The pony in the above video presented for a history intermittent dragging of his right hind leg at the walk. Occasionally, the leg would "lock" in the extended position and the pony would drag his limb for several strides until it "unlocked". On presentation, the pony tended to have a delay in flexion of the right hind when walked in a straight line. However, when walked in a figure 8, the right hind leg would stay in the extended position for a second or two and then suddenly jerk into the flexed position. When the pony was trotted in a straight line he was completely sound however during the downward transition his right hind wound "drop out" at the moment the leg went from the extended position to the flexed position.
This pony is experiencing a condition known as intermittent upward fixation of the patella. The patella (knee cap) is controlled by three patellar ligaments (medial, middle, and lateral patellar ligaments) which originate within the quadriceps muscle. As such, the contracture and relaxation of the quadriceps muscle allows for engagement of the patella to keep the leg in extension and ALSO allow for disengagement of the patella to allow flexion of the limb.
Intermittent upward fixation of the patella is common in young horses when they are first introduced to work under saddle. It is also more common in fast growing horses that tend to have a "straight" or upright conformation through their stifle. In my experience, this condition is more common in female versus male horses and is most often resolved by increasing the intensity of exercise, especially at the canter, uphill. However, some horses experience this condition regardless of increased exercise and require medical/surgical intervention. Years ago it was common place to transect the medial patellar ligament however long term evaluation of these horses suggested a predisposition to arthritic conditions of the stifle joint.
More commonly, these horse are treated with hormonal therapy (estrogen) , iodine blister of the medial patellar ligament, and scarring of the medial patellar ligament via surgical intervention. Mild cases often improve with increased work plus estrogen treatment. However, horses that are "locking" their stifles most likely will need surgical intervention.
Prior to recommending medical or surgical treatment, these horses should be evaluated for soundness and the stifle joint should be radiographed. Horses that have pain associated with OCD conditions of the stifle joint may present in a similar fashion and require a different approach in managing this condition. If your horses feels like he/she is "dropping out behind" when you are at the trot, especially during the downward transition from the trot to the walk or canter to the trot, it is likely that intermittent upward fixation of the patella is playing a role in their poor performance!!