STRETCHING
For decades stretching has been promoted by team sport coaches, physical therapists and personal trainers as a way to prevent injury, decrease muscle soreness, increase performance, strength and muscle gains. It’s not.
Stretching Myths
- Stretching increases flexibility
- Stretching prevents injuries
- Stretching reduces delayed onset muscle soreness (DOMS)
- Stretching improves performance
Does Stretching Lengthen The Muscles?
The reviewers of this study observed that most theories surrounding increased flexibility after stretching tend to involve a mechanism that supposes an actual mechanical increase in the length of the involved muscles. They also noted the more recent suggestion that this increase might occur due to an alteration in sensation, providing a summary of all the different theories while providing evidence in each case.
The 4 theories about the mechanical properties of the altered muscle tissue due to stretching are: viscoelastic deformation, plastic deformation, increased sarcomeres in series and neuromuscular relaxation.
- Viscoelastic deformation: Viscoelasticity is the property of materials that exhibit both viscous and elastic characteristics when undergoing deformation. There has been a proposition by some researchers that the viscoelastic properties of the muscles are responsible for the ability of stretching to increase flexibility. Truth be told, stretched muscle appears to lose its resistance to stretch after being held in a stretched position after a period of time. The reviewers however, observed that such effects are certainly transient in both human and animal studies and that for that reason they cannot account for the long term effects of stretching.
- Plastic deformation: Plastic refers to the type of deformation that is irreversible. Even though elastic materials behave plastically when extended past their elastic limit, after reviewing the studies performed in this area, the reviewers found that the results did not support a plastic deformation but rather a viscoelastic deformation, which is temporary and not permanent.
- Increased sarcomeres in series: Eccentric training is able to change the optimum length of a muscle which is indicative of an increase in the number of sarcomeres in series, as the length-tension relationship of each sarcomere impacts on the length-tension relationship of the muscle as a whole. While animal studies have found changes in the number of sarcomeres in series after placing a limb in a cast at maximum ROM, the same effects have not been found in human studies when investigating the effects of stretching protocols.
- Neuromuscular relaxation: the reviewers noted that some researchers have proposed that static stretches cause an adaptation to the stretch reflex, which enhances the ability of the muscle to relax and not contract. However, long term studies have found no change in passive torque curves as a result of stretching, which indicates that alterations in the stretch reflex appear unlikely to be responsible for the changes in flexibility.
Sensation Theory
This length increase is transient, its magnitude and duration being dependent upon the duration and type of stretching applied. Most of these studies suggest that increases in muscle extensibility observed after a single stretching session and after short-term (3-8 week) stretching programs are due to modified sensation.
The reviewers noted that studies that have been performed to assess the exact mechanism by which muscle length is increased following stretching protocols have found that the only variable that changes following stretching programs in tandem with flexibility is the sensation of pain (i.e. maximum pain and onset of pain) during the stretch. In summary, the conclusion of the above review is that changes in the mechanical properties of muscle do not appear to be responsible for the changes in flexibility that are observed following stretching protocols.
They also concluded that stretching increases flexibility (joint ROM) by altering the sensation of stretching, such that the onset of pain occurs at greater joint angles. Furthermore they concluded that stretching does not change the actual mechanical properties of muscle or the point at which the stretch reflex is activated.
Stretching essentially induces neural adaptations causing an improved stretch/pain tolerance. The more permissive the nervous system, the greater the ROM it allows the muscle-tendon structures to reach.
Another interesting neural effect of stretching on flexibility is that stretching the lower body increases the range of motion of the upper body and vice versa. That means that stretching increases your body’s stretch tolerance systematically. However, stretching doesn’t generally increase your mobility –the ability to produce injury-free movement– during functional tasks.
On the other hand, strength training with heavy eccentric muscle contractions over a large range of motion increases muscle fascicle length considerably. That’s one of the reasons why movements that include the weighted stretch such as farmers walks, weighted pullups and heavy rack pulls are so good at building muscle.
Does Stretching Increase Flexibility/Joint Range of Motion (ROM)?
This review assessed whether stretching protocols are able to generate lasting increases in flexibility across several joints.
A review on hamstring flexibility that incorporated 28 studies and 1.338 healthy subjects concluded that hamstring stretching does increase joint ROM in a variety of stretching duration, positions and techniques (although a relatively poor overall study quality was noted).
The conclusion was that regular stretching can increase joint ROM by a mean 8 degrees for more than 1 day after stopping stretching.
This systematic review of randomized trials investigated the effects of static calf muscle stretches compared with no stretching. 5 of the trials showed that calf muscle stretching increases ankle dorsiflexion, particularly when performed for more than 30 minutes of total stretching duration. In summary, stretching does appear to increase flexibility, as measured by joint ROM, in several different muscle groups. Such increases in flexibility appear to last more than a 1 day at least.
Does Stretching Prevent Injuries?
This 2002 systematic review found 2 studies that assessed the effects of stretching before exercising on the risk of injury. Both studies were performed in new military recruits undergoing 12 weeks of initial training. Neither of the studies reported any change in injury risk as a result of the stretching interventions.
