This is brought about as explained on page 48 by mounting the foot in equinus, and we must here describe the methods by which the commencement of the movement of extension may be communicated to the leg so that the heel may be the first part of the foot to touch the ground. These methods may be termed _knee extending mechanisms_. They assist the passive action of the weight of the leg. In fact the recurrence of extension is brought about by a pendulum movement of the leg, which, at first oblique downwards and backwards, swings into a downward and forward obliquity. But this movement is slow (the pendulum which marks one second is one metre long) and incomplete. The patient can make it complete with a little instruction, by extending the thigh slightly as soon as the foot touches the ground. This may be sufficient if the stump is long; the leverage is good, and while the hip is being flexed a swing can be given to the thigh piece which accentuates the pendulum movement of the leg. But with a short stump some special mechanism is essential to make sure that extension will be complete, otherwise the patient will be obliged to walk with short and calculated steps, to wait whilst the pendulum action produces extension of his knee and allows him to put weight upon his foot. C. _Mechanism for starting extension of the knee during the time the leg is swinging._--There are two methods which are generally combined: 1. Elastic traction by an artificial muscle. 2. The extending sling. 1. _Artificial muscle._--The action of an artificial muscle made of elastic (noiseless) or of a coiled steel spring, is easily understood. (a) The simplest method (that which is commonly used for infantile paralysis affecting the quadriceps) consists in fixing an elastic band divided into two slips, one on either side of the patella between the front of the thigh and of the leg, about the middle of each. (This is represented in figure 98 in our convertable leg.) (b) When the apparatus includes the regular artificial knee the makers generally place this mechanism in the interior of the thigh and leg pieces, using methods which are often very ingenious. Of these we illustrate some on pages 40 onwards, with an explanatory description. In describing these mechanisms, which may be called intra-condylar, it is necessary to speak at the same time of the _stop to limit extension_ because, as will be seen, it is combined with the extending spring. We have already said that rigidity in extension when the limb is vertical is essential, but whilst it is necessary for extension to be _complete_ at this moment it is also necessary to prevent the knee being forced into the _hyperextended position_, as this would quickly strain the joint and render the limb useless.