Question

body part cannot be used as strain unit of measurement

Answer 1

Humans and other animals body parts vary widely in size and shape. Thus, if we were to use body parts as a "standard of measurement" the idea of standard measurement wouldn't exist.

Say we are using "hands" as a measure. One individuals hand will likely not be the same as another persons hands, therefore if we were building a wall 50 hands tall, the actual sizes of the wall would be different.