“What Happens When You Stretch?”
I get asked this a lot. The stretching of a muscle entails a progressive decrease in the normal overlap of thick and thin filaments that make up the muscle fiber. The muscle is lengthened, and the more fibers are stretched within a muscle, the more length is developed.
Once the maximum resting length of the muscle is reached, additional stretching force shifts its effect on the surrounding connective tissues, especially the tendons, ligaments, and fascia. As the tension increases, the collagen fibers in the connective tissues align themselves along the same line of force as the stretch. When this occurs, it helps to realign any rogue fibers like scar tissue, or fibers shortened due to lack of use, into a more normal configuration.
As you can imagine, muscles shorten (contract) and stretch quite often, both voluntarily and involuntarily, throughout the course of our movements and postures. Constant monitoring and adjustment of these stretches and contractions are needed if we are to attain just the right posture or movement in each given situation, as well as to avoid injury. This type of fine tuning is the function of a special group of sensor cells called proprioceptors.
There are 3 main types of proprioceptors:
One lies parallel to the muscle fiber itself, and is sensitive to changes in length, and the rate of change of length, of the muscle. It is responsible for the stretch reflex, which attempts to resist the change in muscle length by causing a stretched muscle to contract. The faster or the more sudden the change in muscle length, the stronger the contraction. This is the basis for “normal” muscle tone. When a stretch is held for a prolonged period of time, these sensor cells habituate and reduce their signal for the stretch reflex, allowing for greater lengthening of the muscle.
Professional athletes and dancers often push their limits by training towards having very little to no stretch reflex in response to a sudden stretch, offering abnormally great gains in flexibility.
The other two proprioceptors are located closer to where the muscle ends and where the tendon begins. These sensor cells are sensitive to changes in tension, and the rate of change of tension, around the points of origin or attachment of muscle tendons. When muscles contract, either on purpose, or due to the stretch reflex, they exert tension at the point where the muscle connects to the tendon. When this tension exceeds a threshold, it triggers a lengthening reaction, which prevents the muscle from contracting any further, and causes it to relax. This is possible only when the tension on these 2 proprioceptors exceed the tension on the first proprioceptor that is telling the muscle to contract. Another reason why prolonged stretching is effective is that it gives enough time for this lengthening reaction to occur.
More often than not, I teach my patients to hold stretches for at least 15 to 30 seconds for these 2 reasons: (1) to decrease signaling for the stretch reflex via habituation of the first proprioceptor, and (2) to allow for the lengthening reaction to occur via signaling from the other 2 propioceptors.
Any discussion on stretching is not complete without mentioning the effect of reciprocal inhibition. Try this: with your right arm dangling at your side, lightly grasp the middle of your right upper arm with your left thumb on the biceps and the other 4 fingers of your left hand on your triceps. Now, slowly bend the right elbow. You will feel the biceps muscle under your left thumb contract into a noticeable bulge, while your other 4 fingers will feel a less dramatic thinning and lengthening of the triceps. This phenomenon allows greater ease of a desired movement of a muscle by forcing a relaxation of the muscle that has the opposite action. In this instance where the desired action is elbow flexion, the bicep acts as the agonist and the tricep acts as the antagonist. Agonist action inhibits antagonist action.
It is easier to stretch a muscle that is relaxed than to stretch a muscle that is contracted. In the example above, I would be able to get a more effective stretch of the triceps if I first ask the patient to contract the biceps against my resistance.
The next time you want to stretch, be clear on which muscle or muscle group you are stretching at that time. Next, identify which muscle or muscle group has the opposite action. Start by moving those or doing isometric holds. Then, go ahead and stretch until you feel some discomfort but not pain. Now hold that stretch. There is no pressing need for high repetitions when stretching under normal circumstances. Four to eight stretches is typical per muscle group, and even less if recovering from injury or surgery.
Still, I cannot emphasize enough the importance of the following two factors in effective and efficient stretching:
(1) Stretch daily. Muscles, tendons, fascia, and (to some extent) bones are elastic tissues. So are elements in our nerves, blood vessels, and internal organs. Thoughtful and regular stretching is an important and often missed practice in attaining and maintaining optimal health. When in doubt, seek professional advice.
(2) Stretch corresponding body parts: left and right sides, or front and back. In doing so, one can identify areas of tightness — which can change from day to day — and use the “good side” as a goal for stretching the other. The desired outcome is always optimal rather than absolute balance.
Happy stretching!