Thoracic "Skipping" as a Consequence of Proximal Patellar Hesitation (PPH)
We often observe "skipping" or "hesitation" in the thoracic limbs is a consequence of proximal patellar hesitation (i.e. PPH) on the contralateral side.
At the trot, the normal horse maintains synchrony in the diagonal pair of limbs. In other words, the diagonal pair of limbs move at the same time. For example, the left pelvic (LH) and right thoracic (RF) limbs maintain concurrent weightbearing and nonweightbearing (flight) phases of the stride. The right pelvic (RH) and left thoracic (LF) limbs do the same. This gait characteristic is termed diagonal synchrony.
In cases of proximal patellar hesitation (PPH), the horse may be unable to flex one or both pelvic limbs from an extended position at the appropriate time during the trot; there is a slight delay in flexion due to proximal (upward) hesitation or "snagging" of the patella. Consequently, the pelvic limb is slower at initiating the flexion phase of the stride. It does not flex as quickly as the contralateral thoracic limb, which flexes slightly sooner than the pelvic limb. Delayed flexion of the pelvic limb relative to the contralateral thoracic limb results in diagonal asynchrony of stride.
The thoracic "skip" or "hop" that we often observe at the trot in horses with PPH represents an attempt to prolong the flight phase of the thoracic stride and allow the hind limb to "catch up". If the hind limb is slow, the contralateral thoracic limb must also be slow in order to reestablish diagonal synchrony of stride. The skip is a strategy implemented by the horse to slow down the front limb.
Thoracic limb "skipping" as a result of PPH is frequently misconstrued as nonweightbearing thoracic limb lameness, since the head and shoulder tend to rise during the flight phase of the stride. Diagonal asynchrony tends to be more noticeable with increased stride length, such as occurs during extended trot work or with the affected limb on the outside of a circle.
Skipping may also be more obvious at the beginning of the riding session and dissipate with time (i.e. the horse may "warm out" of it). Increased tone associated with the distal patellar apparatus as a result of exercise helps to alleviate PPH and the associated diagonal asynchrony.
The best way to eliminate a thoracic "ship" or "hop" that is occurring as a result of PPH is to resolve the PPH.