To stretch or not to stretch
It is generally assumed that stretching prevents injuries and flexibility exercises are recommended for warm-ups and/or cooling-down programs.
However, when reviewing the scientific literature, we find many contradictory findings. Some articles claim that stretching has a positive effect on injuries, while others claim stretching has no impact on the prevention of injuries.
In addition, there is a lot of discussion about the way one should stretch: which stretching exercise is the right one, is ballistic stretching good or rather bad, how long should you stretch, and so on.
The first article on stretching tries to clarify the contradictory findings and discussions on whether or not stretching is useful. The second article discusses the stretching methods in more detail.< br /> What is certain is that when it comes to stretching, there is no clear-cut answer. Stretching is not always good or bad. Everything depends on the sport, the athlete and even the muscle in question.
How does a muscle work?To discover the effects of stretching, we need to understand how a muscle works.
During a movement, a muscle can provide strength by shortening. This is called a concentric contraction. An example is the thigh muscle (m. quadriceps), which contracts when kicking a ball (stretching of the knee).
On the other hand, a muscle can also provide strength by lengthening (excentric contraction), for example the thigh muscle, which ensures your knees slowly buckle rather than immediately giving out.
Once the muscle receives a stimulus from the brain, its strength is transferred via such a concentric or eccentric contraction to the tendon, which then sets the joint in motion.
When great or rapid strength is required, the body also has a specific system to make use of. During these explosive movements (e.g. when jumping), the muscle-tendon unit is stretched and then uses the energy stored to contract even more. You can compare this to a tense elastic that you let go of. In the literature this is called a stretch-shortening cycle (SSC). When you try to jump as high as possible, you will always first bend your knees (the thigh muscle works excentrically and stretches for a very short time) before jumping up. As such, you use the stretch shortening cycle to jump higher.
Less explosive movements or more cyclic movements are less complicated. In this case, the energy generated by various chemical processes in the muscle is immediately transferred to the tendon and joint to make a movement, usually concentric.
For which sport is stretching the most beneficial?If you look at the two systems, it is also logical that for the SSCs (jumping, sprinting, throwing,...) it is more beneficial to have a more flexible muscle-tendon unit than for the more cyclic movements (swimming, cycling, long-distance running,...). With the more cyclic movements energy would be lost if they were too flexible. The intention there is to enable the strength to turn into a movement as quickly as possible.
For sports that require a lot of SSCs, on the other hand, it is better to stretch more in order to promote the elasticity of the muscle-tendon unit. This involves storing as much potential energy as possible in the muscle in order to allow it to transform into kinetic energy (movement energy). However, there is probably an optimum level of this flexibility to carry out a specific task. In other words, if you have a very flexible elastic and you let it go, it won't fly far... So, for an SSC the muscle must be sufficiently stiff, but also sufficiently flexible. Stretching too much or being too flexible may also have a negative effect on performance.
Can stretching reduce the risk of injury?. Before discussing whether or not stretching reduces the risk of injury, it is important that we take a closer look at the muscle-tendon unit referred to above. There is a difference between the muscle that can actively contract and the tendon that can only passively transmit strength.
For SSCs (more explosive sports), the tendon absorbs some of the energy exposed to the muscle-tendon unit. However, when the tendon itself has less flexibility and is therefore rather stiff, less energy can be absorbed by the tendon, exposing the muscle to greater forces. This could then lead to muscle trauma. A tendon that is more flexible, on the other hand, can absorb more energy itself, exposing the muscle to less strength.
Bearing this in mind, stretching could therefore have a preventive effect if we could make the tendon itself more flexible.
The good news is that several studies actually showed that this is possible. Stretching influences the elasticity of the tendon, allowing it to absorb more energy over time.
However, when we look at sports with no or low-intensity SSCs, the reasoning is completely different. In these sports, the tendon does not need to absorb very high forces because they simply do not exist or because they never exceed the absorption capacity of the tendon. In these sports, stretching does not reduce the risk of injury. As discussed above, athletes may even experience a detrimental effect on their performance because the transfer of strength from muscle to tendon is less efficient if the tendon is too flexible.
