Here is my second article on the anatomy of the foreleg 😀 For a reminder of the bones and joints of the foreleg, please head to Anatomy of the foreleg part 1! If you’re already familiar with this article, you can carry on reading now the biomechanics and muscles 😉
Biomechanics of the equine foreleg
From the elbow down to the phalange, mobility is pretty much limited to flexion/extensionmovements. There are some other types of rotation, depending on the joint, but they are very minor and the main movements are flexion and extension; the abduction and adduction of the entire leg is done by the shoulder. A few cool things about the movements of the elbow and carpus follow the basic osteopathic principle stating that the structure and function are inter-related:
- the extension movement of the elbow is limited by a bone formation within the joint.
- the flexion of the carpus is limited by the pisiformus which protects the joint and tendons from hyperflexion
So if the structure of those bones was different, the movement would be different! Love it 🙂On the other hand, if the movement changes, say because of tension in muscles or joints, the structure will change too, with time.
The chock absorption system of the fetlock:
Before I carry on with the muscles, I’d like to take a minute to talk about the mechanism of the fetlock, that I personally love. You can tell a lot on how a horse holds himself/herself by just feeling the structures of the fetlock, and the mechanism in general is pretty cool. The extension of the fetlock during stance phase (when the limb is in contact with the ground) permits to both absorb some of the impact of the hoof with the ground AND store potential energy that will be re-used straight away in the transition to swing phase. It’s done passively by the suspensory ligament, the sesamoid bones, and the deep and superficial flexor tendons. Those structures are going to extend under the different forces applied during stance phase and the elastin fibres contained within the ligament and tendons will automatically seek to come back to normal lenght after the forces are released. It’s such a clever mechanism, I absolutely love it.
Here is my personal interpretation of the movement:
=> hoof hits the ground
=> increase in weight and forces applied on the fetlock as stance phase progresses
=> potential energy stored within the joint, fascia, tendons and suspensory ligament thanks to the elastin fibres, as the extension of the fetlock increases
=> progressive release of thus energy by recoil mechanism at the start of swing phase.
The movement happens every time the horse’s foot hits the ground, including when taking off for a jump and on landing. The amount of energy stored and released depends on the speed of the movement and the forces applied on the fetlock!
How cool is that!!
Unfortunately, the capacity of those structures to deal with impact will decrease with time, as well as depending on the use of the horse. So if I take a racing horse and a hacking horse, the chances of the racing horse to develop inflammation and pathology in the fetlock area (either tendon or ligament) will be much higher due to the over use of the recoil mechanism! It’s all down to how you use your horse 🙂
The muscles of the foreleg
I’ve already explained briefly in my little introduction video over on my FB page that the extensors of the foreleg are at the front of the limb, and the flexors at the back. Like for the shoulder, I’m not going to describe the muscles in details as it would be super tedious, instead I’ve attached some more of my (lovely) drawings 😉
As you can see, the deep and superficial tendons actually start all the way up the leg, around the elbow of your horse. So they have a long way to go before reaching the back on the canon bone, and then the deep flexor isn’t even finished, it still has to go all the way under the coffin joint! Each of those muscles have a flange, one on the radius for the superficial flexor called the radial flange, and one on the carpus for the deep flexor, called the carpal flange. Those structures are here to make sure the muscles and tendons stay against the leg when contracting.
You can also notice that the flexors mirror the dorsal extensor the leg, and that the suspensory ligament actually has 2 flange, one lateral and one medial, that go round the fetlock to the front of the leg and join the tendon of the dorsal extensor. This ensures the perfect movement of the fetlock and helps limit the risk of hyper-extension.
Finally, I’d like to bring your attention on one last thing: the muscles moving the elbow are also muscles moving the shoulder. I haven’t represented in the images here, but the triceps, biceps and brachial muscles are all muscles that move both the elbow and the shoulder. There’s a continuity in anatomy that ensures coordination and flow of movement in the healthy horse, but also ensures compensation processes in horses that display tension. This way, a stiff shoulder can result in a change in the use of the entire foreleg, changing the weight distribution all the way to the feet and coffin joint. Quite a few people think that such processes are linked with the developement of common pathologies such as tendonitis, suspensory ligament injury and navicular syndrome.
That’s all for today, hope you enjoyed all those information!