Effect of Acute Static Stretch on Maximal Muscle Performance: A Systematic Review.

Kay AD, Blazevich AJ. Med Sci Sports Exerc. 2011 Jun 8. [Epub ahead of print]
It is well documented that a warm-up, when preparing for a physical activity or athletic performance, can influence physical performance and risk of injury.  However, pre-exercise routines can include various components (e.g., cardiovascular work, stretching) and can be difficult to figure out which aspects of the warm-up contribute to the benefits. A growing body of literature has indicated that static stretching, a common component of a warm-up routine, may have detrimental effects on muscular performance.  Kay et al performed a comprehensive systematic review of literature to assess the acute effects of static stretching on strength-, power-, and speed-dependent tasks.  The authors also stratified the data according to stretch duration (< 30 s, 30-45 s, 1-2 min, > 2 min).  Out of an initial 4559 potential articles 106 articles were included for review.  55% of the articles reported a significant reduction in strength, power, or speed related tasks while 69% reported no significant reductions.  The conflict in percentages was due to certain studies producing both significant and nonsignificant results concurrently (the authors found 149 findings from the 106 articles).  Closer analysis of the data revealed differences based on the duration of stretch.  When stretch duration was 60 s or longer, significant reductions in muscle performance were reported.  In contrast, a stretch duration of < 45 sec does not result in meaningful reductions in muscle performance.  These findings were consistent regardless of performance task, type of contraction (e.g., isometric, eccentric), or muscle group. However, very few studies assessed changes in maximal eccentric strength and all of these studies used stretch durations >60s. 
This systematic review is interesting because it focused on the acute effects of static stretching and explores potential influential factors (e.g., duration of stretch, contraction type). The lack of data regarding eccentric contractions is interesting and concerning since there is a considerable amount of muscle strain during eccentric loading and these contractions are common in sports. It would be interesting to look at changes in eccentric strength with stretch durations  less than 60s.  Some limitations of the review were a lack of control groups in some studies and possible under-reporting of studies with nonsignificant results. That aside, short duration stretching tends not to result in significant muscular impairments while longer duration stretches do impair performance.  Fortunately, in warm-ups, individuals usually use stretches with shorter durations but longer duration stretches are sometimes used in the clinical setting.  In a clinical setting this review could have implications in terms of the order of activity.  Typically I have patients warm up and stretch prior to exercise and I keep the stretch time to 30 seconds.  Then at the conclusion of the program if joint/muscular stiffness are issues I will address that with low load longer duration stretches.  This pattern of stretching exercises places the long duration stretches at the end of the program so that it cannot negatively impact other exercises. It would be interesting to hear how other clinicians utilize total stretch duration and time with their athletes/patients. This literature review definitely provides some thought provoking data regarding the benefit of static stretching (e.g., increasing range of motion, decreasing musculotendinous stiffness) in relation to injury prevention versus the possible negative effects on muscular performance.  
Written by: Thomas Martin
Reviewed by: Jeffrey Driban

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