As a tennis player, I used to spend hours stretching weekly as I, my peers and coaches believed it would improve my performance and reduce the risk of injuries. Stretching is commonly used in multiple contexts whether before other exercise or sport in the hope of increasing performance and reducing risk of injury or after exercise with the hope of reducing exercise-related soreness. A lot of people including myself report stretching feeling really good and relaxing. Commonly, it is used to relieve a sore or painful muscle or joint. As a physiotherapist I was taught to emphasize stretching as the best option to improve someone’s painful restricted movements. “Especially as we get older and stiffer.” But what does stretching actually achieve? Are there more helpful options that offer similar or additive benefits? 

Stretching is often perceived as requisite for flexibility and flexibility is often perceived as important for health and performance. So, I wish this article by Nuzzo (2020) had been read more widely and written earlier in my development as an athlete, coach, exercise lover and health professional. The main points I’ll outline encompass the science of the last 80 years within the review that should de-emphasis the importance of stretching and flexibility and will hopefully reduce any concern that you need to complete more stretching or be more flexible. The paper also highlights how strength, cardiovascular fitness and body composition are better for predicting most health qualities. In other words, muscle mass, strength and cardiovascular fitness are better training goals for health and performance. 

Before we continue, I would like to emphasise that the evidence summarised below might be interpreted as negative. This is not my intent. I do not want to give more barriers to completing exercise as the population is sedentary enough. I am happy if you enjoy stretching or found it as an active solution to a problem you had. However, the current scientific consensus of the last 80 years strongly supports that your precious time should be spent on resistance and aerobic training which have more robust health benefits.

Flexibility is the intrinsic properties of body tissues that determine maximal joint range of motion and is determined by “stretch tolerance” and “muscle-tendon unit mechanical and viscoelastic properties.”

Health effects of flexibility:

• Flexibility does not predict age of mortality. Body composition, cardiovascular endurance, muscle strength and power are better predictors of age of mortality. 
• Flexibility does not predict falls or ability to perform day to day activities in older adults unlike muscle strength. In older adults, flexibility predicts gait speed (a predictor of disability) equally to muscle strength of the thigh and calf muscles. Lower flexibility might be associated with decreased weekly physical activity. This is interesting as people become more sedentary as they get older and might predict why people feel they are becoming less flexible. Chicken or the egg type question. I’ll address this in future posts as there is evidence of “muscle-tendon unit mechanical and viscoelastic properties” changing with age.
• Flexibility predicts quality of life similarly to muscle strength. In some studies where flexibility did not relate to quality of life, other fitness measures did. 
• When related to back or hamstring flexibility, flexibility does not predict future incidence of back pain in adults or industrial workers.
• Flexibility does not predict blood pressure, resting heart rate or positive blood cholesterol profile. Body composition and cardiovascular fitness correlate more strongly with these cardiovascular outcomes. However, increased flexibility may predict reduced arterial stiffness, which would positively influence blood pressures long term.
• Flexibility does not predict lung function. 
• Flexibility does not consistently predict development of metabolic syndrome.

Sport/Performance effects of flexibility:

• There are multiple studies showing no relationship between flexibility and hamstring injuries in soccer and AFL players or adolescents. Some studies on people with hypermobility, meaning increased lower limb flexibility, show increased risk of injury.
• Flexibility has mixed effects, some positive while some negative, of predicting injury in military cohorts. On the contrary, there is “strong evidence” that poor cardiovascular fitness and muscle weakness predicts risk of injury in the military and non-military population. Body mass index, a measure of body composition, is also a better predictor of injury risk in multiple types of sports. However, the paper highlights to be careful seeking only one measure to assist prediction of injury as injury risk is multifactorial.
• Flexibility does not predict performance in simulated military, firefighting or police scenarios. The statement was made in the review that “body composition, cardiovascular endurance, muscle endurance, muscle strength, and agility are all more important than flexibility for physically demanding work.”
• Flexibility does not consistently correlate positively with other training effects such as body composition, cardiovascular endurance or muscle strength. It was stated that “absence of correlations between flexibility and other fitness components indicates flexibility is a distinct trait, but not one particularly important for health and function”.
• Flexibility does not predict sports performance better than strength and power in some sports. Those participating in American football are less flexible than the non-athletic population and flexibility within this sport does not predict starters vs non-starters (those who start the game on the bench vs on the field). Flexibility is comparable between elite and sub elite athletes in sprinting, handball, climbing and taekwondo. Flexibility did not predict time trial performance in kayaking. Flexibility negatively correlated to faster and more economical runners in some studies.
• Certain cohorts like swimming, gymnastic and ice hockey goalies tend to be more flexible than other sports cohorts and, in these cohorts, might predict the level of competition. However, in these cohorts, so does strength and power.
• Again, the review quotes, “body composition, cardiovascular endurance, muscle endurance, and muscle strength were more strongly correlated with athletic performance”. 

