Equinus is a condition in which the upward bending motion of the ankle joint is limited. Someone with equinus lacks the flexibility to bring the top of the foot toward the front of the leg. Equinus can occur in one or both feet. When it involves both feet, the limitation of motion is sometimes worse in one foot than in the other.  People with equinus develop ways to compensate for their limited ankle motion, and this often leads to other foot, leg or back problems. The most common methods of compensation are flattening of the arch or picking up the heel early when walking, placing increased pressure on the ball of the foot. Other patients compensate by toe walking, while a smaller number take steps by bending abnormally at the hip or knee.

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Often, it is due to tightness in the Achilles tendon or calf muscles. Other patients acquire the tightness from being in a cast, being on crutches or frequently wearing high-heeled shoes. In addition, diabetes can affect the fibers of the Achilles tendon and cause tightness. Sometimes equinus is related to a bone blocking the ankle motion. Equinus may also result from one leg being shorter than the other. Less often, equinus is caused by spasms in the calf muscle.