From The American Quarter Horse Journal
Bowed tendons are one of the most common and most serious injuries in racehorses, and they occur occasionally in other performance horses, as well.
Bowed tendons are the No. 1 reason for lameness after a race, and they are also the most common reason for horses to be scratched after entering a race. The term “bowed tendon” is used to describe the damage resulting in an enlargement or bowing outward of the tendon. The injury may result in a high, middle or low “bow,” or it may be a “full bow,” involving all three areas. Most bowed tendons occur in the front legs, since they carry more weight and are subjected to more stress.
Sprain or strain of the tendons below the knee may cause injury to the deep flexor tendon, the superficial flexor tendon, or both, as well as the surrounding tendon sheath. The attachments of the sheath may be torn from their places in the tendon, causing internal bleeding, inflammation, swelling and rupture, making the tendon “bow” outward.
The injury is painful and swollen, and some of the damage is permanent. A horse with a bowed tendon will never be quite as strong and sound again. But with proper and immediate care, the damage can be minimized, and the horse’s chances for continued usefulness will be greatly improved.
This kind of tendon injury is usually due to excessive strain at a hard gallop, especially if the horse is becoming tired. The most common cause is fatigue while racing or while performing some other fast movement. Sprains like this don’t generally occur when a horse is fresh, but more often when he becomes tired and his muscles are fatigued. When his muscles, which are more elastic than the tendons, cease to contract and expand as rapidly and readily as they should (because they are tired), the strain of fast movement comes more and more on the tendons.
As long as the muscles are fit and readily responding to the stimuli resulting in contraction, a sprain is unlikely. But when muscles tire, the strain comes on the tendons. The natural elasticity of the muscles saves the tendons. When the elasticity is lost, the tendon suffers. The horse is still trying to run as fast as he can, but his muscles may be contracting too slowly, or a little out of phase, or he becomes a bit uncoordinated, and a damaged tendon is the result. Contributing factors that may make a horse more likely to bow a tendon are long, weak pasterns, long upright pasterns (making a shorter tendon to begin with – the too-short tendon is holding the pastern too upright), toes too long, improper shoeing, muddy tracks, an overly accelerated training program, or a horse too heavy for his structure.
If a tendon is severely strained and injured, symptoms of a bow will appear almost immediately. There will be swelling over the whole area (the back of the leg below the knee), but later the enlargement may be more confined to a specific portion. Heat and pain will be evident when the back of the leg is pressed with your fingers. The horse may stand with the heel of his foot off the ground to ease the pressure and tension on the sore tendons at the back of the leg. His knee may be forward as he stands, with just his toe resting on the ground. When he walks, he won’t put his fetlock joint clear down to normal level, because of the pain from weight on the torn tendons.