The posterior of these cords, made of hemp, ends inside the upper third of the leg. It limits the extension of the knee, exactly as described in the Marks leg.
The other two cords extend down to the foot, which is attached in a manner very similar to that shown on [page 54], but with a single rubber cylinder behind, and with the instep cut obliquely so that when the joint is in the resting position of equinus there is an opening in front amounting to an angle of 15° to 20°. The posterior cord, of hemp, is attached in the heel; the anterior, made of catgut with an indiarubber section, enters the foot obliquely and is fixed a little in front of the middle of the sole.
When the knee flexes, the wooden cross piece tilts, its posterior end becoming lower, its anterior higher ([Figs. 91 and 92]), the elastic of the anterior cord is tightened, thus raising the front of the foot, whilst at the same time the heel cord is relaxed. Thus the mechanism which produces extension of the knee acts at the same time upon the foot; when the knee is straight the foot is plantar flexed to 20°, when the knee flexes the foot comes to a right angle. Thus the foot becomes dorsiflexed at the same time as the knee flexes, as in ordinary walking.
If the action in walking is watched, it will be seen that as the limb is swung forward, the toe is raised so as to clear the ground.[7]
[7] The mechanism of this artificial leg resembles that of the "tendon leg," which was in such common use in England before the present war that it is often called the English pattern.—(Ed.)
Fig. 92.—(Figs. 90 to 92. Foot and Knee of Frees.)
In the sitting position the anterior cord is not relaxed, there is no dead point, so that the knee always tends to extend. This is somewhat inconvenient.