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 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 his/her 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.
Here are some useful points:
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 towards 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 spasm associated with exercise.
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 resists 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 that will limit 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:
More about collagen
I mentioned earlier that the connective tissues in and around muscle 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 muscle:
How stretching happens
Several physical properties of viscoelastic tissues help describe how these tissues elongate with stretching. These properties are the creep, load relaxation and hysteresis.
Creep describes the ability of a tissue to elongate overtime 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.
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.
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
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).
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.
I have attempted to give a Readers Digest version of the background to the theory of stretching. Some of the theory may be difficult to grasp and may challenge your existing preconceived ideas of stretching.
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