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Polio paralyzes motor nerves (most commonly those of the legs, but also frequently of the back and sometimes of the arms). The muscles which the paralyzed nerves are supposed to control do not get sufficient exercise and thus begin to atrophy soon after the patient has recovered from the active phase of the disease.
When, as is often the case, the muscles atrophy unevenly on the two opposing sides of a joint, it is called "muscle imbalance" and, if left uncorrected, usually results in the joint coming to favor strongly a particular position (the stronger muscle pulling harder than the more paralyzed one).
For example, often in polio cases, the muscles in the back of the thigh are less paralyzed and retain more strength (atrophy less) than the muscles toward the front of the thigh. Since the back thigh muscles which pull the leg back into a bent position meet little resistance from the front thigh muscles (used to pull the lower leg forward into the straight-leg position), the leg comes to "prefer" a bent position and the back thigh muscles become accustomed to not being fully stretched out very often. Over time, these muscles actually shrink or "contract." It becomes impossible to pull the leg, even manually with force, into the straight position anymore, and the muscles begin to pull tightly on the tendon behind the knee. A leg which can no longer be straightened out at the knee is said to have a "knee contracture."
Similarly the back calf muscle in polio cases is often stronger than the muscles on the front of the shin, causing the foot to be pulled down into a "toe pointing" position. When the muscle contracts as a result of its success in always pulling the foot down, the ankle losses full range of motion and the toe point or "heel contracture" becomes permanent.
The drawing below shows knee and heel contractures of the right leg.
Contractures which are not severe can often be corrected by a serious of casts, each cast serving to straighten the joint a little more, until near full range of motion is recovered (though not muscle control).
Severe contractures usually require a minor surgical procedure to loosen up the tendons that hold the muscles before casting begins. Depending on whether surgery is required or not, and how severe the contracture, the leg-straightening procedure can take anywhere from a couple of weeks to several months. Once done, however, it will not need to be repeated as long as the person continues to wear a brace regularly, since the brace preserves full joint extension.
Ngalula's legs (pictures below) were originally bent back to the point where the calves almost touched the thighs. Straightening required two operations and eight months of successive casts. (The pictures show him first at roughly the half way point, then later when the casts were removed and he was fitted with braces to permit walking.)