Stretching

Increasing flexibility

Researchers at the University of Central Arkansas compared the traditional static stretch with dynamic stretching and concluded that static stretching increases flexibility almost four times as much as dynamic stretching (18). This shows that if flexibility is the goal, static stretching is more suitable than dynamic stretching.

When Japanese researchers studied the effect of static stretching and PNF on flexibility, they saw that PNF produced an even greater increase in flexibility (19).

When the same comparison was made at the University of the State of Rio de Janeiro, however, this difference was not seen. Their conclusion was that static stretching and PNF lead to the same improvement in flexibility (20). This was also the conclusion of Turkish researchers who compared static stretching and PNF (21).

Increasing and maintaining strength

Several studies have shown that static stretching causes short-term strength loss (22,23,24). In addition, static stretching appears to cause more short-term strength loss than ballistic stretching (25,26,27,28) and dynamic stretching (29).

Brazilian researchers, among others, compared static stretching with ballistic stretching and saw that in this comparison too, static stretching caused a greater increase in flexibility (28). However, they also saw that static stretching reduced maximal strength during a leg press, while strength remained unchanged after ballistic stretching. They therefore advise choosing ballistic stretching rather than static stretching immediately before activities that require a lot of strength.

..static stretching exercises produce a greater acute improvement in flexibility compared with ballistic stretching exercises. Consequently, static stretching may not be recommended before athletic events or physical activities that require high levels of force. On the other hand, ballistic stretching could be more appropriate because it seems less likely to decrease maximal strength.Consequently, static stretching may not be recommended before athletic events or physical activities that require high levels of force. On the other hand, ballistic stretching could be more appropriate because it seems less likely to decrease maximal strength.

R.F. Bacurau, School of Arts, Sciences and Humanities, University of São Paulo

Since ballistic stretching is very similar to dynamic stretching, you might suspect that dynamic stretching also leads to less strength loss than static stretching.

That is indeed the conclusion of researchers at the University of Oklahoma when they compared the effect of static and dynamic stretching on strength (Herda 29). They too saw a decrease in strength after static stretching, but not after dynamic stretching.

Overall, an acute bout of dynamic stretching may be less detrimental to muscle strength than static stretching for the hamstrings.

T.J. Herda, University of Oklahoma

Researchers at George Washington University, by contrast, saw an increase in strength after stretching when they compared the effect of static, dynamic and PNF stretching (Manoel 30). Whereas the Brazilians tested maximal strength with a leg press, the Americans tested strength on a bicycle.

However, different muscle fibers are activated for this (type IIA instead of IIB), which may explain a difference in the study outcome. What I find more interesting is comparing the differences between the various stretches. The increase in strength was greater after dynamic stretching than after static stretching and PNF. They saw no difference between the effect of static stretching and PNF on strength.

The Turkish researchers who compared static stretching and PNF also saw no difference in their influence on strength. In their study, they looked at drop jump height (21).

Finally, British researchers also looked at the effect on jumping power a quarter of an hour after static stretching, ballistic stretching and PNF (31). They too saw that strength decreased after static stretching and PNF, but not after ballistic stretching.

So it is important to consider why you are stretching and to decide which stretching method best fits that goal. For the greatest increase in flexibility, you can choose static stretching and PNF. If this must not come at the expense of strength, dynamic and ballistic stretching are more suitable.

The effect of stretching on athletic performance

You can see that stretching can have positive effects (flexibility) and negative effects (strength loss). You may then wonder what this means for athletic performance. As with a warm-up, it is useful here to first consider what "good performance" means. For most sports, athletic performance can be measured in delivered force, speed, distance covered, or things such as points scored. For a weightlifter, it is how much weight he can lift, and possibly how heavy. In some sports flexibility is more important, while in others the emphasis may be on speed, strength or accuracy.

Studies on athletic performance often focus on strength, speed and endurance. This is therefore important to keep in mind if your main goal is to drop from the top of a pole into a split during pole dancing. If you struggle with the split, you should look at the effects on flexibility. If you already struggle to get to the top of that pole, you would do well to focus more on stretching that does not cost strength.

Perhaps even more important: a distinction must be made between the immediate effects of stretching and the long-term effects, since there can be major differences between them. The studies mentioned above on the influence of various stretching methods all looked at short-term effects immediately after stretching.

