Updated 18 November 2014

The science of stretching

What happens to the muscle when you stretch? A biokineticist answers.

Flexibility is defined as the absolute range of movement in a joint or series of joints that is attainable in a momentary effort with the help of a partner or a piece of equipment.

This definition tells us that flexibility is not something general but is specific to a particular joint or set of joints. In other words, it is a myth that some people are innately flexible throughout their entire body.

Being flexible in one particular area or joint does not necessarily imply being flexible in another.

Being "loose" in the upper body does not mean you will have a "loose" lower body. Furthermore, the flexibility in a joint is also "pecific to the action performed at the joint (the ability to do front splits doesn't imply the ability to do side splits even though both actions occur at the hip).

What happens when you stretch
The stretching of a muscle fibre begins with the sarcomere, the basic unit of contraction in the muscle fibre. As the sarcomere contracts, the area of overlap between the thick and thin myofilaments increases.

As it stretches, this area of overlap decreases, allowing the muscle fibre to elongate. Once the muscle fibre is at its maximum resting length (with all the sarcomeres fully stretched), additional stretching places force on the surrounding connective tissue.

As the tension increases, the collagen fibres in the connective tissue align themselves along the same line of force as the tension. Hence when you 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 helps to realign any disorganised fibres in the direction of the tension. This realignment is what helps to rehabilitate scarred tissue back to health.

When a muscle is stretched, some of its fibres lengthen, 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).

According to SynerStretch you should think of "little pockets of fibres distributed throughout the muscle body stretching, and other fibres simply going along for the ride". The more fibres that are stretched, the greater the length developed by the stretched muscle.

8 Types of stretching

1. Static stretching
Static stretching involves gradually easing into the stretch position and holding the position.

The amount of time a static stretch is held, may be anything from 6 seconds to 2 minutes. Often in static stretching you are advised to move further into the stretch position as the stretch sensation subsides. The stretches on our site are all examples of static stretching exercises, such as the "Total body stretch" series.

2. Ballistic stretching
Ballistic stretching uses the momentum of a moving body or a limb in an attempt to force it beyond its normal range of motion.

3. Dynamic stretching
Dynamic stretching consists of controlled leg and arm swings that take you gently to the limits of your range of motion.

Where the event requires a dynamic movement then it is appropriate and perhaps necessary to conduct dynamic stretching exercises. Start off with the movement at half speed for a couple of repetitions and then gradually work up to full speed.

4. Active stretching
An active stretch is one where you assume a position and then hold it there with no assistance other than using the strength of your agonist muscles. Active stretching is also referred to as static-active stretching.

5. Passive stretching
Passive stretching is also referred to as relaxed stretching, and as static-passive stretching. A passive stretch is one where you assume a position and hold it with some other part of your body, or with the assistance of a partner or some other apparatus.

6. Isometric stretching
Isometric stretching is a type of static stretching which involves the resistance of muscle groups through isometric contractions (tensing) of the stretched muscles.

7. Assisted stretching
Assisted stretching involves the assistance of a partner who must fully understand what their role is otherwise the risk of injury is high. A partner can be employed to assist with Partner stretches and Proprioceptive Neuromuscular Facilitation (PNF) techniques.

8. Partner stretches
Your partner assists you to maintain the stretch position or help you ease into the stretch position as the sensation of stretch subsides. You should aim to be fully relaxed and breath easily throughout the exercise. Partner assisted stretches are best used as developmental exercises, with each stretch being held for thirty seconds.

Types of Flexibility
Many people are unaware of the fact that there are different types of flexibility. These different types of flexibility are grouped according to the various types of activities involved in athletic training. The ones which involve motion are called dynamic and the ones which do not are called static. The different types of flexibility (according to Kurz) are:

  • Dynamic flexibility (also called kinetic flexibility) is the ability to perform dynamic (or kinetic) movements of the muscles to bring a limb through its full range of motion in the joints.
  • Static-active flexibility (also called active flexibility) is the ability to assume and maintain extended positions using only the tension of the agonists and synergists while the antagonists are being stretched. For example, lifting the leg and keeping it high without any external support (other than from your own leg muscles).
  • Static-passive flexibility, (also called passive flexibility) is the ability to assume extended positions and then maintain them using only your weight, the support of your limbs, or some other apparatus (such as a chair or a barre). Note that the ability to maintain the position does not come solely from your muscles, as it does with static-active flexibility. Being able to perform the splits is an example of static-passive flexibility.

Research has shown that active flexibility is more closely related to the level of sports achievement than is passive flexibility. Active flexibility is harder to develop than passive flexibility (which is what most people think of as "flexibility"); not only does active flexibility require passive flexibility in order to assume an initial extended position, it also requires muscle strength to be able to hold and maintain that position.

Factors affecting your flexibility

According to Gummerson, flexibility (he uses the term mobility), is affected by internal and external factors.

Internal influences include:

  • the type of joint (some joints simply aren't meant to be flexible)
  • the internal resistance within a joint
  • bony structures which limit movement
  • the elasticity of muscle tissue (muscle tissue that is scarred due to a previous injury is not very elastic)
  • the elasticity of tendons and ligaments (ligaments do not stretch much and tendons should not stretch at all)
  • the elasticity of skin (skin actually has some degree of elasticity, but not much)
  • the ability of a muscle to relax and contract to achieve the greatest range of movement
  • the temperature of the joint and associated tissues (joints and muscles offer better flexibility at body temperatures that are 1 to 2 degrees higher than normal).

External influences

  • the temperature of the place where one is training (a warmer temperature is more conducive to increased flexibility)
  • the time of day (most people are more flexible in the afternoon than in the morning, peaking from about 2:30pm-4pm)
  • the stage in the recovery process of a joint (or muscle) after injury (injured joints and muscles will usually offer a lesser degree of flexibility than healthy ones)
  • age (pre-adolescents are generally more flexible than adults)
  • gender (females are generally more flexible than males)
  • one's ability to perform a particular exercise (practice makes perfect)
  • one's commitment to achieving flexibility
  • the restrictions of any clothing or equipment

Some sources also the suggest that water is an important dietary element with regard to flexibility. Increased water intake is believed to contribute to increased mobility, as well as increased total body relaxation.

Factors that may limit your flexibility
Rather than discuss each of these factors in significant detail as Gummerson does, I will attempt to focus on some of the more common factors which limit one's flexibility. According to SynerStretch, the most common factors are: bone structure, muscle mass, excess fatty tissue, and connective tissue (and, of course, physical injury or disability).

Depending on the type of joint involved and its present condition (is it healthy?), the bone structure of a particular joint places very noticeable limits on flexibility. This is a common way in which age can be a factor limiting flexibility since older joints tend not to be as healthy as younger ones.

Muscle mass can be a factor when the muscle is so heavily developed that it interferes with the ability to take the adjacent joints through their complete range of motion (for example, large hamstrings limit the ability to fully bend the knees). Excess fatty tissue imposes a similar restriction.

(Pea Blaauw, registered biokineticist)


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