Stretching

From Biomch-W

The scope of this document is for performance and injury prevention.  It does not currently consider treatment of, or rehabilitation from, injury.  The purpose of this document is to provide a continually developed resource reviewing the latest knowledge on the subject area.


Contents

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Overview

Stretching has been widely promoted for:

  • Short term increase of range of motion about joints (ROM)
  • Short term increase in muscle performance
  • Short term injury prevention
  • Short term relief of cramp and delayed onset muscle soreness (DOMS)
  • Long term increase of ROM
  • Long term increase in muscle performance
  • Long term injury prevention

This review attempts to assess the data in this area, to see if stretching can achieve these aims and to highlight any negative effects associated with its use.

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Muscle failure

Injury to the muscle-tendon unit (MTU) is in the form of mechanical failure. When a muscle is stretched it creates a passive resistance, just like a spring. Up to a point called the elastic limit the MTU will return to its original length when the load is released. Beyond the elastic limit the muscle will begin to plastically deform and will no longer return to its original length. After further stretching the MTU will rupture (1). Injuries usually occur at the junction of muscle and tendon (3,4,30).

The MTU is more complicated than a spring model, however, because it is viscoelastic whereas a spring is simply elastic. Viscoelasticity means the MTU has a higher resistance to faster stretches. The elastic limit is determined by the amount of strain energy that the MTU can absorb in the stretch. Higher resistances cause higher strain energies (energy = force x distance) and so a fast moving stretch will fail at a shorter length than a slow stretch. See figure 1

Image:StretchingFig1.jpg Figure 1: MTU failures. Slow versus fast stretching


Figure 1 shows failures based on the passive resistance of muscles. Active resistance produced by the contractile elements of muscle increases the amount of strain energy that can be absorbed by the muscle without effecting the elastic limit (30), see fig. 2. As such, the active components can work as an independent breaking mechanism protecting the muscle from stretching to the failure point.

Image:StretchingFig2.jpg Figure 2: MTU failures. Active versus passive stretching


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Stretch definition

The passive elements of muscle/tendon units in situ will be under tension throughout the angle range of their attached joints.  From this respect muscles are always stretched with respect to their unloaded length.  However, what most people regard as stretching of muscles involves increasing their length to the upper range, with respect to the range of motion of the attached joint(s).  However, it should be noted we have not identified a formal definition in the literature.  In future it may also be useful to use 'conscious perception of stretch sensor feedback' as a formal requirement for stretching in research unless specific angles/forces are specified.

Stretching can be done passively (e.g. using support or weight of body segments) or actively (using activation and consequent shortening of contralateral muscles).  Duration of stretch and speed of going from neutral length to stretched length are variable depending on protocol.  During static stretching, the muscle is lengthened slowly to the target position, held for a predetermined amount of time and gently released.  During dynamic/ballistic stretching the length is changed rapidly with no hold time.  Anecdotally we have found that practitioners differentiate between dynamic (moderate pace) and ballistic ('dangerous' pace).  However, in looking at research papers these terms are often used interchangeably.

Stretches can be repeated, again determined by protocol.

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Short term stretching (preparation for performance)

The criteria for using short term stretching are:

  • It reduces the passive stress at extreme strain leading to:
  • reduction in injury rate and/or improvement in performance.
  • Improvements are not outweighed by negative effects.

If muscle acted like a simple spring, there would be no way for stretching to reduce the stress (force/area) for a given strain (length/original length) whilst remaining in the elastic zone (i.e. before failure).  However, muscle is more complicated and demonstrates viscoelastic properties (8,11,35).  This means the muscles have a flow property that resists changes in length.  The faster the change the more resistant it is. Therefore change of length will initially produce a higher passive resistance which will then reduce to a base level related to the strain.  This is sometimes called stress relaxation.

The viscoelastic properties, due to the complicated non homogenous nature of muscle, are probably quite complicated. Viscoelasticity introduces a historical component to the behaviour of muscles.  A stretch that is held produces stress relaxation and this means a subsequent release and stretch encounter less resistance for a given strain.

