How Stretching Works
by Sang H. Kim

Your skeletal muscles are designed to move your body. By contracting (getting shorter) your muscles pull on your bones and create movement. If the muscles and their surrounding tissues (fascia) are elastic, the joint is able to move through its maximum range of motion. If the muscles are tight, the range of motion is limited. Although muscles are generally long enough to accommodate the full range of motion, they are limited in elasticity by the conditioned response of the myotatic reflex.

When a muscle lengthens beyond a certain point, the myotatic reflex causes it to tighten and attempt to shorten. It’s a simple process wherein the muscle fibers and spindles register a sudden unexpected increase in their length and fire off a message via the nervous system to contract the affected muscle, thereby stopping the lengthening and potentially heading off a catastrophe that might result in a muscle tear. This is the tension you feel during stretching exercises.

Although it may seem like a hindrance to flexibility, the myotatic reflex is desirable because it prevents, in many cases, muscle strains and tears. Without it your muscles would be allowed to overextend and tear easily. Through stretching, you not only lengthen your muscles, but you reset your myotatic reflex to a higher tolerance.

Reciprocal Inhibition

Reciprocal inhibition can be used to your advantage in stretching. Your skeletal muscles operate in pairs, with one contracting (the agonist) while the opposing muscle relaxes (the antagonist). The function of the agonist is to move the bone or joint while the function of the antagonist is to both allow the movement and to slow it if necessary. For example, when you bend your leg at the knee, the hamstring muscles at the back of your thigh receive a message from the nervous system to contract.

The opposing quadriceps muscle at the front of your thigh does not receive a message to contract so it is therefore allowed to relax and lengthen as the leg bends. If the quadriceps contracted or did not relax simultaneous to the contraction of the hamstring, your knee would be unable to bend.

To feel reciprocal inhibition in action, set your hand on a table in the position of a knife hand strike and push downward. With your other hand, feel the triceps muscle in the back of your arm. It should be tensed with the effort of pushing on the table. Now feel how relaxed the antagonist muscle—the biceps—is as it allows the triceps to contract.

The result of this function is that the muscle that is not contracting is inhibited, i.e. its ability to contract is suppressed. This principle can be used to enhance the results of your stretching program by inducing the inhibition response just prior to stretching a muscle. For example, when stretching your hamstring muscles in a forward bend, focus on contracting your quadriceps by pulling upward, as if you were pulling your kneecaps up toward your hips. The contraction of your quads signals your hamstrings to release further, allowing you to achieve a deeper stretch. This technique also develops strength in the quads. If you apply this principle to each stretch, you will build both strength and flexibility in opposing pairs of muscles, a nearly perfect model for increasing your flexibility.