I have not given the citation for each finding to improve ease of reading. It’s a free paper if you wish to find links to the 313 citations within the study. There was an element of cherry picking of studies throughout this review. In that respect it differs from a Cochrane review.  However, given how many papers were identified that support the authors precepts, the review highlights the need to be sceptical about the importance of being flexible as a training goal to improve health, performance or reduced injury risk. 

The Nuzzo (2020) review also highlights the usefulness of (or lack of) static stretching or prolonged fixed lengthening of muscles or joints through the available range of motion. The commonly reported mechanisms of improved flexibility with static stretching include increased “stretch tolerance” and “muscle-tendon unit mechanical and viscoelastic properties.” Dynamic stretching or repeated active lengthening of a joint was not discussed in this review despite its effects being measured commonly by a static flexibility test.

Health effects of stretching:

• A quote from a meta-analysis was “there is conflicting information regarding both the relationship between flexibility interventions and functional outcomes or daily functioning in older adults.” It was quoted that “in contrast to stretch training, aerobic and resistance training are known to positively impact a range of health outcomes.”
• Stretching “does not produce clinically important reductions in delayed-onset muscle soreness in healthy adults.”
• Stretching does not improve muscle contractures, movement-limited neck pain disorders and recovery after an ankle fracture. 
• Stretching is inferior to aerobic and resistance training for improving pain and physical function in individuals living with fibromyalgia. 
• Stretching can induces blood vessel growth, reduces disuse-induced muscle atrophy, increases metabolic turnover, and improves markers of cardiovascular function. This is not most people’s rationale for encouraging stretching. This is also a function of any activity that lengthens muscles and tendons such as…….. wait for it…. aerobic and strengthening exercise which also induce these cardiovascular benefits.
• A reply article by Nuzzo (2020) in response to criticism quotes “The United States does not have a nationwide problem in musculotendinous stiffness. It has a problem with obesity. Stretching will do little to resolve this issue. Americans should prioritize cardiovascular endurance and muscle strength. They should prioritize physical exertion—aerobic exercise, muscle-strengthening activities, sports participation, etc. Functionally relevant levels of flexibility will be achieved or maintained along the way”.

Sport/Performance effects of stretching:

• Resistance training involving repeated actions of eccentrically lengthening and concentrically shortening of the muscle through the full available range of motion improves flexibility equally to stretching.
• In agreement with the relationship between injuries and flexibility in military and athletic cohorts, stretching does not prevent injuries in military training, running and most studies in sports. This evidence has not changed since 1999 despite popular cultural opinions and further study. The author’s opinion was that in current professional sport “coaches are already weary of flexibility assessments” and priorities assessing muscle strength, running speed, muscle endurance, and body composition. This fits with my experience working with coaches in Australian football, tennis and athletics. 
• Aerobic exercise for 10–24 weeks increases lower back and hamstring flexibility. No comparison to static stretching was given although static stretching and aerobic exercise are combined, short-term flexibility can be improved (Takeuchi et al., 2020).
• Some studies show stretching reduces strength if stretches are held for greater than 60 seconds.
• A study reviewed highlighted that stretching can increase strength when “participants completed 2 h of stretching per week for 10 weeks.” I think we can deduce what the effect of 2 h of resistance training extra per week would have on strength instead of completing stretching.

