Equinus is a condition involving the position of the foot relative to the leg at the ankle joint. The word 'equinus' is derived from 'equine', meaning horse. A horse's leg is essentially positioned such that the equivalent of the human 'foot' is striking the ground as the primary pressure point, rather than the equivalent of the human 'heel'. When someone has a contracted position of the foot in which more of the ball of the foot initially hits the ground than the heel, the foot is said to be in an equinus position. Equinus is a condition that in and of itself does not directly cause problems, but contributes to the development of a wide range of issues due to the abnormal position of the foot.

Equinus has several causes. The most common is simply tightness of the calf muscles, which leads to more tightness of the Achilles tendon. Some children are born with tightmess due to neuromuscular disorders that cause the muscles to spasm, or create tightness by toe walking when they first begin to walk. Adults who have normal calf muscles can develop this tightness due to excessive foot flattening, the long term use of high-heeled shoes, and the use of a cast to immobilize the leg for an injury. Another cause of equinus is due to abnormal bone formation on the front of the ankle that actually blocks the joint from moving upward. A third type is seen in those who have very high arches.

Symptoms that equinus can lead to include low back pain, heel pain both on the bottom of the heel and the back of the heel, arch pain, early bunion formation in children, toe contractures, and an inability to stand for long periods of time. The greater pressure to the ball of the foot can result in the formation of painful calluses, joint pain (metatarsal), as well as nerve inflammation (neuroma). In diabetics, the greater pressure can result in wound formation, which can lead to infections and amputation if not properly treated.

Equinus treatment can be simple or complicated, depending on its severity and ultimate cause. Simple cases of equinus caused by tight calf muscles can be stretched out with an aggressive home stretching program, the use of night splints to maintain stretching while sleeping, or a specific treatment program by a physical therapist. Correction takes time and the exercises need to be strictly adhered to for success. More difficult cases require surgical intervention that lengthens the Achilles tendon or the tissue that lies above the tendon. This lengthening allows the foot to assume a more normal position, reducing pressure to the ball of the foot. If the ankle joint bone is causing the problem, surgery is needed to remove the bone blocking the ankle from bending upward fully.