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stretching

Stretching: what’s the point?

Sean Fyfe trawls the evidence for practical advice on the pros and cons of stretching.

Sports practitioners, trainers, coaches and therapists have for decades routinely prescribed stretching in pursuit of various fitness goals for recreational and elite athletes alike. But what are the evidence-based benefits of stretching? Do we advise and practise it out of habit, outdated beliefs or current best practice? This overview of the research on stretching should help therapists to update their understanding of what works and why – as well as what doesn’t.

The basics

There are three commonly used stretching techniques:

  • Static: sustained pressure is applied to a muscle or muscle group in a lengthened position;
  • Dynamic or ballistic: repeated bouncing movements are made at the end of range of the muscle length, stimulating the stretching reflex;
  • PNF (proprioceptive neuromuscular facilitation): combines a series of isometric muscle contractions and static stretches performed according to several specific protocols.

Stretching is normally done to achieve one or more of five aims:

  1. to increase muscle length
  2. to reduce the risk of injury
  3. to enhance prospects of healing for injured tissues
  4. to enhance performance
  5. to reduce pain associated with muscle and joint stiffness.

The many permutations of goals and techniques for stretching make it hard to draw overall conclusions from the research about what works in which circumstances. Below we draw out some of the main themes and conclusions – but it is clear that more research is needed in many areas (the role of stretching in hamstring flexibility/rehab, for instance, is relatively well covered, whereas other muscle groups and other objectives are far less well researched).

Stretching for flexibility

i. Static stretching works

The research seems pretty clear in its support for the efficacy of the single most common use of stretching: static stretching to achieve an increase in range of movement (7,8). Increases in muscle length are thought to be achieved through two mechanisms (26):

i. Increased muscle-tendon viscoelasticity results in a direct decrease in muscle stiffness (less force is needed to produce a change in muscle length).

ii. Inhibition (overcoming) of the stretching reflex reduces the active resistance of the muscle to the force applied.

How stretching works

Muscles (and their tendinous attachments) have properties of elasticity (resilience), plasticity (pliability) and viscosity (internal friction). Strength training over time generally bulks up and shortens the muscle; stretching lengthens it. The ‘stiffness’ of a muscle-tendon complex describes how readily it stretches when a given force is applied: a stiff muscle moves less far than a compliant one, which means it is less flexible, but on the other hand has greater potential elastic recoil and therefore stored power. The speed of movement also matters: the slower the loading, the more pliable and less resistant the quality of movement.

The muscle’s spindle cells govern the nervous system response to maintain contractile properties, most notably the ‘stretch reflex’ – the automatic contraction of a muscle which has relaxed and lengthened to the edge of its safe range of movement. Stretching for flexibility uses various slow, static and end-of-range movement techniques to overcome the stretching reflex and encourage a gradual lengthening of the muscle and tendon fibres and/or greater nervous system tolerance of the stretching at the end of range.

So in principle stretching is effective in increasing muscle flexibility. What is the appropriate exercise prescription? This depends on whether you are seeking short-term or long-term flexibility gains.

Short-term gains are what we all expect when we undertake conventional ‘preparatory’ stretching prior to activity, with the hope of enhancing performance or decreasing the risk of injury. Long-term changes might be the goal during the management of a chronic injury, rehabilitation from an acute injury or to achieve a change in range of movement which will facilitate an improved sporting technique that was previously inhibited by muscle inflexibility.

Magnusson (19) demonstrated in his 1998 study of passive properties of human skeletal muscle during stretching manoeuvres that long-term increases in joint range of motion resulted from a change in stretching tolerance rather than increased viscoelasticity. To complicate the picture further, different muscles have been shown to adapt differently to stretching and some muscle stretches may need to be held for longer than others.

In a 2004 literature review, Shrier and Gossal (25) suggested the following for static stretching flexibility protocols.

  • Static stretches should be held for 15 to 30 seconds.
  • For short-term changes there is no benefit in holding a stretch for longer then 15 seconds (18).
  • There is no added benefit in repeating a stretch more than 4 to 5 times on one particular muscle (3,10).
In a highly practical 1994 study, Bandy and Irion (2) looked at long-term flexibility changes. They found that muscles stretched for 30 seconds a day continued to yield gains in their range of movement for up to six weeks before the ROM reached a plateau. If the daily stretches were held for 15 seconds, it took 10 weeks to achieve the same degree of flexibility change. Stretching for greater ROM is more effective after a jogging warm-up (21,29) – and it is reasonable to suggest this would be true of other modalities of warm-up, too. Stretching is also more effective when heat or ice is applied (13), with heat having much the same effect as a warm-up and ice having an inhibitory effect or decreasing pain so as to enable greater stretch tolerance. Putting all these findings together, then, we can say that long-term isometric stretching programmes should produce changes in less than six weeks if stretches are done daily after a five-minute warm- up, held for 30 seconds and performed four to five times.

ii. PNF is even better

Research comparing the effectivess of PNF stretching versus static stretching produces varied results. However, there seems to be consensus on the superior effectiveness of PNF stretching for increased flexibility, particularly with hamstring muscles.

