Stretching and its effect on the muscle-tendon-unit

Stretching is a commonly used tool in pre-match preparation as well as in rehabilitation. There are different stretching techniques, which might benefit one or the other purpose. A good example here would be the utilization of static vs. dynamic stretching in a pre-match warm-up or to increase range of motion in a flexibility session.

While the research mainly focused on the muscle (and even more on its force producing capability), it seems relevant to ask what actually happens during stretching and which part of the muscle-tendon unit (which includes the tendon, the connective tissue or myotendinous junction (MTJ) = the part in which the “muscle turns into the tendon (1 - see references below)” and the muscles) is stretched? Anecdotally it was believed that stretching would alter tendon length, which would therefore cause instability to a given joint and further increase injury risk?

As a result, the following paragraphs will focus on a) is it possible to stretch individual parts of the muscle-tendon unit, b) if so, what are the effects on each part, c) are there result differences between various types of stretches and finally d) how to use these information in a football setting.

Is it possible to stretch individual parts of the muscle-tendon unit?
First of all I would like to spend some thoughts on the figure below showing how muscles, tendons and bones interact. As it seems, it is not very easy to determine what part is stretched and therefore altered after the stretching.

 

 

The first figure (from left to right) show the normal “at-rest” state (without stretching or contraction of muscle) of a joint. The picture in the middle, shows the same joint before receiving a stretch. With the vertical bone being rotated, the muscle, the connective tissue and the tendon will be stretched. The right picture shows the same stretch, however, now the muscle also contracts “pulling” in the opposite direction to the stretch (which is basically a PNF stretch).

From a logical point of view it seems that all three parts are stretched to a more or lesser extend. It was shown that the viscoelastic properties are responsible for length changes through stretching (2), also in the tendon (3-5). Viscoelastic means that a materials which exhibits deformation will return to its origin state once the stress is removed (6) (which is also called hysteresis and here everyone can make up his own mind if a tendon can really be stretched or not). For example it was shown that even during walking the gastrocnemius tendon was stretched (7) for 7 mm (8).  During a drop jump the entire MTU was lengthened 3.8 (± 1.1) cm, while the tendon itself showed a stretch length of 3.1 (± 1.5) cm (9).

 

Research shows that the passive tension (or stiffness) of a stretched muscle-tendon unit was reduced after stretching (10), showing the positive effect of stretching, even without impairing the force and speed capacities of the muscle (10). It was reported that despite the changes, the stiffness of the Achilles tendon itself was unchanged (11), suggesting that the increased range of motion is based on the connective tissue (10). In another investigation it was concluded that the tendon stiffness stayed the same and was even greater immediately after the static stretching (12). In contrast, it was also reported that tendon stiffness decreased (13).

Guissard et al. (10) suggested adaptations of the mechanical (namely the force producing part or the muscle) properties of the muscle–tendon unit would explain the increase in flexibility to long-term stretching.
As it seems, the little research is conflicting and it seems more accepted that connective tissue as well as the muscle itself account for (length) changes compared to changes to the tendon.

 

The effect a more compliant tendon are seen it its energy storage capacity (14). A less stiff tendon can store more energy and it is thought that less force is therefore taken by the muscle, which in return might reduce injuries in that muscle (15). Usually, a more compliant tendon, can tolerate more force. However, with a low compliant tendon, more force needs to be tolerated by the muscle, which was thought to explain the noted association between reduced flexibility and injury occurrence during SSC motion.

In addition, it seems that more compliant tendon structures in knee extensors favored the performances during jumping (16).

However, a more compliant tendon also will “delay” the force transmission to the bone and therefore might not be desired in sports without SSC (14), as the force in “low” or non-SSC movements will most likely not exceed the force-capability of the tendon. For example jogging (which is performed to great extend in footballers) was characterized as a “low” SSC movement, for which a high stiffness was deemed to be more beneficial, as more energy is released quicker from the muscle, to the tendon into the bones (14), which ultimately moves the player.

There seems to be gender differences (17, 18) in tendon stiffness with male experiencing greater tendon stiffness. In addition, females demonstrates greater ability to adapt to a stretch stimulus by decreasing tendon stiffness and therefore an increase in compliance (17). It was suggested that a more compliant tendon increase the capacity to tolerate load and therefore lower the injuries risk for the tendon (19).

