The Science of Stretching

Stretching is widely used as a means of increasing flexibility, but what actually happens to the musculoskeletal system when you stretch? Do different kinds of stretching achieve different results? Can stretching be harmful? Is it better to stretch with a partner, or is stretching by yourself just as effective? Over time various myths regarding stretching have emerged while advances in sports medicine have revealed new insights.

Background

Nothing could be more natural than stretching; many mammals stretch instinctively to increase their flexibility, especially on waking from sleep or after being in a cramped position. Stretching is practiced by athletes before and after exercise, by anyone undergoing rehabilitation, and when you simply feel stiff.

The benefits of stretching have been realised for many years (Cornelius W., 1990) and while these are generally believed to include improvements to joint range of motion, reduction of tissue trauma, injury reduction, warm up, motor performance, fitness, and relaxation, not everyone agrees on the best forms of stretching for achieving specific goals. Furthermore, recent scientific research suggests that some of these perceived benefits may be over-stated.

 

Flexibility

 

To understand stretching we need to understand something about flexibility. Movement depends on the range of motion in synovial joints. This is determined by joint geometry, capsuloligamentous structures that surround joints, and passive and active muscle tension. Muscle flexibility is the ability of the muscle to lengthen to permit the joint to move through its range of motion, and a decrease in this ability results in a loss in flexibility.

 

What happens when you stretch?

 

Skeletal muscles consist of muscle fibres, which consist of tubular muscle cells or myofibrils. The myofibrils are made up from repeating sections of sarcomeres. Muscle fibers are composed of tubular myofibrils which contain repeating sections of sarcomeres. Sarcomeres are filaments of fibrous proteins that slide past each other during muscle contraction and relaxation. A single muscle fibre may contain around 100,000 sarcomeres.

When the sarcomere contracts, the myofilaments overlap increases and when the sarcomere stretches the overlap decreases. When all the sarcomeres are stretched to their maximum any additional stretching stresses the surrounding connective tissue. This causes the collagen fibres in the tissue to align in the same direction as the tension. This stretching of the connective tissue results in a realignment of disorganized collagen fibres when helps restore the health of the tissue.

 

Types of stretching

 

There are many different types of stretching. Some important ones include:
• Static stretching – this involves stretching the muscle and maintaining the position. Typically, the position is maintained for a few seconds to a few minutes. This is considered to be the safest stretching technique.
• Ballistic stretching – this relies on the movement of a limb to force it further than its usual range of motion. The muscle to be stretched in held stationary and is lengthened by subjecting it to a bouncing motion. The most effective technique is the use of low force over an extended time rather than high force over a shorter time. This kind of stretching carries a higher degree of risk
• Dynamic stretching – this allows the muscle to elongate in its relaxed state. This is achieved by contracting the antagonist muscle and then slowly moving the joint. For instance, for dynamically stretching the hamstring, you would contract the quadriceps and extend the knee causing the relaxed hamstring to elongate.
• Proprioceptive Neuromuscular Facilitation (PNF) – here the aim is to promote the neuromuscular mechanism by stimulating the proprioceptors in order to increase strength and flexibility and improve coordination. This is achieved with brief isometric contractions before static stretching. The technique involves a partner who resists the isotonic contraction undertaken by the subject. The subject then relaxes and the partner moves the joint through its maximum range.
• Self-administered PNF – while PNF is usually associated with using a partner, it can also be self-administered. An extensive study of the benefits of self-administered PNF (Wicke et. Al., 2013) demonstrated that, when practiced over a four-week period, it significantly improves hamstring flexibility; hip range of movement and hip, back and shoulder flexibility are also improved.
• Eccentric Flexibility Training – this is a used to particularly to increase hamstring flexibility (O’Sullivan et al. 2012). The technique makes use of a resistance band and prepares the athlete for situations in which hamstring muscles are subject to eccentric forces such as when decelerating or landing.

 

Some benefits of stretching may be overstated…

While stretching has many potential benefits, research suggests that some of these might have been overstated (Nelson R, Brandy W, 2005), for instance:
• Warming the muscles – this improves blood flow, increases metabolic rate, and improves muscle function. However, it requires active and dynamic stretching; static stretching has no effect on muscle warming.
• Injury risk reduction – unfortunately, and despite many scientific studies, there is no evidence that stretching really does reduce to risk of injury, for instance (Pope et al. 2000)
• Improvement athletic performance – again, unfortunately there is no evidence of this. In fact, several studies have shown that, in certain circumstances, stretching can reduce athletic performance, for instance (Young and Behm, 2002)

 

Finally – a word on breathing

The importance of correct breathing in the context of stretching and flexibility was pointed out by Welge (2000). With abdominal or diaphragmatic breathing in which the abdomen rises while inhaling, the lumbar spine extends slightly, resulting in increased lordosis and anterior pelvic tilt, with the opposite happening during exhalation. This can help stretch specific muscles. As an example Welge cites the hamstring. Here the region to be stretched is supported and, with the back and shoulders straight, the hamstring is slightly stretched and held for half a minute while abdominally breathing. The exercise is most beneficial when practiced on a daily basis.

 

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References
Cornelius W., (1990), Benefits from Flexibility Exercise, national strength and Conditioning Association Journal, 12(5):61-64.
Nelson R, Brandy W, (2005), An Update on Flexibility, National Strength and Conditioning Association, 27(1):10-16
O’Sullivan K., McAuliffe S., DeBurca N., (2012), The effects of eccentric training on lower limb flexibility: a systematic review. British Journal of Sports Medicine 46(12):838-45.
Pope, R.P., Herbert R.D., Kirvan J.D., and Graham B.J., (2000), A randomized trial of pre exercise stretching for prevention of lower-limb injury. Med. Sci. Sports Exerc. 32:271–277.
Welge M., (2000), Don’t Forget to Breathe: Using Deep Breathing to Help You Stretch, National Strength & Conditioning Association Journal, 22(5):69
Wicke J., Gainey K., Figueroa M., (2013), A Comparison of Self-Administered Proprioceptive Neuromuscular Facilitation to Static Stretching On Range of Motion, Journal of Strength and Flexibility, Conditioning Research, 28(1)/168–172.
Young, W.B., and Behm D.G., (2002), Should static stretching be used during a warm-up for strength and power activities? Strength Cond. J. 24:33–37.