It appears then that the first criteria for using stretching pre-performance may be met although more data is needed for how the muscle reacts during performance - it is a possibility that the stretch gains are undermined when the muscle contracts often.  With regards to reduction of injury rate and improvement of performance supporting data is lacking.  Two papers summarised in (22) both looked at injury rates in endurance exercise/training in stretching and non-stretching groups.  There was no statistically significant difference between the two regimes.  [I have seen reference to a study on marathon runners that links stretching to increased injury rates in Caucasian males but I have been unable to source the paper.  I think its by a Dr Lally.  Can anyone help?]

A major factor in the research is that stretching causes a decrease in force generating capacity (e.g. MVC, maximum electrically induced or stretch shortening cycle) (12,14,15,16,25,31,33,34,39 but not in 8).  The effect can initially be as large as 28% with effects lasting in excess of 60 minutes (12).  This will clearly be a negative influence on performance for those sports that require high force levels.  Also, it will be interesting to see research carried out examining injury risk associated with stretching in high force sports.  Although there appears no link to injury rates in endurance sports, high force sports (e.g. sprinting) may be negatively affected with the peak force reduction.  Biomechanical errors may increase the injury rate.  As yet this is a hypothesis and needs to be tested.

Another possible problem is that long held stretching may cause plastic deformation (31,33,37).  It was suggested in (37) that plastic deformation of supporting collagen fibres may be a protective mechanism against high loads.  The benefit would presumably that it is easier to repair or reverse the plastic deformation that it is to repair a tare.  However, ideally the muscle should not be exposed to plastic deformation.

Reasons for maximum force reduction may include: unfavourable shift in torque/fascicle length curve (12), neurological drive reduction (see discussion in 31 for insignificant reduction versus significant in 32 with large strains, also 31,33 for changes to proprioceptors due to mechanical change) and plastic deformation (see discussion in 31).  The torque/fascicle relationship may be explained by the results in (17)

Note that interestingly (5,35,36) showed that repeated contractions produced a similar stress relaxation to passive stretching.  This raises the question as to whether improved viscoelastic states can be achieved through warm up instead of stretching.

Another potential problem with both short and long term stress reduction in the tendon is that it would reduce the rate at which force generated by the muscle can be transferred to skeleton.  This latency may be costly for fast movement performance.  There is a question mark over whether acute stretching affects dynamic movement at all which is of prime relevance to movement sports such as Badminton (18).

Another safety concern is over ballistic movements that, due to their nature, have an associated level of risk (11). 

A property of muscles that increases resistance is thixotropy. Thixotropy is a property of some gels, such as the MTU, where stiffness is reduced as the material is mixed up. It is thought (1) that stable bonds form in muscles when they are inactive. Until these bonds are broken there will be an increase in resistance. The way to break these bonds is through movement. This will automatically happen in a sports specific warm-up routine.

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Long term adaptations

The same criteria can be used for justification (or not) of a stretching protocol over the long term. For long term changes it would be highly unlikely that viscoelasticity could provide changes, but maybe biological adaptations could.

In (23) significant changes were achieved in range of motion. The paper concludes the changes were due to decreased stiffness and increased strength in extreme positions.  No reductions in MVC or rate of force development were found.  It is surprising that no reduction in rate of force development was found as this should be an inevitable consequence if the tendon adapted.  Perhaps adaptation was only present in the muscle.  Another possibility was that changes were due to plastic deformation (as previously discussed).  If this were the case then it is possible that the muscles would be at greater risk of injury.  (24, 38, and 39) also found increases in flexibility following chronic stretching training.  Further research needs to be conducted to see if these results can be confirmed and to note the adaptations that occur at the mechanical level.

One study (20) associated stiffness with superior concentric and isometric performance (eccentric was unaffected).

Conversely, for performance in stretch shortening cycles (SSCs): (6,7,9) found that increased compliance was associated with improved performance in counter movement jump (6,9) and sprint (7), these being examples of SSCs.

