A physiotherapist's view on flexibility

Chris Mallac provides an overview of the theoretical basis of stretching routines.

Most coaches, athletes, and sports medicine personnel use stretching methods as part of the training routine for athletes. Many would agree that it forms an integral part of training and preparation. However, most of the theoretical and practical factors in stretching are often incorrectly applied.

What is flexibility?

De Vries defines it as the range of motion available in a joint, such as the hip, or a series of joints such as the spine. This encompassing definition considers several important aspects of flexibility. That is, it deals with a joint or series of joints used to produce a particular movement, and it considers that flexibility is both static and dynamic.

It is important to highlight some points regarding flexibility. First, flexibility is joint-specific. That is, you cannot say someone is flexible just because they can touch their toes. The same person may not even be able to reach around and scratch the small of their back because their shoulder has poor flexibility. Second, flexibility is sport-specific. You would not expect a front-row rugby forward to have the same flexibility as an Olympic gymnast, because it is not required for his sport. In a contact sport like rugby, being that flexible would be detrimental to his body.

Components of flexibility

Flexibility has two important components: static and dynamic flexibility.

Static flexibility describes the range of motion without consideration for the movement. This is the maximum range a muscle can achieve with an external force such as gravity or manual assistance. For example, holding a hamstring stretch at an end-of-range position.
Dynamic flexibility describes the use of the desired range of motion at the desired velocity (usually quickly). Dynamic flexibility is the range athletes can produce themselves. For example, a javelin thrower or baseball pitcher needs a lot of shoulder rotational flexibility, but they also need to be able to produce it at rapid speeds of movement.

Here are some useful points:

Good static flexibility is a necessary pre-requisite for good dynamic flexibility; however, having good static flexibility does not in itself ensure good dynamic flexibility
Dynamic flexibility is vitally important in those high-velocity movement sports such as sprinting, kicking, and gymnastics
Dynamic flexibility is limited by the ability of the tissues to lengthen quickly, and the inhibition of what is called the ‘stretch reflex', which if present will act to limit the range of motion (more about this later)

Why is flexibility important?

Adequate flexibility allows the joints to improve their range of motion. For example, flexibility in the shoulder musculature allows a swimmer to ‘glide' the arm through the water using shoulder elevation. This allows the joints to easily accommodate the desired joint angles without undue stress on the tissues around them. It, therefore, is essential for injury prevention.

Stretching also forms an integral part of rehabilitation programs following injury. For example, it is accepted that a muscle tear will heal with scar tissue. This scar tissue tends to be functionally shorter and has more resistance to stretch than normal healthy muscle tissue. Therefore, stretching is used at an appropriate time in the healing process to assist in lengthening this contracted scar tissue.

Good flexibility improves posture and ergonomics. Our bodies tend to allow certain muscles to tighten up, which will affect our posture. Vladimir Janda, a Czech rehabilitation specialist, describes a group of muscles in the body that universally show a tendency toward tightness and also being overactive in movements. Some of these include the hamstrings, rectus femoris, TFI, piriformis, adductors, gastrocnemius, and quadratus lumborum. These muscles are often implicated in postural syndromes causing musculoskeletal pain.

Flexibility, because it allows a good range of motion, may improve motor performance and skill execution. Think of a sprinter who needs flexibility in the hip flexors to allow good hip extension at toe-off, and good hip extensor flexibility to allow necessary knee drive in the leg recovery phase of sprinting. Skill execution and reduced risk of injury will be greatly enhanced if the body has the flexibility needed for that particular sport. There is also an argument that stretching may reduce post-exercise muscle soreness, or delayed-onset muscle soreness (DOMS), by reducing muscle spasms associated with exercise.

Relative flexibility

Shirley Sahrmann, an American physiotherapist, uses the term ‘relative flexibility' to describe how the body achieves a particular movement using the relative flexibility available at a series of joints. She believes that for the body to accomplish a particular range of motion, it will move through the point of least resistance, or area of greatest relative flexibility.

