Traditionally the concept of stretching before vigorous physical activity has been endorsed by health professionals in order to increase joint range of motion (ROM), thus reducing the risk of sports related injuries. However, current clinical scientific evidence testing this assumption is limited and at best clouded by misleading and conflicting research. As such, the purpose of this critical review is to examine whether pre-exercise stretching as part of a warm up prior to vigorous physical activity reduces the risk of sports related injuries. This critical review included randomised control trials that were written in the English language, which examined the effects of stretching as a tool for reducing the risk of injury. These studies were only reviewed if the subjects being examined were human and over the age of 17. Studies were identified by searching the EBSCO, CINAHL (Cumulative Index to Nursing, and Allied Health Literature), SPORTdiscus, Pubmed, Science Direct and Scopus data bases between January 1980 to November 2014. Key words searched in the databases where ‘warm up’, ‘stretching’, ‘trial’, ‘randomised’, “prevention of injury’, ‘incidence of injury’.
Two checklists which were chosen to support our quality analysis of randomised control trials in order to identify the risk of bias. These checklists were then further modified to make our criteria more specific and sensitive. If the criteria of our modified check list was satisfied, a high score would indicate a more robust qualify of evidence. Post this exclusion criteria six articles were critically reviewed.
The results of these articles found that subjects with a dorsiflexion range of 34 degrees, were 2.5 times more likely to be injured compared with those of average flexibility. However, the overall results of this study found no statistical significant effect on risk of injury and this was attributed to low statistical power. A further second study conducted by the same authors concluded no significant effect of stretching on all lower limb injuries, despite good statistical power. However, incidental findings showed subjects who were aerobically less fit and agile at running were fourteen times more likely to sustain lower limb injuries compared with fitter subjects. Conversely short durations of stretching was found to significantly increase passive ROM, passive extensibility and maximum passive muscle moment of the hamstrings. This study attributed these findings to an increase in stretch tolerance, while the course of passive muscle stiffness curve did not change. In contrast, test results from another study showed statistical significant difference in lower extremity overuse injuries, with a P value =0.002 and a P value =0.001 in hamstring flexibility. Both of which are significantly lower than a P value of 0.005 threshold certainty. Whereas as an internet based study found that there primary outcome of stretching did not clinically or statistically have a significant reduction in risk of all injuries. However the results of their secondary outcome did produce statistically significant reductions in the risk of muscle, ligament and tendon injuries, which should be warranted.
While most studies examined the effects of stretching and whether it would be a useful tool in reducing the risk of injury. Another study examined the efficacy of a prophylactic program that used stretching with other variables and reported a 75% reduction of injuries in their intervention group. The only flaw with this study was that they had no way to quantify and determine which variable within the program had a true and total effect. Inferences have been made that a decrease in muscle or muscle tendon unit extensibility (stiffness) would reduce flexibility, thus limiting joint ROM. This is believed to a strong predictor contributing to the risk of injury, and for this reason stretching is prescribed. However this is in contrast to the physiological concept that a stiffer muscle would provide more efficient transmission of contractile force production. Thus making runners for instance more economical with their running. Whereas a more compliant muscle tendon unit would need more contractile force to transmit to a joint compared with that of a stiffer joint. This contradicts the idea and aim of stretching which is to increase muscle tendon unit compliance. Perhaps the casual relationship found by one of these studies, which used the 20-metre shuttle, run test would correlate with this concept. Nonetheless, further investigations are needed to determine whether this outcome measure would work as a strong predictor in risk of injury.
Unfortunately the use of outcome measures with most of these studies were not specific and sensitive enough to test these existing theoretical assumptions. For example those who examined torque and angle curves, load and elongation behaviour and passive stiffness of stretched muscle groups immediately after stretching. Reported incorrect definitions of stiffness, failed to calculate the slope in the torque and angle curve and were unable to accurately measure muscle group and joint stiffness. The only conclusion that can be made here is that there is no clear evidence to suggest that stretching reduces the risk of injury by acute decreases in muscle stiffness.
Five papers which were critically reviewed, two of those studies indicated that 20-30 seconds of stretching did not produce a significant difference in reducing risk of injury. While the remaining three studies showed that stretching does increase joint ROM significantly, they attributed this to an increase in stretch tolerance. While one of three studies further indicated that stretching can be useful in conjunction with a holistic exercise program. Many flaws within the methodology subsequently impacted the results of these studies which has made it difficult to conclusively state what the effects of stretching are on risk of injury. These studies were unable to answer the question they were seeking to answer and as result the validity of these studies could be questionable. It must be noted that stretching as a method of injury reduction is very dependent upon the type and mechanics of sporting activity a person undertakes. As a recommendation more sensitive outcome measures should be used to test these theoretical assumptions. In addition changes in sensation modification attributed to increase in stretch tolerance does not directly decrease risk of injury. Stretch tolerance alone cannot treat muscle impairments resulting from a loss of extensibility. If the increase in extensibility is perceived as an increase in stretch tolerance, the most appropriate question would be to consider whether the effects of stretching is due to a change in sensation modification, can this reduce the risk of injury and if so how?
Lastly, if our ultimate goal as clinicians is to improve movement and function, short-term intermittent stretching is not enough to create structural changes in muscle length. Especially if we are seeking to cause plastic deformation with patients who have muscle shortening and contractures. In addition, active stretches like proprioceptive neuromuscular facilitation and nerve glides could also be used as first line agents in treating injuries and restoring tissue function
By Sevda Onder (Fitness/Physio Geek)