So, should you never stretch when practicing sports such as jogging, cycling or swimming?. Here too, as you probably expected, there is no clear-cut answer. In each of the sports mentioned above, one has to look at the movements that have to be performed.
Concretely, we need to look at each individual muscle. The hip flexors of a cyclist should not be stretched too much as they always have to work in a shortened position. The hamstrings, on the other hand, sometimes do need stretching because the cyclist needs to be able to sit in a specific position (e.g. in a time trial).
In swimming, for example, we need to look at the most commonly used swimming technique. For example, breaststroke swimmers are more likely to benefit from a large inward rotation of their hips (endorotation of the hips) as this allows them to move more water and thus move forward faster.
A lot depends on the techniques used by each individual athlete. Many will remember Kim Clijsters’ slide, where she almost does a split to reach a distant ball. The muscles on the inside of her thigh (adductors) need to be very flexible for her to do this. For other tennis players this could be less beneficial or even disadvantageous as their elasticity would increase too much, which would negatively affect the SSC.
How come very agile and flexible athletes suffer muscle injuries?.
In practice, we see that very agile and flexible athletes sometimes get muscle injuries. This may seem strange as you might assume that someone who has a very flexible muscle-tendon unit has enough ability to absorb the energy.
The very simple reason why these athletes still get muscle injuries is that a stiffer muscle-tendon unit is not the only risk factor for muscle injuries in sports with a lot of SSCs, for example.
. In other words, muscle injuries are caused by a variety of factors. Fatigue, lifestyle, behavioral habits, metabolic processes, etc. may all influence muscle injuries. More research is required to clarify this further.
To stretch or not to stretch?As you have read above, stretching is anything but straightforward. You need to find a balance between optimum performance and avoiding injuries. It is assumed that the biological composition of muscles and tendons in terms of flexibility, among other things, is also one of the reasons why athletes from certain countries or even tribes are better at long-distance running or sprinting. The composition and flexibility of their muscle-tendon unit may be optimal for the efficient transfer of the energy that is supplied into a movement.
For top athletes we need to have a detailed look at each individual sport, muscle and athlete.
For recreational athletes and younger athletes, we can give some general guidelines for stretching:
Always try to stretch a little after exercising
When exercising, your muscles frequently contract. That is when the well-known actin and myosin filaments slide over each other. When you stretch after exercising, you pull these filaments apart, bringing your muscle back to its original length.
Don’t stretch too much just before sports with lots of SSCs:
If you stretch a lot just before sports with a lot of SSCs you will lose some of the ability to store energy in the tendon because the tendon temporarily becomes too flexible. As a result, you may become a little less explosive. It's a matter of finding the perfect level of flexibility to perform your sport as efficiently as possible.
Make stretching a key goal during growth spurts
During a growth spurt or immediately after, many injuries occur (muscle injuries or overloading of the attachment between tendons and bones) because the size of the skeleton no longer corresponds to the length of the muscles. After all, the skeleton is growing and the muscles must be given the time to adjust to these new dimensions. It is important to rebalance this as soon as possible in order to prevent injuries.
Wilson GJ, Murphy AJ, Pryor JF. Musculotendinous stiffness: its relationship to eccentric, isometric and concentric performance. J Appl Physiol 1994; 76 (6): 2714-9
Wilson GJ, Elliott BC, Wood GA. Stretch-shortening cycle performance enhancement through flexibility training. Med Sci Sports Exerc 1992; 24: 116-23
Ettema GJC. Muscle efficiency: the controversial role of elasticity and release of series elastic energy in skeletal muscle during stretch-shortening cycles. J Exp Biol 1996; 199: 1983-97
Witvrouw E, Mahieu N, Danneels L, McNair P. Stretching and Injury Prevention. Sports Med 2004; 34(7): 443-9
Witvrouw E, Mahieu N, McNair P. The role of stretching in tendon injuries. Br J Sports Med 2007; 41: 224-6