Sady et al (1982) compared flexibility training using ballistic, static and PNF on shoulder, trunk and hamstring muscles and found PNF the most effective (24). Etnyre and Abraham (1986) studied PNF versus static stretching on one-joint muscles using the plantarflexors of the ankle and reached the same conclusion (9). They also studied the two most common forms of PNF – contract-relax (CR) and contract-relax-antagonist-contract (CRAC) – and found the latter to be more effective. As submaximal contractions during PNF are just as effective as maximal (11), it is advisable to use these, because of the lower risk involved. PNF stretching is equally effective whether contractions are held for three, six or 10 seconds (4).

Stretching to reduce injury risk

The research evidence here is contradictory. It is also even harder to make valid comparisons between studies on this topic, as it splits between acute and chronic injury, as well as between the efficacy of stretching as part of an activity regime versus stretching regimes performed separately from any other workout.

stretching linked to activity

Two literature reviews shed some light on stretching in the context of activity, both pre-exercise and afterwards. In their 2004 review, Thacker et al (27), reported: ‘There is not sufficient evidence to endorse or discontinue routine stretching before or after exercise to prevent injury among competitive or recreational athletes’ and called for more well-conducted randomised controlled trials. In 2002 Herbert and Gabriel (14) examined the hypothesis that ‘stretching before exercise does not seem to confer a practically useful reduction in the risk of injury’ but also found little evidence to support that contention.

A study of military recruits between 1996 and 1998 who practised a series of 18 static stretches before and after training, compared to a control group who performed no stretches, demonstrated a significantly lower rate of muscle-related injuries, but no difference in the rate of bone or joint injuries (1). The way this study differentiates between different types of injury makes it exceptional among the research. There is certainly scope here for further inquiry.

stretching in isolation

In the context of overall stretching programmes, a 2004 survey of flexibility training protocols and hamstring strains in professional football clubs in England conducted by Dadebo et (7) found that ‘hamstring stretching was the most important training factor associated with HSR [hamstring strain rate]’. The most common technique used was static stretching and the authors concluded that HSR went down in inverse relation to the amount of stretching incorporated into training.

But the results of another hamstring flexibility study apparently show the opposite effect. ‘Does the toe- touch test predict hamstring injury in Australian Rules footballers?’ investigated in 1999 how far flexibility was a factor in hamstring strains. The authors (3) concluded that it wasn’t.

Turl and George (1998) reported something similar (28). They assessed rugby players with a history of repeated grade I hamstring strains versus a control group and found no variation in hamstring flexibility between the two groups. They also, however, found adverse neural tension in 57% of the hamstring injury group, against 0% for the controls. It is common to find a confused understanding of the difference between hamstring flexibility and adverse neural tension (restricted movement of a nerve as it passess along its tract) when stretching for increased flexibility is prescribed as part of a hamstring injury rehab programme.

We can summarise the knowledge on stretching to reduce injury risk thus:

  • It is proven that static stretches have a short-term beneficial effect on muscle stiffness and that muscle tone and flexibility can affect the movement of a joint.
  • An overall static stretching regime can help prevent muscle-related injuries, but not necessarily because it promotes increased muscle flexibility.
  • It is not necessary to perform static stretching before or after exercise to prevent injury.

From these conclusions, it is reasonable to extrapolate that a chronic joint injury may benefit from injury-specific static stretches prior to activity; for example, stretching the shoulder’s external rotators before swimming or a throwing activity. On the flip side, there are chronic injuries where excessive flexibility is an underlying cause of pain and in these cases stretching will often be detrimental. The role of stretching in chronic injury management is certainly an area needing more research.

Stretching for injury rehab

The aim of stretching during rehabilitation is to aid extensibility of the healing site and return normal muscle length as early as possible. Malliaropoulos et al (2004) assessed the role of stretching during rehabilitation from grade II hamstring strains and concluded that the group ‘which carried out a more intensive stretching programme, was found to have a statistically significant shorter time of regaining normal ROM and rehabilitation period’(20).

The importance of a controlled progression of stretching during rehabilitation from muscle strain is widely accepted and backed by the research. What is less clear is whether we should stretch injured muscle tissue in the same way as non-injured tissue. We also need more investigation into what role, if any, there is for ballistic stretching during rehabilitation.

Stretching for performance gains

To maximise power, don’t stretch

Plenty of research data is available on this question. Most of it relates to the effect of preparatory static stretching on the performance of activities involving the stretch reflex or maximal voluntary contraction (MVC) – in other words, sports demanding explosive power such as sprint, high jump or basketball.

While the evidence varies, the balance of data finds that static stretching has a negative effect on the subsequent performance of activities involving the stretch reflex. Power et al (23) found that a preparatory bout of static stretching decreased the isometric force output of the quadriceps muscles for the next two hours. This would certainly suggest that for optimum performance in explosive sports, pre- activity stretching is not a good idea.