Closing remarks from the Nuzzo (2020) paper indicated that stretching should not be removed but “de-emphasized” from all exercise prescriptions as only the “middle range of flexibility levels” might be desirable for health and function and stretching is not the only exercise modality that can achieve this. This is just like Goldilocks principles around recommendations for optimal body fat levels. Globally, national health institutes recommend removing stretching from suggested training interventions and not assessing flexibility due to its unimpressive track record and “lack of evidence for an association between flexibility tests and health outcomes” (Pillsbury et al., 2013). These same institutes have questioned why stretching has been popularly encouraged for decades and “on what evidence were such original recommendations made”. The answer according to Nuzzo (2020) is “very little”.

We should consider stretching’s limited unique benefits, time allowance, and personal preference when considering weekly stretching vs more aerobic and strength exercises. Studies supporting the use of stretching reported benefits only after 20-60 min protocols completed multiple times a week! That’s way more time than most people spend stretching. Millions of people globally, nonetheless, are probably encouraged to be flexible and stretch each week. Some people spend time at the start or end of a workout stretching. However, most people report time constraints to being consistently physically active. We should de-emphasis telling people who report time barriers to exercise to stretch and relocate their time toward more aerobic or resistance exercise. Additionally, as access to equipment is a commonly reported barrier to exercise, the Nuzzo (2020) review points out that just like stretching, “other forms of exercise, such as body weight squats, vertical jumps, push-ups, and sit-ups, can also be performed without equipment.”

“But Tom, I completed x,y,z stretch and it improve my x,y,z pain.” I have no doubt that stretching helped you. I congratulate you for finding a solution to a problem you have had. Stretching does feel good. I believe this feeling is influenced by the perceived benefits you feel while completing stretching. We know exercise can have a placebo effect. There does not seem to any strong negative effects of stretching so I don’t think it is harmful. If you are meeting the Australian physical activity guidelines (which don’t include stretching) or sports performance training requirements, add in stretching. The concern is that it commonly replaces or is completed instead of other nationally recommended forms of exercise. According to the evidence, stretching compared to light resistance exercise completed through full range of motion or aerobic exercise, isn’t more helpful for most health, injury, and performance outcomes. If we recognise that stretching is just a low intensity form of resistance training, putting a low amount of tension on joints and muscles for prolonged periods, we should encourage stretching in situations where low intensity forms of resistance training are appropriate.

For “warm ups”, instead of static stretching prior to exercise, I complete movements of reduced intensity of the intended task, like starting your run at a lower pace or speed for 5-10 minutes or completing the sports skills needed at a lighter intensity or doing warm up sets of strength exercises in a progressive fashion. This dynamic “warm up” of preference might reduce your risk of injury by improving your psychological readiness to exercise, the potential common denominator of “effective warm-up programs” for improving performance and reducing injury (Podlog et al., 2022).

Stretching continues to be studied and emerging evidence for its benefit will be highlighted at ACE in the future. We love having our beliefs challenged and changed when strong evidence becomes available. Flexibility and stretching should be the lowest on most individuals’ fitness hierarchy so for the moment, be sceptical of strong opinions about the benefits of being “flexible” or encouragement to choose stretching over other forms of exercise. 

Written by Tom Murphey, DPT.

“Unfortunately for our community, scientific scaremongering is common, easy to believe and hard to heal. Research is often messy, and strong stances or beliefs can be both erroneous and dishonest. I aim to produce honest reviews of some high-quality research to provide informed insight so you can make up your own mind on the science you value.”

References:

Nuzzo, J. L. (2020). The case for retiring flexibility as a major component of physical fitness. Sports Medicine, 50(5), 853-870.

Nuzzo, J. L. (2020). Reply to Kruse: Comment on:“The Case for Retiring Flexibility as a Major Component of Physical Fitness”. Sports Medicine, 50(7), 1409-1411.

Pillsbury, L., Oria, M., & Pate, R. (Eds.). (2013). Fitness measures and health outcomes in youth.

Podlog, L., Wadey, R., Caron, J., Fraser, J. J., Ivarsson, A., Heil, J., … & Casucci, T. (2022). Psychological readiness to return to sport following injury: a state-of-the-art review. International Review of Sport and Exercise Psychology, 1-20.

Takeuchi, K., & Nakamura, M. (2020). Influence of aerobic exercise after static stretching on flexibility and strength in plantar flexor muscles. Frontiers in Physiology, 1604.

https://www.youtube.com/watch?v=YyS_phAhPpY&ab_channel=ClinicalAthlete – podcast interview of the author Dr. Jim Nuzzo.