"Stretching has no direct, positive effect on athletic performance"

From the studies mentioned above, we can already conclude that stretching immediately before activity carries the risk of a loss in strength.

Canadian researcher Ian Shrier has done a great deal of research on stretching, both his own studies and so-called comparative studies in which other researchers' work is compared. He reviewed several studies on both the immediate effect and the long-term effect of stretching (32). He compared 23 studies on the immediate effects of stretching on performance, including three of the studies I also mentioned above (22,23,24). He mainly looked at factors such as delivered isometric force and jumping power.

He concluded that 22 of the 23 studies showed that stretching has no positive effect on athletic performance immediately afterward. This also included studies showing that it had a negative effect, as can be seen in the image alongside the article. To the left of the line you see the studies that indicated a negative effect, or at least the given score, with the names of the studies all listed on the right, and to the right you see the scores of studies that indicated a positive effect.

An acute bout of stretching does not improve force or jump height, and the results for running speed are contradictory.

I. Shrier, SMBD-Jewish General Hospital

"Regular stretching does have a positive effect on athletic performance"

Shrier then compared nine studies on the effects of regular stretching on performance. In other words, the long-term effects. This looked a lot more positive (32).

Of these nine studies, seven indicated a positive effect of regular stretching. The two studies that did not show a positive effect focused only on running efficiency. No study indicated a harmful effect, as was the case with the immediate results of stretching before activity.

Regular stretching improves force, jump height, and speed, although there is no evidence that it improves running economy.

I. Shrier, SMBD-Jewish General Hospital

"Long-term stretching lowers injury risk, stretching immediately before activity does not"

My karate/jiu-jitsu/kobudo teacher sometimes says during stretching before class: "You stretch for the next training session". By this he makes clear that stretching does not provide an advantage for the training at that moment. You have seen that this is correct with regard to athletic performance.

If you look at the many studies that examine the effect on injuries from immediately following activities, this is also correct. Several studies have been conducted on the immediate influence of stretching on the risk of injuries during a training session or competition. The question asked is: if you stretch before training, will this lower the risk of injuries during that same training session? In itself this is a logical question, because muscle stiffness is often seen as a risk factor for injuries (11).

Although there are studies showing that stretching lowers the risk of injuries, in these cases there were often interventions other than stretching (33,34,35). Stretching was often not the only factor influenced by the researchers. In many cases, a warm-up was also performed alongside stretching. Although these are often combined in practice, it is not useful when you want to measure the effects of one or the other separately.

Shrier rightly points this out in his study titled "Should people stretch before exersize?" (36). He refers to other studies in which the effects were measured separately and the results led to a different conclusion. In all these cases, stretching turned out to have no positive effect on the number of injuries during the following activity (37,38,39). A distinction was made between contact-related injuries and other injuries. After all, just like the effect of a warm-up, stretching will make little difference when, during soccer training, your knee is kicked in two by an opponent's sliding tackle.

Why stretching has no direct, or even a negative, effect on injuries

Shrier gives a number of reasons why stretching before a training session or competition can have no effect, or even a negative effect, on injuries (27). First, he points out that most injuries happen during the eccentric part of a movement. Think, for example, of bending your knees, where standing back up is the concentric or positive part. In this example, most injuries will occur on the way down, not once you have already arrived there. Stretching increases the previously mentioned range of motion, allowing you, for example, to sink even deeper through the knees. This shifts the endpoint of the movement. Since the endpoint is not the point where most injuries occur, shifting it adds little to injury prevention. This is different, of course, if you do ballet or gymnastics and need to be able to do the splits. Then it does make sense to increase range of motion, but even in those cases it is not that single stretching session before training that makes this possible.

In addition, Shrier points to the danger of the so-called analgesic effect of stretching. This is the pain-relieving or numbing effect of stretching (40,41,15). By stretching the muscles before training, you reduce the signaling function of pain. If you then do not notice, or only notice later, that there is a problem, which the pain signal is trying to inform you about, complaints can develop that you might otherwise have been able to prevent.

Finally, he refers to a study showing the possibility that even light stretching can lead to muscle damage at the cellular level (27,42). In the study he refers to, "light stretching" meant lengthening the resting muscle length by 20%.

