Vol. I: VII. On the Proportions and on the Movements of the Human Figure: 351.
The Da Vinci Notebooks
When the arm is bent at an angle at the elbow, it will produce some angle; the more acute the angle is, the more will the muscles within the bend be shortened; while
the muscles outside will become of greater length than before. As is shown in the example; d c e will shrink considerably; and b n will be much extended.