The Science of Stretching

One of the most common reasons that people come to yoga is to gain flexibility. As we age, certain areas of our bodies become stiff and we fall into habitual movement patterns that exacerbate the problem. For example, sitting for long periods of time will greatly reduce the range of motion at the hips for most people. One of the best ways to counteract all that sitting is to get up and move, and preferably practice yoga. Aside from feeling better, increasing flexibility can also promote healthy joints, muscles, and bones. It also ensures that we stay active, which in turn promotes cardiovascular health.

Anatomical basis of flexibility

In order to increase flexibility, we have to understand what potentially inhibits flexibility. There are 4 main elements that contribute to your flexibility: skeletal structure, connective tissue (ligaments, tendons and fascia), muscles, and the central nervous system.

Let’s start with your skeleton. There is a range of variation in length, width, and angles of the skeleton. Just as someone’s humerus (upper arm bone) can be longer than another person’s, the angle of the shoulder joint can also differ. One person’s scapula (shoulder blade) may have the joint surface facing somewhat forward, and thus the humerus will sit forward in the joint. Of all the things that contribute to range of motion, this is one of the most difficult to change. Although bone is living tissue and does adapt to habitual strain (in a limited sense), it will be difficult if not impossible to overcome skeletal barriers to your flexibility.

There are different types of connective tissues. Tendons connect muscle to bone. Ligaments connect bone to bone. And fascia surrounds most body tissues, including muscles and joints. Tendons are relatively stiff compared to the other types of connective tissue, but they work in concert with the muscles they attach to as muscle-tendon units (MTUs) which I’ll discuss in the next paragraph. Typically, it isn’t possible to change the length of a ligament in a healthy manner through stretching. Although a ligament will stretch, it will also quickly reach the point of no return - when stretched too much it loses elasticity and will no longer function properly.

Muscles are composed of contractile units and arranged in fascicles, or long tubes of muscle fibers. It is possible for fascicles to add length, thus increasing the length of the muscle. The responsiveness of the MTU to different types of stretching, and different types of activity, is still being explored. There are no studies that track long-range stretching (such as a yoga practice over many years), so it is difficult to assess how much change in muscle structure is possible, and under what parameters.

The central nervous system (CNS) can limit flexibility through an increased sensitivity to stretching. The CNS includes your brain and spinal cord and is responsible for all actions of the body, voluntary and involuntary. There are several proprioceptive cells in your muscles and tendons which sense where you are in space and how quickly you are moving. Signals from these cells travel to the CNS, and the CNS responds to moderate the degree of movement so that the chances of injury are reduced. Higher sensitivity of this system can mean less flexibility.

Is it possible to increase flexibility?

It is possible to increase the range of motion (ROM) of our joints. There are two hypotheses to explain how we increase range of motion. The first, called the ‘sensory’ hypothesis, posits that the sensitivity of your nervous system to stretch is lessened through habitual use. The second, ‘mechanical’ hypothesis, posits that increases in muscles length and/or decreases in muscle stiffness are responsible for an increased range of motion.

The sensory hypothesis states that the responsiveness of the nervous system has an influence on the range of motion at a joint. There are specialized cells within your muscle called muscle spindles. When you stretch a muscle, the muscle spindles also stretch. When they experience a stretch, they send a signal back to your central nervous system (spinal cord), sort of like an alert. The CNS responds by sending a corresponding signal back to the muscle to contract so that there is not too much movement at the joint, or possible joint damage. It’s an effective system for preventing injury.

When you begin a stretching regime, one of the first things to change is the responsiveness of this signaling system. There is a desensitization that allows for a greater range of motion across that joint. Current research indicates that a stretching routine of 3-8 weeks acts primarily on the nervous system, supporting the sensory hypothesis. This process has been shown in several studies, although they are all short-term studies (hours to weeks).

The mechanical hypothesis states that stretching changes the length and/or stiffness add greater range of motion to the joint. It is possible for muscles to add to fascicle length through activity. When you exercise or stretch, you subject your muscles to high strain, and microtears in the muscle fibers can happen. These microtears are repaired through the activation of satellite cells, which are stem cells living in the fascia of the muscle. These satellite cells activate and head to the area of damage, where they can add nuclei (muscle cells are multinucleate) or additional cellular units to the muscle, increasing its size. Muscle has an effective self-healing mechanism that could result in adding muscle length in response to regular stretching.

Does it matter how it works?

As yet, there is no consensus as to which hypothesis is correct, although there is some evidence for both. It is likely that both sensory desensitization and mechanical changes to the muscle are contributing to a greater range of motion when a person undertakes a stretching routine (or practices yoga).

We don’t know the effect of a regular stretching routine on long-term ROM or joint health. More studies need to be done to fully explore what is capable. But what myself, and other long-term yoga practitioners have experienced, is a greater range of motion at all joints. A greater range of motion usually translates into healthier joints, and greater mobility as we age. However it works, we know that stretching helps us feel better and maintain health.