What we’ve learned through science is that it isn’t just one thing. Stretching is actually pretty dang complicated.
First the Anatomy
Each muscle fiber is wrapped up in fascia, a material a little like the plastic wrap you could see surrounding a leftover chicken leg in the fridge. Each individual muscle fiber wrapped up in its fascia is then collected into a group with another coating of fascia holding the group together. Then several of those groups of muscle fibers are bundled together in one big group of muscle surrounded by a bigger, thicker layer of fascia.
As the muscle nears a bone, it thins and becomes tapered. The fascia covering each fiber as well as that surrounding the groups of fibers continues and becomes a tendon that forms the connection of muscle to bone. Scientists call this entire structure the muscle-tendon complex. It’s considered one unit because muscle and connective tissue (fascia and tendons) are so intimately connected and intertwined that studying only one or the other is difficult. Having said that, they’ve been able to tease out what is happening to each of them when we stretch.
The Muscle Component
Stretching muscle causes a reflex mechanism in the spinal cord– sort of like the reflex a doctor elicits when she taps your knee and your leg jerks. Sensitive receptors known as muscle spindles are located throughout the length of the muscle. Muscle spindles note a change in muscle length during a stretch as well as how fast the stretch has occurred. They send this information to the spine. That triggers the stretch reflex which attempts to resist the change in muscle length by causing the stretched muscle to contract. The more sudden the change in muscle length, the stronger the muscle contractions will be. (And that is one reason you want to go slowly into a stretch without any rapid sudden movement.) This reflex helps to maintain muscle tone and upright posture and to protect the body from injury. The longer you hold an asana and stretch the muscle, the less the muscle spindles can do their job. They only work for a short while. After time, their effect goes away. When that happens, you get a little more length during the stretch because the muscle stops contracting.
There’s another reflex mechanism involving sensory receptors called golgi tendons which lie at the tapered ends of muscles where the tendon is forming from the fascia. These sensors are activated when a muscle is stretched a little farther. Their job is to send information to the spinal cord to elicit muscle relaxation to prevent excessive strain and subsequent injury.
Hundreds of sarcomeres make up each individual muscle fiber. They are divided into bands composed of either actin or myosin. During contraction of a muscle, those bands slide over one another causing the muscle to shorten. The actin and myosin bands are connected to each other by chemical bonds. Stretching causing some of those bonds to break allowing the muscle to lengthen.
Sarcomeres are also elastic. Over half of each sarcomere is composed of a protein called titin that gives it elasticity. Titin acts like a rubber band. It has the ability to increase the length of a muscle when stretched and then to shorten to the original length when the stretch is released. First its overall shape changes to become elongated and then with increasing force of a stretch, the protein unfolds from its three dimensional structure. The result is a lengthened sarcomere and therefore a longer muscle.
Connective Tissue Component: Fascia and Tendons
More than half of an initial change in length, the give of releasing into a stretch, is due to elasticity of the connective tissue. It’s like a rubber band.
In addition to its elastic component, connective tissue creeps with longer duration stretches. That is, stretching for a few minutes causes a reorientation of the collagen fibers within it to a more ordered array. They line themselves up in parallel to provide more lengthening.
Tendon fibers are pretty much already in parallel to one another, but fascia fibers are more willy-nilly. Most of the creep is due to reorientation of the collagen fibers in fascia to an ordered parallel arrangement. The fibers begin to line up like rows of soldiers coming to attention, providing additional lengthening and stretching.
Creep is what gives connective tissue its ability to maintain length, and therefore flexibility, over the long term. The effect doesn’t go away a few minutes after you release the stretch. It’s the more permanent aspect of increased flexibility.
All those other stretch mechanisms occur first. Then, with time, the creep of connective tissue begins. Exactly how long one needs to maintain a stretch to produce creep isn’t clear. It may occur in four or five minutes. By ten minutes it is more likely to be happening.
Fascia creep can progressively increase length at time frames of at least up to one hour. In fact, in one experiment, an entire third of the total lengthening from a stretch took place during the latter part of the hour.
The mind plays a role here. Some researches believe that much of our ability to lengthen muscle-tendon complexes and to increase flexibility comes from our brains’ ability to alter how we feel in response to a stretch. Usually our stretches are stopped by a sensation of discomfort or even, when the stretch is forced beyond that point, outright pain.
Over time the exact point at which a stretch makes us feel uncomfortable may tend to increase with a period of repeated stretching exercises. In other words, the maximum amount of stretch we could tolerate last week might be much less that we can tolerate today because our mind simply doesn’t act to stop the stretch because of a perception of pain.
Pain is a protective response. We quickly pull our hand away from a burner because the brain knows that a burn that will destroy our tissue. Maybe the brain actually “learns” that a stretch to a certain point is ultimately not harmful and so it has figured out that stretching to that point is okay and it no longer elicits a sensation of “ouch.”
Are longer duration stretches beneficial?
Stretching fascia is the primary way to increase the range of motion of joints in a lasting way. While the other stretch mechanisms provide temporary lengthening, it is the creep effect of fascia which contributes the most to long-term flexibility.
Especially for those with a reduced range of motion from disease or an extended period of inactivity, prolonged stretches can greatly improve function. This has been seen clinically with stretches of the rotator cuff and with a tendon on the bottom of the foot that often shortens and tightens causing pain and disability. Longer stretches incorporating the creep effect can be very healing.
There’s also some evidence that gentle stretching elicits an anti-inflammatory response within the muscle and connective tissue. With more forceful stretches, there’s a counterproductive pro-inflammatory effect. Long gentle stretches may help to reduce inflammation leading to tendonitis.
Bit of Caution
Bobbing stretches that are rapid and short can strain the muscles and connective tissue resulting in tears. Don’t bounce.
Warm muscles stretch better than cold ones. Temperature can significantly influence flexibility. A five minute warm-up period of general increased activity is wise for best effects and less injury.
There may be a relationship between long duration stretches and the maximal amount of force a muscle can provide. If you really need absolute maximum power, longer duration stretches may not be for you. One group of researches found that a small surgical release of fascia resulted in a 15% reduction in force production due to a lowered compartment pressure. That may translate to a reduction of maximum strength after prolonged stretching. Some body builders, though, like to stretch their fascia to make room for greater muscle growth since their concern is bulk and not power.
Traditional Yoga texts suggest that asanas were initially intended as postures for meditation. The greatest yogis spent prolonged periods of time in classic postures like pascimottanasana, padmasana, and bhadrasana.
Holding a stretch for an extended period in a relaxed manner provides an opportunity for meditation and allows for a deeper level of awareness of the energy body. On a musculoskeletal level, these longer duration stretches primarily release fascia, and they can significantly improve long-term flexibility and function.
Stretching the mind by keeping it out of forgone conclusions and repetitive thought processes is similarly intricate and beneficial. We’ll discuss these benefits in another article.