Woods has returned to action at the Masters after a five-month break

Tiger Woods has admitted to using platelet rich plasma (PRP) therapy, also known as "blood spinning", in his rehabilitation from knee and Achilles injuries. Here, we explain the methods and how it works.

What is the procedure?

Blood is removed from the body, spun in a centrifuge and reduced to a high-concentration platelet solution. It is then reinjected into the body to act as a catalyst in the healing process. In sport, PRP is permitted only to treat tendon and cartilage problems, not muscle tears.

How does it work?

The concentrated platelet solution contains growth factors pivotal in the repair and regeneration of tissues. These special proteins also help the healing process by stimulating growth.

How effective is it?

Subject to debate, with further research required. It is theoretical and controversial, with many positive results anecdotal. A recent study published in the Journal of the American Medical Association into the treatment of chronic Achilles problems found that PRP compared with a placebo saline injection and "did not result in greater improvement in pain and activity".

How ethical is the practice?

It is a well-accepted practice which has been used for some time, including in NFL American football and Major League Baseball. A problem would be if practitioners were to include the storage of blood or blood products but there is no suggestion that this is happening.

Is it legal in sport?

The practice, when "administered by intramuscular route", is prohibited by the World Anti-Doping Agency (Wada). However, a treatment to a knee or Achilles would not be counted as such an approach and is therefore permissible in sport. However, Wada will be watching the outcome of future research with a keen eye and this could change.

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