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

“What happens is if a given muscle is stretching really far, those sensory neurons, those spindles within the muscle will activate and will send a electrical potential, literally a bit of electricity along that wire's length into the spinal cord. And then, within the spinal cord that sensory neuron communicates through a series of intermediate steps, but to the motor neuron and makes sure that that motor neuron contracts. Now, why would that be useful? Well, what this does is it creates a situation where if a muscle is or is stretching too much because the range of motion of a limb is increased too much then the muscle will contract to bring that limb range of motion into a safe range again.”

“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.”