In all the animal kingdom, there is no other part of the anatomy that is as complex as the legs of a horse. They are physiologically intricate to the nth degree, resulting in a fragility that, if compromised, will end a horse’s life. They are also regularly called upon to execute feats of physical power, strength, and endurance no other animal could accomplish. Because of this, it is essential to a horse’s livelihood to have sound legs and hooves.
In this article, we will take a closer look at the amazing composition of horse leg anatomy.
How Horse Limbs Are Constructed
Horse limbs first include two apparatuses: the suspensory apparatus which carries weight and absorbs shock, and the stay apparatus, which locks joints in the limbs and allows for sleeping while standing. Their limbs are highly specialized, each with one digit, the third, and with the main muscle mass of the limb situated close to the body’s trunk. The forelimbs differ in construction from the rear limbs, and the top halves of the fore- and hindlimbs differ from each other. From the knees down, construction in both front and rear legs is largely the same. That being said, the muscular architecture can vary a bit between breeds. For example, a Quarter Horse is bred for acceleration, and an Arabian is bred for endurance, so they can have inherently different masses of the same muscle.
Because a horse’s legs are such an intricate system of bones, joints, ligaments, tendons, muscles, and connective tissue, all designed to carry a relatively heavy body, both good conformation and healthy limbs are extremely important for proper function.
The key to a horse’s performance and agility is the soundness of its front legs. They deal with two/thirds of the horse’s weight. Most conformation issues in the legs are found in the forelimbs and are huge factors when it comes to lameness or injury.
Anatomically, the forelimb starts with the scapula up top and runs down to the navicular bone in the hoof, the scapula being the upper-most bone of the forelimb. The scapula connects to the humerus via a ball and socket joint, and the humerus then connects to the radius. The size of the humerus bone is what gives a horse “choppy” or sustained movement. The longer the humerus, the more range to the horse’s movement. Along with the ulna and various other tendons and ligaments, this essentially comprises the top half of the horse’s forelimbs.
Unlike the front legs, the rear legs of a horse are directly attached to the spine via the pelvis, making the pelvis part of the hind leg anatomy. This is by design, having the effect of maximizing power to propel the body forward. But as you can imagine, the design also comes with risks. The direct attachment to the spine puts it at risk of injury. This risk is mitigated by the many angles within the hindlimb construction. Where the front limbs are almost perfectly straight, the hindlimbs have an open zig-zag design meant to prevent the spine from becoming concussed.
After the pelvis, moving downward, there’s the point of buttock, femur (thigh), patella, fibula, tibia, calcaneus, tarsals, and metatarsals. This is where the differences between the front and rear legs end. After the hindlimb metatarsals, the structure is cannon bone to coffin bone as in the front legs. The hindlimb equivalent of the knee is the tarsus area, more commonly referred to as the “hock.” It’s a problem area because there’s an intricate system of small bones that make up the tarsus, all interwoven with an equally complex system of ligaments. The hock takes a great deal of strain and is therefore highly susceptible to injury or unsoundness.
Anatomy Of The Lower Legs
About 95% of lameness issues in the leg occur from the knee down. As with a Swiss timepiece, efficient movement is entirely dependent upon the infinite precision of the inner workings. The ability to move most efficiently is what keeps horses safe and their legs sound.
The knee joint is what allows for movement in the first place. Stretching out from the knee joint is the horse’s cannon bone, which connects to the fetlock. The cannon bone has splints on either side that aid in support. Behind the fetlock, we have the ever-important sesamoid bones, and there are two of them. The sesamoids are the pulley system for the lower leg. Then we have the pastern system, intricate in its own right, comprised of both joints and bones, long and short, and connecting to the coffin joint. Even those only tangentially involved in horsemanship may recognize these anatomical names as they are commonly associated with health and soundness issues.
The Key to Sound Horse Leg Anatomy: Hooves
A horse’s feet (hooves) are more susceptible to soundness issues than any other part of it’s lower anatomy. This is problematic because they play the most vital role in the horse’s ability to function and survive. You wouldn’t know by looking at them, but hooves are incredibly complex. It’s a small space for several different features, all doing different things, working together to keep the horse healthy. For ease of understanding, a hoof can be divided into three parts: outside, inside, and underside.
The outside consists of the hoof wall, the coronary band, the periople, and the inner wall. The hoof wall, the part we see on the outside, is the hard, keratinous surface that protects all the interior parts. It has no nerve endings or blood vessels, mercifully, as it also supports the entire weight of the horse and absorbs the shock of its movements. If not naturally worn down, horses must have their “feet done” by farriers who are professionals trained in filing down and maintaining the health of the outer wall. This procedure is required because the outer walls are ever-growing at a rate of about 3/8 of an inch per month.
Outer walls are either black or white, and one is not stronger than the other. In healthy horses, they have no cracks or rings. They are also inflexible, which is a major contributing factor to lameness: when there’s an injury inside the hoof that causes swelling, the wall does not expand to allow space for it.
The coronary band is where the hairline of the leg meets the hoof. It contains a deceptively large blood supply and is instrumental in the health of the outer wall. Just below this, there’s a soft area of newly formed hoof wall tissue. This soft area is protected by the periople, which gives it time to harden.
The inner wall of the hoof does provide some flexibility that the outer wall does not, which is helpful with the shock absorption. Its most important function, arguably the most important function performed by any part of the horse leg anatomy, is the protection of the laminae. Both brilliant and deadly in its composition, it’s an interwoven system that bonds the outer wall to the coffin bone and is full of nerves and blood vessels. Ever heard of laminitis ? It’s an inflammation of those delicate laminae, which can result in the wall bonding to the pedal bone and is often fatal. More than 7% of all equine deaths in the world can be attributed to laminitis.
The underside consists of the sole, central sulcus, bars, and the frog, the sensitive area to be mindful of when cleaning a horse’s feet after riding. Inside the hoof is the digital cushion, coffin bone, and navicular bone, also known as the distal sesamoid. The coffin bone is the largest in the hoof and helps determine the hoof’s shape. Any issue with or injury to the coffin bone can result in lameness, including issues with horseshoeing.
Two Major Tendons
Last but not least, two hugely important pieces of the horse leg anatomy puzzle are the extensor tendon and the deep digital flexor tendon, also referred to as simply “DDFT”. The extensor tendon attaches to the front of the coffin bone and straightens the leg. The DDFT has the all-important job of bending and flexing the leg. It does so by running down the back of the leg and wrapping around the navicular bone.
Understanding Horse Leg Anatomy Makes You A Better Horse Person
In this article, we focused on the anatomy of the legs and why they are such a special part of a special animal. Why systems fail and why they succeed is important information every horse person should know, because information and education make all of us better horse people.
Horsemanship comes with great responsibility. We are privileged to have these magnificent creatures among us. When trust and bonds are built, they will give their all to their humans, sometimes to their own detriment, so it’s our responsibility to advocate on their behalf tirelessly and to care for them diligently.