Range of motion is often thought to be desirable. However, this is a very sport specific conjecture and there has not been any research conducted in Badminton to identify where an increased ROM may be useful. It is reasonable to conclude that for Badminton the notion of ‘reach’ is most applicable here. The only action identified by the author that could be limited by ROM is the lunge. Stride length is probably limited more by optimisation of force moments by the nervous system than ROM. Arm reach is limited by arm length rather than joint ROM.

Also some of the perceived gain in ROM following a stretch could actually be due to stretch tolerance (10), presumably from desensitising of proprioceptors. This brings question marks of the theoretical effectiveness of stretching to alter ROM and also raises safe concerns (19). Could stretch tolerance inhibit stretch reflex and allow damage to occur to antagonist MTUs and joint structures?

There has been research associating different muscle tightness levels with varying injury rates and this may provide a use for chronic stretching. According to a comprehensive review (21) there are varying profiles for injury rate against tightness for different muscles in the body. Sometimes there was a statistically significant U shape to the graph with too tight or too loose relating to increased injury rates. It may be found that chronic stretching of the ‘too tight’ muscles may lead to fewer injuries but this needs to be supported by research, including establishment of ideal tightness levels. Care should also be taken that increases in flexibility do not result in performance decreases such as strength.

The hypothesis that more flexible individuals would suffer less from short term performance reductions following strethcing, was tested in (39).  They failed to find a correlation between flexibility and deficits, or any benefit from long term (3 weeks) flexibility training (via stretching) and acute stretching induced performance loss.


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Possible other areas of injury prevention

  • Strength gain: as strain is inversely proportional to cross-sectional area, any training regime that causes muscle hypertrophy should theoretically reduce injury rates. Also, strength is by definition an increase in active resistance so again should reduce injury rates. However, literature searches have failed to find supporting data for (or against) this. This lack of evidence was also noted in (21). However (21) does find evidence linking left/right and agonist/antagonist strength imbalances to higher injury rates but the same review could not establish support for corrective training regimes resulting in injury improvements
  • Warm-up: although there is conflicting evidence over the effects of warm ups (13) there is support for an increase in the length of failure (ROM) (2,3,4). This should allow more time for active resistance to absorb the strain energy. Warming up of a muscle is done by activating it so warm-ups must be sport specific
  • Activity specific endurance training: reduction in fatigue will benefit active resistance and biomechanical safety. This document currently does not investigate this area of injury prevention
  • Action technique: technique (e.g. shot technique) may have an impact on biomechanical safety. This must be a consideration for action design but is not covered by this document. It would be interesting to see if there is a trade-off between safety and performance.
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Cramp and DOMS

Cramp is the last element to consider. It is clear to everyone who experiences cramp that stretching causes immediate relief. However, we do not have research that cramp is prevented through stretching.

Relief from or prevention of DOMS by any form or regime of stretching has repeatedly not been found (22, 26, 27, 28, 29).


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Conclusions

Stretching is a surprisingly complex subject.  However, it is clear now that stretching as an acute preparation for performance is counter productive.

Chronic stretching may offer some potential benefits in the future.  There may be a trade off between basic contraction performance and SSC performance.  However, a lot more research is needed in this area.  Also, the safety of long term stretching needs to be established.  Also, more research should be conducted into the effects of stretching on resistance versus stretch speed.  Most research to this date has looked at static force/length testing.

Risk of injury in general has not been demonstrated to be affected by stretching either way. However, studies need to be carried out to understand the effect of risk in high power sports like Badminton. Stretching may reduce injury when targeted to abnormally stiff muscles or unbalanced left/right muscle pairs.

Until more research is done, there is too little data to specify a long term stretching regime.  For researchers, however, there are interesting avenues that need to be explored.

For coaches and players we advise that, instead of stretching, acute preparation should consist of a sport specific warm that activates all the muscles that will be used in the performance.  Also, we would advise against long term stretching regimes until their efficacy, safety and perhaps even sport specific requirements are established.


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Avenues for research

  • In depth analysis of mechanical changes due to long term stretching
  • Analysis of changes to resistance versus stretch speed
  • More data for long term affect on SSCs and MVC
  • Real performance task changes over long term (firstly classics like counter movement jump, then a range of sport skills to understand interplay of SSCs, MVCs, etc...)


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References

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