But we must not exclusively attribute to the action of the muscles of the leg this uniformity in the translation of the foot. In fact, we know that, during this translation, two distinct causes are working:
1. The angular movement which the leg has around the pelvis.
2. The horizontal translation of the pelvis itself; that is to say, of the point of suspension of the leg while the latter oscillates.
We may conceive that, by the combination of these two movements, the motion of the leg may tend to become uniform; this will happen if the minima of the velocities due to the first-named species of motion correspond with the maxima of the second kind of motion. It therefore becomes very interesting to determine what is really the motion of the trunk of the body during different gaits.
The apparatus already described has also served for the solution of this problem.
A cord attached to the waist transmitted to the registering apparatus the motion of translation of the trunk. By experimenting successively on various gaits, we obtained the following figure, whose analysis gives some interesting results:
Fig. 2.
The undulations are far greater when the walking is very slow than when it is more rapid. Thus, the motion of the body becomes more uniform by reason of a higher velocity. This is the reverse of what happens with the vertical oscillations of the body, which increase with the velocity of progression and with an increase in the length of the steps.
The number of alternations of motion is double that produced by the movement of a single foot, as shown in Fig. 1. This is readily understood when it is remembered that the two feet, repeating the same acts, give alternately to the body a new impulsion.
To make clear this action we have traced, parallel to the line 2 of Fig. 2, the curves P, produced by the motions of the right and of the left foot. These curves, of which one is dotted, and the other full, are at once recognized as similar to those of the line 2 B, of Fig. 1. In fact, on observing the superposition of the different parts of these curves with the curve of translation, we see that the body receives an
