Last time we talked about carpal arthrosis, pointing out that too much bowing–dorsiflexion–of the foreleg at the knee was the immediate cause of damage to the articular cartilage. Further, with a bit of mechanics, we saw that too much dorsiflexion at the knee occurred because the resisting moment (turning force) generated by the muscles and tendons on the back of the forearm and carpus was not enough to balance the turning force generated by the body weight acting on the foreleg.
While the next, obvious question is why the resisting moment is too small, let’s hold that for a moment. It has long been well-known to horsemen that the horses with the conformation back-on-the knee are not a good risk because of the almost inevitable development of carpal arthrosis. The reason for that now becomes crystal clear. If too much dorsiflexion is the cause of carpal arthrosis, as it is, the horse which starts with the foreleg in this position is predisposed to the damage. The foreleg is already dorsiflexed, ready to be damaged. Looking at that in terms of the equation for equilibrium of moments, with the leg already dorsiflexed the value of 1 is greater than with the normal leg. That is, the moment tending to cause dorsiflexion, F1, is greater to start with in the horse which is back-on-the-knee.
Let’s return now to the question of why the resisting moment is too small. There are two, intimately related answers: the moment, F1 becomes too large and/or the moment, Tp, becomes too small. The moment, Tp, becomes too small because the muscles and tendons on the back of the foreleg (for those who would like to know: the deep flexor, superficial flexor, ulnaris lateralis, and flexor carpi ulnaris) become fatigued. After the horse has been galloping along at a high rate of speed for some time, the muscles (and tendons) begin to tire, no different than you or me running. The tiring of the tissues means that the value of T, the muscle/