure than normal.

In addition to the blood, other body fluids including the lymph and cerebrospinal fluids are infused with the healing properties of the oxygen.

This promotes the healing process as more oxygen helps the injured vessels regenerate and accelerates fibroblast (the cells contained in connective tissue in the hamstring) activity.

David Beckham is also reputed to be undertaking oxygen treatment, but for a different reason. Because he will remain inactive his foot mends, his fitness will suffer. The treatment offered is that he spends time in an environment lower in oxygen than normal.

When the body is forced to make do with a limited amount of oxygen, it steps up production of red blood cells. The net effect is to "artificially" increase the body's capacity for exercise. In Beckham's case, it may help cancel out the fact that he cannot train.

By Johnny Watterson