Final answer:
Stretch reflexes maintain muscles at their optimal length.
Step-by-step explanation:
The reflexes that maintain muscles at their optimal length are known as stretch reflexes. The muscle spindle proprioceptive receptors play a crucial role in this process. When a muscle is stretched, these receptors activate and send a signal through the spinal cord, which leads to the contraction of the stretched muscle and inhibition of antagonist muscles, thus maintaining muscle length at a desirable level. The knee j*rk reflex, which is tested during a physical exam, is a common example of a stretch reflex.
Thus, stretch reflexes maintain muscles at their optimal length by activating muscle spindles that cause the muscles to contract and inhibit the antagonist muscles.