A good example is to think of a rower at the bottom of the catch position. In this position, the rower must have his hands (and the oar) past his feet to generate the drive necessary to transfer force from his body to the oar. If for some reason the rower has excessively tight hips and can't bend up (or flex) the hips (usually due to gluteal tightness), his body will find somewhere else to move to compensate for that lack of hip flexibility. More often than not, this rower will flex the lumbar and thoracic spines to make up for the lack of hip flexion. That is, the back has more ‘relative flexibility', and therefore contributes to the overall range of motion. In this case, however, the back will exhibit movement that is more than ideal, possibly leading to lumbar and thoracic dysfunction and pain.

The concept of relative flexibility is vital when understanding movement dysfunction in athletes. It is imperative that joint movements are not looked at in isolation, for other more distant joints will influence that movement. Try this simple test to highlight this point. Sit on a chair with your upper backed slumped (that is, assume a poor posture). Now, maintaining this position, try to elevate both arms above your head. Now straighten yourself up (assume a good posture) and try it again. Unless you have gross shoulder dysfunction, you will be able to elevate more with a straight back than a curved one. By assuming a slumped position, you prevent the upper back (thoracic spine) from extending. This extension of the upper back is necessary for full range elevation. Without an extension, it is difficult for the shoulder to elevate fully.

If you do this for long enough (months to years) eventually the lack of movement will attempt to be taken up elsewhere (such as the lower back, or the shoulder itself). This may subsequently lead to a breakdown of these joints due to the excessive movement they may eventually demonstrate.

What factors limit flexibility?

Flexibility can be limited by what is called ‘active' or ‘contractile' and ‘passive' or ‘non-contractile' restraints. Muscle contraction is one of these ‘active/contractile' restraints. Flexibility can be limited by the voluntary and reflex control that a muscle exhibits while undergoing a stretch, in particular, a rapid stretch that activates the ‘stretch reflex'. As a muscle is rapidly stretched, a receptor known as a 'spindle' causes the muscle to contract to prevent any further stretch reflexively. If left unchecked, the stretch reflex would work to avoid elongation while the muscle was being stretched. A benefit of ballistic or fast stretching is that the nervous system learns to accommodate by delaying the stretch reflex until closer to the range of movement.

Furthermore, a resting muscle does not always mean that it is ‘resting'. Muscles usually exist with a certain degree of muscle ‘tone'. An increase in tone will increase the inherent stiffness in muscles. If you are scientifically minded, this describes the way actin and myosin remain bound and thus resist passive stretching of the muscle. The actin and myosin stay bound because of a constant low-level discharge in the nerves supplying that muscle. With actin and myosin unbound, a muscle should (in theory) be able to stretch to 150% of its original length.

‘Passive/non-contractile' restraints in the form of connective tissues will also limit flexibility. The passive restraints include the connective tissues within and around muscle tissue (epimysium, perimysium, and endomysium), tendons, and fascial sheaths (deep and superficial fascia). The important microscopic structure to consider in passive tissues is collagen. The way collagen behaves with stretching will be discussed shortly.

Other passive restraints include the alignment of joint surfaces. An example of this is the olecranon of the elbow in the olecranon fossa which will limit the full extension (straightening) of the elbow. Other joint constraints include capsules and ligaments. The joint capsule/ligament complex of the hip joint is important in limiting the rotation of the hip.

The nerves passing through the limbs can also limit flexibility. As a limb is taken through a full movement, the ropey nerve tracts also become elongated and become compressed. The nerve endings and receptors in the nerves trigger a reflex response that causes the muscle to increase its resistance to stretch.

In addition to the points mentioned above, many other factors influence flexibility:

An older muscle has more inherent stiffness due to the morphological changes in the muscle and collagen in the connective tissues
A muscle that has been immobilised with a cast will demonstrate an increase in stiffness over time (longer than four weeks)
Excessive training causes more cross-linking to occur between collagen fibres and therefore increases stiffness
Excessively repeated muscle contractions cause high volumes of neural discharge. A muscle can remain in a state of high resting tone following training sessions
An increase in temperature causes a decrease in muscle stiffness. This can be environmental temperature or temperature increases induced by the friction of muscle contraction. We, therefore, tend to be less stiff around two o'clock in the afternoon
Finally, an increase in intramuscular fluid (fluid in the muscle cell) can increase stiffness due to a splinting effect. This is the proposed reason why the use of creatine monohydrate tends to make muscles feel stiffer

More about collagen

I mentioned earlier that the connective tissues in and around muscles are considered to be ‘passive' or ‘noncontractile'. The principal structure in these tissues that we need to consider is collagen. A key term used in physics and biomechanics to describe the way collagen behaves is ‘viscoelasticity'.