It does not seem prudent to decrease one's tolerance to pain, possibly create some damage at the cytoskeletal level, and then exercise this damaged anesthetized muscle. Of note, no basic science evidence suggests that stretching would decrease injuries. Finally, some basic science data suggest that a warmup may help to prevent injuries

I. Shrier, SMBD-Jewish General Hospital

Practice: how long should you stretch?

If you stretch statically, how long should you hold the stretch? If you stretch dynamically or use PNF, how often should you repeat it? This too has been studied extensively in various studies.

Static stretching: how long?

The previously mentioned researchers at the University of Central Arkansas looked in another study at hamstring flexibility after stretching (43). In 1994 they looked only at the duration of stretching. They had their subjects, men and women aged 21-37, stretch for 15, 30 and 60 seconds to measure the difference in effect on flexibility (range of motion). Stretching was done five days a week for six weeks.

In the end, the researchers saw that 30 and 60 seconds of stretching resulted in a greater increase in flexibility than 15 seconds. There was no significant difference between 30 and 60 seconds, leading them to conclude that 30 seconds is sufficient for an optimal result.

The results of this study suggest that a duration of 30 seconds is an effective time of stretching for enhancing the flexibility of the hamstring muscles. Given the information that no increase in flexibility of the hamstring muscles occurred by increasing the duration of stretching from 30 to 60 seconds, the use of the longer duration of stretching for an acute effect must be questioned.

W.D. Bandy, University of Central Arkansas

Three years later, the same researchers again compared the difference between 30 and 60 seconds, but also the frequency of stretching (44). They divided their subjects, aged 21-39, into different groups that did not stretch, stretched for 30 or 60 seconds, or stretched 1 or 3 times per day. They saw that all groups that had stretched had more flexible hamstrings than the group that had not stretched. Between the groups that had stretched, they saw no difference.

Their conclusion was therefore that once per day, 30 seconds per muscle group, is sufficient, since stretching more often does not lead to a better result.

This still does not make clear whether less than 30 seconds could perhaps be just as effective. An answer to this was given by research from the Vrije Universiteit Brussel (45). Twenty women between the ages of 20 and 30 performed stretches for the hip joint for ten weeks. This time the difference in the effect of 10, 20 and 30 seconds of stretching was examined. Just as the Arkansas researchers saw no difference between the effect of 30 or 60 seconds, the Belgians saw no difference between 10, 20 and 30 seconds.

They therefore concluded that, at least for the hip joint, 10 seconds is already sufficient.

At Duke University in North Carolina, this was also examined as part of a broader study on stretching (1). However, they used laboratory animals, New Zealand White rabbits to be precise. Of these, the tibialis anterior (TA, 20 pieces) and the extensor digitorum longus (EDL, 40 pieces) were studied. Although undoubtedly killed "humanely", these at least sixty rabbits gave their lives so that you know how long you should stretch. Just so you know :)
In any case, they chose these muscles because they are easy to access and have only one point of origin and insertion, so on both sides only one tendon connected to bone, making muscle length easier to measure.

The muscles were stretched in a sort of miniature version of a medieval torture device (image on the right) to see the tension (stress) on the muscle and the extent to which this tension decreased (relaxation). Very simply put: the more the muscle relaxes in response to the tension from the stretch, the more this leads to a longer muscle and tendon. In this way they measured the so-called "stress-relaxation" over the time during which the muscle and tendon were stretched. This showed that stress-relaxation was greatest during the first 12 to 18 seconds. After that, the decrease in tension was much smaller.

From the North Carolina study you can therefore infer that 12 to 18 seconds is sufficient. This may be different when the same type of study is conducted among people over 65 (46). Sixty-two older adults with an average age of 84 were divided into groups. There was a control group that did not stretch, and the others were divided into groups that stretched 5 times per week for 6 weeks, for 15, 30 or 60 seconds. The 60-second stretch produced the largest increase in range of motion in these older subjects.

If you compare all these studies, you can say that you should hold a stretch for at least 10-12 seconds and at most 30 seconds. People over 65 would be wise to hold the stretch longer, namely 60 seconds.

Practice: how often should you stretch? Number of repetitions per muscle, number of stretches per week

Just as in strength training you usually work per muscle group with different sets of a certain number of repetitions, you can also stretch a certain muscle several times for a better result. The question, of course, is: "How often?"

As far as static stretching is concerned, multiple researchers have given different answers, ranging from one to twenty times (47,48,49). In itself, all are correct, since all these repetitions contribute to greater flexibility through reduced muscle tension. However, the extent to which each repetition of a stretch contributes to more suppleness becomes smaller as the number of repetitions increases.

