The (basic) physiology of static stretching
Within Eastern movement theory, we identify five different states of the muscles:
- Contraction
- Relaxation
- Stretching
- Releasing
- Loosening - Prepare Contraction
In the article “The (basic) physiology of static stretching” from ACRO Physical Therapy & Fitness institute, the advantages and disadvantages of stretching are clearly described. Within Tai Chi, however, we use stretching during the exercises. By doing so, we only get the benefits of stretching.
The working of the muscles is very complex but can be explained in a simplified way with some pictures. Keep in mind that the article was written in 2017, and the knowledge of fascia was limited at that time. More on fascia in future blogs.
SOME ANATOMICAL BACKGROUND INFORMATION:
The musculoskeletal system:
Muscles, bones, ligaments, tendons, and connective tissue/fascia comprise what is called the musculoskeletal system of the body. The bones allow for upright posture and provide structural support for the body. The muscles allow the body to move (by contracting and thus generating tension). Muscles are attached to the bone by tendons, and when they contract, they create movement. The point where bones connect to one another is called a joint, and ligaments, joint capsules and connective tissue support this connection.
Muscle composition:
At the highest level, the (whole) muscle is composed of many strands of tissue called fascicles. (These are the strands of muscle that we see when we cut red meat or poultry.) Each fascicle is composed of fasciculi, which are bundles of many microscopic muscle fibres. The muscle fibres are in turn, composed of tens of thousands of thread-like myofibrils, which can contract, relax, and elongate (lengthen). The myofibrils are (in turn) composed of up to millions of bands laid end-to-end called sarcomeres. Each sarcomere is made of overlapping thick and thin filaments called myofilaments. The thick and thin myofilaments are made up of contractile proteins, primarily actin and myosin.
THE (BASIC) PHYSIOLOGY OF MUSCLE CONTRACTION:
The way in which all these various levels of the muscle operate is as follows: Nerves connect the spinal column to the muscle. The place where the nerve and muscle meet is called the neuromuscular junction. When an electrical signal crosses the neuromuscular junction, it is transmitted deep inside the muscle fibres. Inside the muscle fibres, the signal stimulates the flow of calcium, which causes the thick and thin myofilaments to slide across one another. When this occurs, it causes the sarcomere length to shorten, which generates a force (a.k.a contraction). When billions of sarcomeres in the muscle shorten all at once, it results in a contraction of the ENTIRE muscle fibre.
Now, one important concept to understand is this: when a muscle fibre contracts, it contracts completely. There is no such thing as a partially contracted muscle fibre. Muscle fibres are unable to vary the intensity of their contraction relative to the load against which they are acting. Rather, muscle contraction force varies in strength from strong to weak based on the NUMBER of fibres involved. Basically, more muscle fibres are recruited, AS they are needed, to perform the job at hand. The more muscle fibres that are recruited by the central nervous system, the stronger the force generated by the muscular contraction. This concept boils down to energy efficiency. It takes energy to contract muscle fibres, and the body loves to conserve energy. So, it recruits fibres on a one-by-one basis looking for the fewest needed to move the load. This means you use fewer muscle fibres to pick up a bottle of water than you do to lift a gallon of milk.
THE (BASIC) PHYSIOLOGY OF STRETCHING:
The stretching of a muscle fibre begins with the sarcomere, the basic unit of contraction in the muscle fibre. As the sarcomere contracts, the overlap between the thick and thin myofilaments increases (discussed above). As it stretches, this area of overlap DECREASES, allowing the muscle fibre to elongate. Once the muscle fibre is at its maximum resting length (all the sarcomeres are fully stretched), additional stretching places force on the surrounding connective tissue. As the tension increases, the collagen fibres in the connective tissue align along the same force line as the tension. So as you continue to stretch, the muscle fibre is pulled out to its full-length sarcomere by sarcomere, and then the connective tissue takes up the remaining slack. When this occurs, it may help to realign any disorganized fibres in the direction of the tension. This realignment may be what helps to rehabilitate scarred tissue back to health (during recovery from muscle injury/after surgery).
Some of its fibres lengthen when a muscle is stretched, but other fibres may remain at rest. The current length of the entire muscle depends upon the number of stretched fibres (similar to the way that the total strength of a contracting muscle depends on the number of recruited fibres contracting). One way to visualize this is to think of little groups of fibres throughout the muscle body stretching, while other groups of fibres are simply “going along for the ride". As such, the more fibres that are stretched in this process, the greater the length developed by the stretched muscle.
Relative to the process of stretching, it is also important to understand how the brain/neural components of the musculoskeletal system adapt to stretching. (FYI – this is the simplified version, so please just appreciate that there are other factors involved). When the muscle is stretched, so is the muscle spindle (a nerve control point located among groups of muscle fibres). The muscle spindle records the change in length of the muscle and how fast this change occurs. It then sends signals to the spine, which then conveys this information to the brain. Initially, this information 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 (why you don’t “bounce stretch.”) This basic function of the muscle spindle helps to maintain muscle tone and to protect the body from injury.
The Golgi Tendon Organ (GTO)
Now, if the force and suddenness of the stretch exceed the muscle’s ability to safely contract for protection (a.k.a. exceed it’s strength), another neural component, the Golgi tendon organ (GTO), goes into action and takes power over the muscle spindle.
Basically - when muscles contract (possibly due to the stretch reflex), they produce tension at the point where the muscle is connected to the tendon. This is where the Golgi tendon organ is located. The Golgi tendon organ then records the change in tension and the rate of change of the tension, and sends signals to the spine to convey this information. When this tension exceeds a certain threshold, it triggers the lengthening reaction, which inhibits the muscle’s contraction and instead causes it to relax and lengthen. The lengthening reaction is possible only because the signalling of the Golgi tendon organ to the spinal cord is powerful enough to overcome the signalling of the muscle spindles telling the muscle to contract. Think of the two systems as a “double fail-safe” that ultimately helps decrease your injury risk.
Why we stretch slowly and for a prolonged period of time:
One of the reasons for holding a stretch for a prolonged period is that as you hold the muscle in a stretched position, the muscle spindle habituates (becomes accustomed to the new length) and reduces its signalling. Gradually, you can train your stretch receptors to allow greater lengthening of the muscles. This, in turn, also increases your “flexibility” as the muscle spindle now allows your muscle to stretch farther before contracting.
Another reason for holding a stretch for a prolonged period is to allow the lengthening reaction (caused by the Golgi tendon organ) to occur, thus helping the stretched muscles to relax.
It is easier to stretch, or lengthen, a muscle when it is not trying to contract.
