The equine hind limb is actively stabilized during standing.

Horses spend much of their life standing, and they are believed to be able to keep their limbs straight without muscular effort. We tested the hypothesis that the stifle (knee) and hock (tarsal) joints could be stabilized merely with the help of a passive lock mechanism whereby the patella is secured behind a hook, formed by the medial femoral trochlea. In anaesthetized animals and isolated limbs the stifle and hock flex readily under compression. In isolated limbs this collapse was prevented by a small force applied to the patella, mimicking the action of the vastus medialis muscle. In vivo, when the limb was planted loosely on the ground none of the muscles with a connection to the patella was active. However, during weight-bearing the vastus medialis (but no other muscle) was active, providing the necessary traction to stabilize the stifle. The required tension was estimated to be less than 2% of the force that would be needed in absence of a lock mechanism. Diagnosis and treatment of patellar fixation should include the possibility of overactive vastus medialis muscle as a possible cause of the disorder.