We already saw above in the Central Arkansas study that there was no significant difference between stretching once or three times (44).
The North Carolina researchers who stretched the muscles from the rabbits saw that tension on the muscles decreased significantly only after the first four stretches (1 Taylor). From this they concluded that performing a given stretch more than four times per session is not effective.

The first stretch in particular contributes most to reducing muscle tension.

Researchers at the University of Plymouth also wondered whether all those repetitions made sense and looked at the effect of one stretch, both static and PNF. In the study, contract-relax was performed (50). They too looked at the increase in hamstring flexibility. They saw that both a single static stretch and PNF contribute to flexibility. They also saw that PNF contributes more to flexibility than static stretching. Unfortunately, they did not look at whether more repetitions would lead to a better result.

Several studies with one shared conclusion: the first stretch produces a considerable result. Where multiple repetitions have been studied, all further repetitions appear to produce relatively little extra result, and the difference is no longer significant after four repetitions in any case.

Personally, I would therefore rather advise using the time to stretch multiple muscles than stretching one muscle several times. Unless you have personal or athletic reasons to prefer a little extra result for one specific muscle over a large result for other muscles.

The preventive effect of stretching on muscle soreness

Sometimes stretching is done and/or recommended to prevent muscle soreness. Although there are studies showing that stretching can reduce muscle soreness (51), the effect appears negligible.

Danish researchers had healthy, untrained women perform eccentric exercises for the quadriceps (52). Eccentric training has been shown to lead to the most microtrauma, small tears in the muscle, and partly because of that to the most muscle soreness. This muscle soreness occurs especially quickly in untrained people. They had the women train the quadriceps in two different periods, the first time without stretching and one to two years later with stretching, before and after training. The researchers saw no difference after the second period in the muscle soreness reported by the women, nor in the physiological indicators for it.

There was no difference in the reported variables between experiments one and two. It is concluded that passive stretching did not have any significant influence on increased plasma- CK, muscle pain, muscle strength and the PCr/P_i ratio, indicating that passive stretching after eccentric exercise cannot prevent secondary pathological alterations.

H. Lund, University of Copenhagen

We know, however, that studies under different circumstances can give different answers to the same question. Australian researchers therefore performed a comparative study, as Ian Shrier did for the effects on athletic performance (53). They compared a total of fourteen studies on the effect of stretching on muscle soreness. In studies that looked at the effect of stretching on muscle soreness one day after the activity, stretching appeared to have hardly any effect. On a 100-point scale, stretching after the activity made only a one-point difference. Stretching after training made only half a point of difference. When they looked at studies focused on the perceived peak of muscle soreness in the period from half a day to three days after the activity, the difference was four points.

Now, a four percent gain in athletic performance is something most people would gladly sign up for, but whether this difference in muscle soreness is worth it seems unlikely. Statistically, the differences cannot be called significant in any case.

The evidence from randomised studies suggests that muscle stretching, whether conducted before, after, or before and after exercise, does not produce clinically important reductions in delayed-onset muscle soreness in healthy adults.

R.D. Herbert, The George Institute for Global Health

Stretching therefore does not produce a significant reduction in muscle soreness.

Summary

• Muscles and tendons are elastic and viscous. Flexibility determines the size of the movement path, the so-called range of motion.
• Muscles and tendons have self-protective mechanisms such as the stretch reflex, inverse stretch reflex and reciprocal inhibition.
• Various ways of stretching make use of these mechanisms. Stretching is done in different ways for different goals, with different results.
• In addition, there is a difference between the effects of stretching immediately before activity and the structural effects of regular stretching.
• Stretching improves flexibility, but it can also reduce strength before activity.
• Static stretching and PNF produce a greater increase in flexibility than ballistic and dynamic stretching, but also produce more short-term strength loss.
• Regular stretching can improve athletic performance. The same difference exists between the immediate effect of stretching on injuries during immediately following activities and the long-term effect on injuries.
• Depending on age, you should hold a stretch for at least 10-12 seconds and at most 30 seconds. For people over 65, 60 seconds is recommended. The first stretch will lead to the greatest increase in flexibility; from the fourth stretch onward, flexibility no longer increases significantly.
• Stretching has no significant effect on muscle soreness.

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