Viscoelastic tissues are made up of viscous and elastic properties. A viscous tissue will deform and stay deformed permanently – if you pull on a piece of play dough, for instance, it will keep that shape. An elastic tissue will return to its original length when the force is removed. For example, pulling on a rubber band and letting go – the band snaps back to its original length.

Viscoelasticity describes a property of tissues (collagen is one of those tissues) whereby deformation/ lengthening of tissue is sustained, and the recovery is slow and imperfect when the deforming force has been removed. That is, it will stretch, then stay stretched for a while before slowly returning to its original length.

Viscoelasticity tells us many practical things about stretching the connective tissues in muscles:

Studies on the cyclic loading of tissues suggest that most deformation occurs in the first stretch, and after four stretches there is little change in ultimate length. Therefore, there is no extra benefit from stretching a muscle ten times in one session
It takes 12-18 seconds to reach stress relaxation, so there is no need to hold a stretch for longer than 20 seconds
Greater peak tensions and more energy are absorbed the faster the rate of stretch. This means that tissue will generate greater tension if the rate of stretch is faster and therefore not achieve the same length as a tissue undergoing a slow stretch. That is, do passive stretches SLOWLY
Once elongated, length changes are not rapidly reversible due to the viscous nature of the tissue. However, deformations are not permanent because the elastic properties will eventually bring the tissue back to its original length. Lasting changes come from the adaptive remodelling of the connective tissues, not mechanical deformation. One study in South Africa showed that stretching every four hours was the most effective way to achieve elongation in a muscle. This may suggest that the temporary change in length following a stretch may start to regress after four hours (Grace Hughes, unpublished study).

How stretching happens

Several physical properties of viscoelastic tissues help describe how these tissues elongate with stretching. These properties are a creep, load, relaxation, and hysteresis.

Creep describes the ability of a tissue to elongate over time when a constant load is applied to it. For example, if we applied 10kg of force to our leg to stretch our hamstring, we might initially get our leg to 90 degrees before our tissues prevented further movement. If we sustained that load, we would find that our leg would gradually ‘creep' a few degrees over some time.

Load relaxation describes how less force is required to maintain a tissue at a set length over time. Using the above example again, if we applied 10kg of force to get our leg to 90 degrees, we would find that less force would be needed (9, 8, 7kg, etc.) to keep it at 90 degrees.

Hysteresis describes the amount of lengthening a tissue will maintain after a cycle of stretching (deformation) and then relaxation. Again, let's assume that if we gained an extra 10 degrees of range in hamstrings after the stretches described above, we would maintain that range for some time after the load was removed.

Neuromuscular considerations

Certain neuromuscular mechanisms acting on muscles influence ‘tension' and have important implications for the value of stretching. These mechanisms include the stretch reflex, autogenic inhibition, and reciprocal inhibition.

A long thin receptor governs the stretch reflex in muscles called a 'muscle spindle'. The spindle's role is to let our feedback systems know about muscle length and the rate of muscle lengthening. When a muscle is rapidly stretched, the spindle (via a loop of nerves) triggers a reflex contraction of the muscle undergoing stretch. A high-speed stretch will, therefore, trigger the spindle and a reflex contraction of the muscle will limit its ability to stretch
The spindle is also responsible for the phenomenon known as reciprocal inhibition. What happens here is that if a muscle contracts, the opposite or antagonistic muscle will relax to allow the movement to occur without resistance. For example, if the quadriceps are contracted, the hamstrings should relax to allow the knee to straighten
The Golgi tendon organ (GTO) is the important receptor to consider in ‘autogenic inhibition'. The role of the GTO is to provide information on tension increases in muscles. This tension can come from contraction or stretch. The GTO connects with a small nerve cell in the spinal cord that inhibits or relaxes the muscle where the GTO is found. The GTO will trigger if a stretch is sustained (for longer than six seconds) or if the muscle contracts forcefully.

