The Spinal Cord Reflex Arc
In the belly of all skeletal muscles, there is something called the muscle spindle stretch receptor (MSSR). The MSSR detects the muscle’s changes in length and tension. When a muscle is lengthened and tensed (which is what happens during stretching), the MSSR sends a message to the spinal cord. The body’s nervous system responds in turn by telling the muscle to contact and resist against the stretch. This built in reaction is called the spinal cord reflex arc and is designed to prevent the muscle from tearing or becoming over-stretched.
For this reason, you never want to force yourself into a stretch. That will just make the MSSR fire even more, and cause your muscle to contract and fight against the stretch. Instead, you want to work with the spinal cord reflex arc. Wait for your body to ease into the stretch, so that the MSSR stops firing, and then move further in.
Golgi Tendon Organ
Another important sensory receptor is the Golgi tendon organ (GTO), which is located at the juncture of muscle and tendon. Like the MSSR, the GTO also detects changes in muscle tension. While the MSSR tells the muscle to contract in order to fight against the stretch, the GTO has a different reaction. Its aim is to protect the tendon from bearing too much pressure, and so it sends a message to the muscle telling it to relax, and thereby reduce pressure on the tendon.
Physical therapists often use a technique called proprioceptive neuromuscular facilitation (PNF) to use the GTO to their advantage. In PNF, we can get deeper into the muscle being stretched by momentarily contracting it. This stimulates the GTO, which in turn tells the target muscle to relax. Once the muscle contraction is released, you’ll then be able to ease deeper into the stretch.
BUT – as you work with the GTO using PNF, you don’t want to stretch so much that the MSSR becomes stimulated and tells the muscle to contract. It’s a delicate balancing act to be played on the tightrope of muscular receptors.