This 2003 systematic review assessed the effectiveness of stretching for preventing exercise-related injury. From the total of 7 studies that were reviewed 3 of the 4 randomized clinical trials found that stretching did not reduce the risk of exercise-related injury, while 1 randomized clinical trial found that it did. All 3 controlled clinical trials found that stretching reduced the risk of exercise-related injury. Based on the available literature the reviewers concluded that they were not able to conclude on whether stretching does prevent exercise-related injury.
This 2004 systematic review assessed the effectiveness of stretching. 6 controlled studies were reviewed, 3 of which assessing the stretching of specific muscle groups (2 on the heel and calf and 1 on the hamstring) and the other 3 assessing the stretching of multiple muscle groups. The meta-analysis of these studies concluded that stretching was not significantly associated with a reduction in total injuries.
In 2008, another systematic review concluded that there is moderate to strong evidence that routine application of static stretching does not reduce overall injury rates.
Additionally, this 2010 systematic review used 7 studies to assess the effects of stretching on injury risk. 3 studies found no effect and 4 studies found some effect. The reviewers noted that the studies finding no effect also displayed a low prevalence of muscle strains while those finding an effect had a high prevalence of muscle strains. Their conclusion was that while the literature is conflicting, there is some evidence that pre-exercise stretching may reduce injury risk in high-risk environments.
In summary, the literature is conflicting and it appears that regular stretching is ineffective for reducing the risk of sports injury, whether the stretching is performed immediately prior to exercise or at another time.
Stretching & Recovery
This systematic review assessed the effectiveness of post-exercise stretching in short-term and delayed recovery of strength, range of motion and delayed onset muscle soreness (DOMS). The conclusion was that there isn’t sufficient statistical evidence proving that stretching has an influence on recovery when compared to other post-exercise recovery methods like passive recovery (i.e., rest), low intensity cycling or running, massage, and cold-water immersion.
Herbert has performed 3 systematic reviews [1, 2] all of which concluding that stretching before or after exercising does not confer protection from delayed-onset muscle soreness and does not produce clinically important reductions in muscle soreness in healthy adults, nor does stretching before exercise confer a practically useful reduction in the risk of injury.
Stretching is a poor form of active recovery, as it can reduce IGF-1 production during strength training and it either does not affect or actually reduces the flow of blood and oxygen to your muscles and tendons.
Stretching & Performance
This review concluded that “The detrimental effects of static stretch are mainly limited to longer durations (≥ 60 s), which may not be typically used during preexercise routines in clinical, healthy, or athletic populations. Shorter durations of stretch (<60 s) can be performed in a preexercise routine without compromising maximal muscle performance“.
The reviewers also estimated that the mean reduction in performance following stretches lasting 1-2 minutes as 4.2 ± 5.0% and they estimated the reduction following stretches lasting >2 minutes as 7.0 ± 5.7%.
Dynamic stretching appears to either have no effect or a slight acute improvement in performance, especially when performed for longer durations.
In summary, static stretching performed for more than 45 seconds appears to lead to meaningful acute reductions in performance tasks whereas dynamic stretching performed for a long duration appears to lead to either no improvement or small improvements in the same type of actions.
Shrier performed a systematic review on the chronic effects of static stretching on performance measures. From the 9 studies that he reviewed, 7 of which displayed a positive effect, 2 displayed no effect, while no study displayed a negative effect. The 7 studies that displayed a positive effect included measures of maximum voluntary isometric strength, contraction velocity, eccentric and concentric force, counter movement jump height and 50 yard sprint time.
Rubini performed a systematic review on the chronic effects of static stretching on measures of strength. Both 2 studies that were reviewed, demonstrated a significant improvement in strength following a long term stretching program. The reviewers suggested that the increase in strength is believed to occur as a result of hypertrophy of the stretched muscles, as being observed in several animal studies.
These studies have observed hypertrophy following chronic stretching protocols, although such protocols involved greater durations of stretching. Chronic stretching programs appear to lead to meaningful improvements in performance tasks, as well as measures of strength.
Additionally there is a trend in research showing that combining stretching, even dynamic stretching, with strength training yields to decreased strength development.
Summary
Tldr; Stretching does not lengthen the muscles. All it does is essentially induce neural adaptations causing an improved stretch/pain tolerance.
Static stretching performed for more than 45 seconds appears to lead to meaningful acute reductions in performance tasks whereas dynamic stretching performed for a long duration appears to lead to either no improvement or small improvements in the same type of actions.
Stretching does not reduce overall injury rates, even when performed routinely.
Stretching is a poor form of active recovery, as it can reduce IGF-1 production during strength training and it either does not affect or actually reduces the flow of blood and oxygen to your muscles and tendons.
Static stretching over a minute has detrimental effects in performance, whereas dynamic stretching either has no effect or a slight acute improvement in performance.
Combining stretching, even dynamic stretching, with strength training yields to decreased strength development.