 

It seems that every form in which the entire MTU is stretched, the muscle is shortened whilst the entire MTU remains its length or is lengthened will have an effect on the connective tissue and/or the tendon.

Static stretching
Conflicting results was seen when applying static stretching and its effect on tendon stiffness. While the stiffness was not changed (20, 21), it was also reported that the stiffness decreased (22). In order to complete the frustration, it was stated that that static stretching training affected the viscosity of tendon structures (21, 22) but not the elasticity.

However, it seems also warrant to have a more in depth understanding as the duration (23) (and probably the intensity as well) affects different attributes/characteristics of the MTU. For example, moderate-duration static stretch reduces active and passive plantar flexor moment, but not Achilles tendon stiffness or active muscle length (23).
Ballistic stretching
During ballistic stretching the muscle is contracting while the tendon is stretched (similar to eccentric exercise movements) and this occurs in a repetitive manner. Six weeks of ballistic stretching was shown to reduce tendon stiffness significantly (24).

Acute effects of PNF stretching on the ankle extensors showed a decrease in passive tension by up to 18%. Contract-relax stretching performed twice a day for 3 weeks lowered the passive tension in the plantar flexors by up to 36% (25).
Furthermore, static stretching including muscle contraction (not a classical PNF stretch as the muscle was only contracted without stretching while contracting) was more beneficial (significantly decreased the stiffness (–8%) and hysteresis (29%) (6)) to the tendon compared to the static stretching only group (13).

Eccentric training
Several studies have shown the superior results of eccentric exercise protocols in the treatment of tendinopathy (15) and it was suggested that eccentric programs can lead to a decreased tendon stiffness, which would then increase the tendon energy capacity.

It was seen that eccentric training program results in changes to some of the mechanical properties of the plantar flexor muscles and the changes were thought to be associated with modifications to structure rather than to stretch tolerance (26). However, 6 weeks of training did not change the Achilles tendon stiffness (26). Chronic training load will change the cross sectional part of the tendon (here the Achilles tendon) showing the possibility to influence the structural components (27).

Also a longer period (Twelve weeks) of eccentric training did not alter Achilles tendon properties in healthy footballers (28). However, a 4-week hard training intervention was sufficient in altering collagen synthesis (building of tendon structures) in untrained adults (29), showing the necessity for altering loads and exercises appropriately to the athlete loading/training capacity.

Plyometric training
12-weeks of plyometric training seems to result in no (30), but also an upward trend in stiffness (31). Furthermore there were no changes observed in the cross sectional area of the Achilles tendon. Plyometric training therefore seemed to enhances the muscular tension transmission through a reduction in energy dissipation by the tendon (31).

Isometric voluntary contraction
8-seconds of maximum voluntary isometric contraction (MVC) showed decrease in tendon stiffness (32). Interestingly, muscle force recovered, however, tendon stiffness did not, showing a possible increase in injury risk through accumulated increase in tendon compliance in the presence of fully recovered force production (19).

 

The importance of the tendon was seen in many activities (33, 34). It was estimated that the contribution of the tendon to the mechanical work was 6% in walking (33) and 16% in single-leg jumping (35). Furthermore, it is suggested that in stretch shortening activities such as the countermovement jump, the muscle “only” act isometrically, and the actual storage and release of energy is solely based on the tendon (34).

Given this information, training the MTU and more importantly the tendons seems very important with regards to an injury prevention and a performance enhancement perspective.

As suggested previously, ballistic stretching, furthermore PNF stretching and (especially) eccentric muscle training should be incorporated into training routines for adult players (after PHV) with a decent fitness and resistance training history. As this type of work increase the “stimulus” for the MTU, which might increase the likelihood for injuries in “unfit” players,an individual progressive increase in load seems to be paramount. Otherwise the stress (excessive repetitive force (36)) might result in muscle strains, tendinopathy or other injuries related to the muscle-tendon unit.

 

• The type of stretch influence on what part (muscle vs. muscle-tendon vs. tendon) is stretched.
• Stretching the tendon might even be a “good” thing to do as it alters stiffness and compliance and therefore might reduce injuries
• Sports (such as football) involving SSC need to incorporate dynamic and probably furthermore ballistic stretching to prevent injuries but also to possibly potentiate performance
• Eccentric training can be utilized to alter force-angle relation of the muscle but also to provoke beneficial changes in the connective tissue and the tendon
• A progressive increase in load (whilst stretching and training) seems paramount for injury prevention and performance enhancement

 

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