The Da Vinci Notebooks at sacred-texts.com
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.
187:172 : See Pl. XIX, No. 2.