The nervous system signals muscles, which signal back to prevent excessive limb stretching and potential injury.

“Now, within the muscles themselves, there are nerve connections. And these are nerve connections that arise from a different set of neurons in the spinal cord that we call sensory neurons. ... a particular kind of sensory neuron that comes out of your spinal cord and into your muscles, which are called spindle neurons. They create or they actually wrap around muscle fibers, kind of corkscrew around them and give kind of a spring-like appearance. ... These spindle connections within the muscle that wrap around the muscle fibers sense the stretch of those muscle fibers.”

“nervous system, neurons, meaning nerve cells, communicate with muscles, and those muscles communicate back to your nervous system to make sure that your limbs don't stretch too far, they don't move too far such that you get injured.”