The way these mechanisms are utilised will be discussed below under the heading of proprioceptive neuromuscular facilitation (PNF) type stretching.

The theory behind different stretching types

Static

Held static stretches are done so that the joints are placed in the outer limits of the available range and then subjected to a continuous passive stretch (gravity, weights, manual). One obvious benefit is that the chance of injury is minimal. This type of stretching is ideal to stretch the connective tissue/non-contractile elements since it makes use of the viscoelastic properties to cause elongation of the tissue. Furthermore, it makes use of autogenic inhibition to trigger relaxation in the muscle (remember the six-second rule).

Dynamic

Dynamic range of motion - This describes a type of stretch whereby a muscle is taken through a full, slow and large amplitude movement. The opposing muscles are used to produce the force in this type of stretching. This type of stretching is done under control and is not jerky.
Ballistic - The type that is done fast and rapidly and through large ranges of motion. An example is leg swings to stretch the hamstrings. The benefit of this type of stretching is that it is sport-specific to ballistic sports and it allows integration of the ‘stretch reflex' if done quite often over some time. As the neuromuscular system adapts to this stretching, the stretch reflex will minimise its contribution to limiting muscle range.
Bouncing - Similar to ballistic, but it is performed in small oscillations at the end of the range. The dangers of (2) and (3) are that they can lead to significant muscle soreness caused by the rapid lengthening of the muscle. This in itself initiates the stretch reflex and increases muscle tension. Furthermore, it fails to provide adequate time for the tissues to adapt to the stretch.

PNF (Proprioceptive neuromuscular facilitation)

PNF uses the concept that muscle relaxation is fundamental to the elongation of muscle tissue. In theory, it is performed in a way that used the proprioceptive abilities of the GTO and muscle spindle to relax or inhibit the muscle from gaining a more effective stretch. It does so, using autogenic inhibition and reciprocal inhibition.

PNF stretching exists in many different forms, but the only ones discussed here will be the contract-relax (CR), hold-relax (HR), and contract-relax and antagonist contraction (CRAC) methods.

Contract-relax (CR) The muscle to be stretched is passively taken to the end of the range. Maximum contraction of the muscle to be stretched is performed against resistance (usually another person). With this form of contraction, the muscle is allowed to shorten during an isotonic contraction. This is continued for at least six seconds (which allows autogenic inhibition to occur). The muscle is then relaxed and taken to a new range and held for about 20 seconds. This can be repeated 3-4 times
Hold-relax (HR) Very similar to contract-relax as above, but the contraction type is static/isometric. The muscle to be stretched is passively taken to the end of the range. Maximum contraction of the muscle to be stretched is performed against resistance (usually another person). With this form of contraction, the muscle does not shorten during its isometric contraction. This is continued for at least six seconds (allowing autogenic inhibition to occur). The muscle is then relaxed and taken to a new range and held for about 20 seconds. This can be repeated 3-4 times
Contract-relax antagonist contraction (CRAC) The first part of this stretch is similar to the CR method above; however, when the muscle to be stretched is relaxed after its six-second contraction, the opposite or antagonist muscle is contracted for at least six seconds (allowing reciprocal inhibition to occur). The antagonist is then relaxed, and the stretched muscle is taken to a new range.

Final thought

I have attempted to give a Readers Digest version of the background to the theory of stretching. Some of the theories may be difficult to grasp and may challenge your existing preconceived ideas of stretching.

Article Reference

This article first appeared in:

MALLAC, C. (2003) A physiotherapist's view on flexibility. Brian Mackenzie's Successful Coaching, (ISSN 1745-7513/ 8 / December), p. 5-8

Page Reference

If you quote information from this page in your work, then the reference for this page is:

MALLAC, C. (2003) A physiotherapist's view on flexibility [WWW] Available from: https://www.brianmac.co.uk/articles/scni8a3.htm [Accessed