Inflammation of a tendon. The inflammation is often caused by mechanical damage (tearing) of the fibres which is colloquially known as 'spraining' a tendon. Mechanical damage is typically caused by repeated cycles (or on occasions a single incidence) of overloading, making tendinitis a typical injury of horses involved in athletic sports such as racing. Tendinitis may also result from direct trauma to a tendon (eg a horse striking into itself). The tendency of a horse to develop tendinitis can depend on individual factors (breeding/age), the type of exercise/sport which it is engaged in, the manner in which and the ground on which it is worked.  

The deep and superficial flexor tendons are most commonly affected by tendinitis. Affected tendons are typically warm, swollen and painful, and horses are often lame. Tendinitis is regarded as a very serious, even career threatening injury for an equine athlete, as once damaged tendon fibres are not able to return to their original state. Scar tissue within the tendon will change the physical properties of the tendon in such as way that it is unlikely to regain its full tensile strength.

Tendinitis requires a horse to be rested for up to a year to give the tendon the best change or recovery. There are various forms of treatment, including oral anti-inflammatory medication and various forms of local therapy.