At the walk and trot, the normal horse maintains similar movement (i.e. 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.
Diagonal synchrony is an important consideration during lameness evaluation, because what affects the movement of one limb can also affect the movement of the contralateral limb on the other end of the horse. For example, application of a toe extension to the LH foot as a means of delaying breakover of the limb and lengthening the LH stride will result in similar gait alterations to the RF limb.
During lameness evaluation, the practitioner must determine whether the origin of each gait deficit is primary in nature or a result of the horse's attempt to maintain diagonal synchrony with another limb. In the latter case, the lameness would be considered to be referred.