The Physiology of Yoga

Although yoga is not a science, it can be studied. The principles of its function -- how it has the effects it does have -- can be understood. After all, fishing is not a science but we can investigate why it works and how to improve it, how to make it more effective. In fact, yoga’s traditional practice of stretching muscles and joints and remaining in positions for extended periods of time exploits powerful reflexes and produces a number of salutary effects. These nearly universal responses are as good a place as any to begin.

Reflexes

Three common reflexes are used in most of the hundreds of widely practiced yoga poses. The first two reflexes, a system of checks and balances, are almost always a part of yoga. You might have learned about them in high school.

Every muscle in the body has a pair of reflexes that govern its activity. Whenever a muscle is stretched, one reflex -- he stretch or myotatic reflex -- stimulates the muscle to contract; the other -- the Golgi tendon reflex --inhibits muscular contraction in response to stretch. Both reflexes start from tiny sense organs within each muscle and tendon that relay information back to the spinal cord.

When the reflexes that promote or facilitate muscle contraction get triggered, the muscles pull back. They are the ones that respond when the doctor strikes the tendon just below your kneecap. That quick little stretch of the quadriceps muscle results in a tightening of the muscle, contracting it and raising your lower leg. These reflexes are initiated by sense organs, the intrafusal fibers, but these organs also have tiny muscles within them! This allows them to adjust how strongly they stimulate the contraction of large skeletal muscles.

Another set of sense organs in each muscle’s tendon -- the Golgi tendon organs -- inhibit muscle contraction. When there is a tug on a muscle, be it by the hand of a good friend, the swing of a tennis racket, gravity, or yoga, these sense organs contact the spinal and pontine motor centers in the brain to modulate down the tone and actual contraction of every one of the body’s muscles.

One basic mechanism in many yoga postures -- entering them slowly and holding them -- utilizes the fact that the intrafusal (stimulating) fibers are dynamic. Their response is adjustable and proportional to the speed of the stretch. Since they adjust internally to tension, they generally respond less to slow movements and have their greatest influence early in the process of muscle stretching. Their influence fades fast, though, especially if the muscle just stays at its new length. The inhibitory Golgi tendon organs damp down muscle contraction with a force that is weaker than the positive contraction­producing stimulus of the intrafusal fibers. But they continue to exert the same amount of inhibitory influence, at their original strength, over long periods of time. After a while, their constant input outstrips the diminishing influence of the intrafusal fibers. This naturally tends to reduce a muscle’s contractile force as a yoga pose is held.

After a short time -- less than two minutes -- the muscle will become quiet and stretch more easily and less