The frog is a part of a horse's hoof, located on the underside, which should touch the ground if the horse is standing on soft footing. The frog is triangular in shape, and extends from the heels to mid-way toward the toe, covering around 25% of the bottom of the hoof.
The frog acts as a shock absorber for the foot when it makes impact with the ground, decreasing the force placed on the bones and joints of the leg.
Purpose of the frog:
The frog is not only a shock absorber, but it is also an important part of the horse's circulatory system — it pumps blood up the horse's leg each time the frog makes contact with the ground.
The blood flows down the horse's leg into the digital cushion, a fibrous part of the inner hoof located just above the frog which contains a network of blood vessels. The horse's weight then compresses the frog on the ground, squeezing the blood out of the digital cushion, and pushing it back up the horse's legs.
Therefore, farriers keep the frog as large and ground-covering as possible when they trim or shoe the feet.
When a horse has certain types of lameness, the farrier may use the frog for support, using specialized shoes that help keep correct pressure on the frog so that less force is transmitted to the wall and sole of the foot or to the navicular bone, coffin bone, and deep digital flexor tendon.
The frog may also decrease in size if does not receive constant pressure, which can occur if the heels of the horse become contracted.