Bowed Tendon

It is said that a horse is "bowed", or that it has a "bowed tendon",when there has been tearing of the superficial digital flexor tendon in the middle of the cannon bone region. This tear causes a curved, bow-like swelling on the back of the leg between the knee and the ankle. Although the swelling is usually in the middle of the cannon bone, it could be behind the knee, at the level of the ankle, or it may extend from the knee to the pastern.
Tendon injuries such as "bowed tendons" do not only happen to racehorses, but any breed or type of horse, performing almost any activity, can be at risk of tendon injury. They can be more serious than some types of fractures because the tendon heals very slowly and replaces torn tendon fibers with fibrous scar tissue. The healed tendon is less elastic, leaving it vulnerable to re-injury. When there is a significant injury, the horse may not be able to return to its previous level of performance due to persistent weakness of the tendon.

Fatigue, overexertion, unequal weight distribution on the feet and legs, poor surface conditions, racing around turns and lack of conditioning, combined with poor conformation, all can result in bowed tendons.