So I was looking at some pictures of me riding my horse at the canter, and I noticed that when he puts weight on a certain leg his pasterns go down and it looks like his fetlocks are almost touching the ground! You're probably thinking that it's because he has long/ weak pasterns, (that was what first came to my mind) but I realized that's not the case- his pasterns are normal length. I measured them once and they were fine. So what else could it be? Would protective leg wear help? I really have no idea.
How long and how hard had you been working your horse when these pictures were taken? What you are seeing is called "overdorsiflexion" of the pastern joint. It can be caused by fatigue. If your horse is fatigued it can lead to injuries like bowed or torn tendons and ligaments.
I read a really good explanation of how this happens in The Bowed Tendon Book
by Tom Ivers. Tom Ivers (1944-2005) was a equine exercise physiologist. He trained thousands of Thoroughbred and Standardbred race horses during his career and he became an expert on what causes tendon/ligament damage. This excerpt is taken from Chapter 5: "Why Horses Bow" "Fatigue There are two kinds of fatigue in the performance horse: chemical and mechanical. And there are two forms of chemical fatigue: lactic acid fatigue and fatigue caused by fuel depletion. In both forms of chemical fatigue, the muscles of the forearm, and all the other propulsion and stabilization muscles of the body, lose their ability to contract and , instead, relax. When the muscles relax, the tendons must take up the slack, absorbing rapidly increasing forces as the fetlocks droop lower and lower toward the ground. Worse, the lowered fetlock overdorsiflexes the leg, the knee bends backward, and the toe refuses to break over at the end of the contact phase of the stride. This throws the gait into increasingly erratic motion and missteps begin to occur. Some fibers in the tendon pull apart with the first fatigued stride, weakening the tendon. With each new stride, the fetlock sinks further toward the ground. At this stage of fatigue, broken bones ala Go For Wand often occur. Considerable injury to the tendon is virtually guaranteed. Prevention of chemical fatigue is a matter of developing a level of fitness appropriate to the demands you expect to make on the animal. Later we’ll examine this in detail. Fuel depletion can be fought on two fronts, appropriate exercise combined with fully supportive nutrition. Again, in depth later. Mechanical or structural fatigue can be likened to what happens to a paper clip when you bend it back and forth over and over again. At some point, it breaks. Concussive stresses, especially those involved in jumping or running at maximum speed, sudden loading of thousands of pounds, stride after stride – these must cause at least some damage in the toughest of tendons. Normally, organic structural materials like bone and tendons repair themselves, and given time, come back stronger then they were originally. But if you fail to notice structural fatigue as it’s coming on, of you cover it up, and if you continue on with the exercise that’s causing it, then failure is preordained. Mechanical failure occurs more gradually in the tendon, with far less chemical fatigue involved. You work a horse hard today, a few fibers give up the ghost and the repair process begins. In a few days you have a little heat and filling. You cover up that repair process with a leg bandage, keeping the leg “tight”. The regular hard workouts continue, causing more damage. You add icing to your daily therapy and the leg looks tight as a drum. More work, more damage. Then, the final big workout or competition and the remaining viable fibers in a few bundles taking the most abuse give way – you’ve built yourself a nice, juicy bow. You have a hole in the tendon that an ultrasound section scanner has not trouble finding no matter how much wrapping, firing, blistering, cortisone injecting and Bute feeding you can marshall. If you’ve injected cortisone before the full bow, then you can thank yourself for hastening the disaster along."
Chemical and mechanical fatigue are not the only things that cause tendon/ligament damage and injury. The book discusses other causes as well: improper shoeing/trimming, unnatural demands (asking the horse to perform beyond his normal capacity), sudden stress, concussion, accidents, conformation, pushing an already injured horse, and owner, trainer, rider indifference (they just don't care). The Bowed Tendon Book
is a good book I highly recommend.
Another factor in why a horse's pastern join may look very low in motion is the composition of the suspensory ligament itself. Here's some information from a book called Equine Photos & Drawings for Conformation and Anatomy
. "The suspensory ligament is more elastic than other ligaments. This is because it contains tendinous tissue. In young horses, it even has a small amount of muscle tissue in its deep part (which is why it is also known as the interosseous muscle). Its main function is to support the fetlock, guarding against extreme hyperextension (or overdorseflexion) of the joint. The two branches that join the common digital extensor tendon limit extreme flexion of the pastern joint."
Here are some photos of horses that are exhibiting overdorsiflexion of the pasterns. As you can see, overdorseflexion can happen anytime - while under saddle or at liberty - when the horse is in motion and the leg is loaded with weight.