In a separate 2005 study, Cramer et al (6) also demonstrated a decrease in force production and muscle activation in the rectus femoris and vastus lateralis muscles after static stretching.

But even here, the issue is not clear- cut, because Power et al’s study also recorded no similar adverse effect on the plantarflexors or on overall jump height.

To put this decrease in force output into functional terms, we can refer to a 2004 study that assessed the effect of different static and dynamic stretching protocols on the 20m sprint performance of rugby union players (12).

The static stretching warm-ups resulted in a decrease in performance; whereas dynamic warm-ups improved performance. A dynamic stretching programme features eg, swinging movements of the arms and legs that take activity-specific muscles through a range of movement, imparting a stretch at the end of range which is not held (also known as ‘elastic stretching’).

The authors speculate that the static stretching increases compliance within the muscle-tendon unit, which in turn reduces the unit’s capacity to store elastic energy. The beneficial response to a dynamic warm-up is thought to relate to the rehearsal of specific movement patterns, which may help increase the coordination of subsequent movement.

In a study of the effect of static prep stretching on performance of vertical jump, Knudson et al (16) found no changes in the kinematics of the vertical jump, despite a decrease in vertical velocity in 55% of the subjects. They concluded that it is neuromuscular inhibition rather than reduced muscle stiffness after stretching that is responsible for the changes in performance.

But a 2003 study measuring the effect of static preparatory stretching on concentric isokinetic muscle action of biceps brachii, revealed the opposite (10), with no measurable change in EMG amplitude. The researchers suggested that deficits in force production after stretching were related to muscle stiffness changes rather than neuromuscular control.

So while it is reasonable to conclude that static stretching can have a negative impact on performance of activities involving the stretch reflex or maximal contractions, we cannot confidently say why.

Stretching isn’t always best

What can we conclude about how static stretching influences other aspects of performance? Nelson et al (22) demonstrated a significant reduction in muscle strength endurance after static muscle stretching.

Many sports, such as the tennis serve or golf swing, combine complex movement patterns with a need for maximum force and accuracy. Very little data seems to exist on how stretching might affect these activities – so this is another area ripe for further investigation. The best advice in practical terms is likely to be based on an extrapolation from tests involving simple movement tasks. Beyond this there seems to be just one study, Knudson et al (17), on the performance of a tennis serve after static stretching, which used speed of serve and accuracy as outcome measures. It demonstrated no change in performance.

Stretching to reduce pain

It doesn’t work on DOMS

It is a common belief that recreational athletes suffer with DOMS (delayed muscle onset soreness) because they fail to stretch sufficiently before activity; it is also said that DOMS can be alleviated with subsequent bouts of stretching. In reality DOMS is linked to an athlete’s physical ability to tolerate the eccentric loading of a particular activity and preparatory stretching cannot relieve this soreness (15). Neither stretch, cryotherapy nor electrotherapy has any effect in relieving DOMS (5). DOMS can only be relieved by NSAIDs, exercise and possibly massage.

More generally it is very hard to undertake objective assessment of the benefits of stretching in reducing injury-related pain. If an injury is associated with a restriction in muscle flexibility, then stretching is a viable treatment option. However, if muscle inflexibility is not considered an influencing factor, for example in an instability problem, then clinical reasoning should rule out stretching as a treatment tool. Thus the use of stretch is going to be determined by the specific individual’s injury profile.

In summary

Sports therpists lack a harmonised ‘best practice’ approach to stretch based on the available research. Here’s what we know:

  • Static stretches with the aim of achieving short-term range-of-movement gains should be held for 15 to 30 seconds and for maximum effect should be repeated four to five times. These short term changes will last between 60 and 120 minutes.
  • For long-term changes, stretches should be held for 30 seconds and also repeated four to five times. Long-term results should take six to seven weeks at most before the changes start to plateau.
  • PNF can achieve range of movement changes, and is often more affective then static stretching, but is also more difficult for athletes to perform on their own.
  • It is reasonable to assume that PNF techniques will achieve the same results as those produced by the various research studies into static stretching.
  • Stretching is a vital part of recovery from muscle strains and a progressive stretching programme can decrease rehabilitation time.
  • Preparatory stretching is not necessary to prevent injury. However, from practical experience, it is the author’s belief that specific static prep stretches can be beneficial in the management of chronic injuries.
  • A regular stretching routine as part of an overall training programme can help prevent injuries and this should be individualised and sport specific.
  • Static stretching should not be included in warm-ups, as it seems to decrease performance of explosive actions and activities demanding muscle strength endurance.

We are still some way from having a clear picture of the pros and cons of stretching. However, there is still solid evidence from which we can draw conclusions and base our management and training programmes. In many cases, the research challenges traditional approaches, which underlines the importance of sports practitioners keeping up to date with the science, so they can adjust their practice and programming in line with the best available evidence.

Sean Fyfe is a physiotherapist, tennis coach and director of TFP (Tennis Fitness Physio), a Queenslandbased company specialising in sports medicine, elite tennis player development, strength and conditioning and childhood motor learning programmes

Stretch: the research - References

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