Habitual stretching: A tool to adapt muscle-tendon structure?

Regular static stretching (=stretching where the joint is held in position) is one of the most common tools in sports practice to increase the range of motion (RoM) of a joint. The mechanism behind the increase of RoM following static stretching at the muscular and tendinous level, which to date has been investigated in studies including interventions of up to eight weeks, is controversial. It has been shown that static stretching of up to eight weeks increases RoM; however, it is very likely that this can be explained at a sensory level (adapted perception of pain) rather than by changes of the muscle-tendon structure (i.e. more compliant muscle tissue). Nevertheless, there is clear evidence that habitual shortening (e.g. using high-heeled shoes) or lengthening (e.g. ballet dancers) of the muscle-tendon unit for several months or years leads to structural changes. This strongly suggests that static stretching of up to eight weeks might not be stimulus enough to induce changes of the muscle-tendon structure, and it is therefore necessary to observe a longer period of stretching. Regarding the effect of static stretching for 10 weeks on muscle performance, there is evidence that it can positively affect dynamic movements (i.e. jump performance). However, the mechanism behind these adaptations is unknown. Concerning detraining of stretching (stopping stretching or reducing the weekly amount of stretching), there is a lack of intervention studies in the literature. Therefore, the aim of this project will be to systematically investigate the effect of a static stretching intervention on the muscle and tendon function (e.g. RoM, jump performance) and structure (e.g. muscle stiffness, tendon stiffness, fascicle length) of the leg muscles for six months and the effect of a three-month detraining phase. Beyond the investigated parameters, we want to understand the mechanism behind the increased RoM and the possible increase in muscle performance. For the project, we will recruit 60 subjects and they will be tested four times: 1) baseline; 2) after three months of stretching; 3) after six months of stretching; 4) three months after stopping stretching or reducing stretching. Subjects will stretch collectively in teams, and every stretching session will be monitored by the investigators. The required parameters will be determined with state-of-the-art methodology, including shear wave and B-mode ultrasound, dynamometry (motor driven machine), a motion capture system, and force plates. In summary, this randomized controlled study will be the first to investigate the effect of a monitored static stretching intervention of six months duration, where structural effects on the muscle-tendon unit are highly probable. Furthermore, we will analyze the effect on performance and the effect of a three-month detraining phase.

The main goal of this project was to investigate if long-term static stretching can influence muscle-tendon properties (e.g., stiffness) as well as its function (e.g., range of motion) in the lower leg muscles. We have seen that static stretch training applied with a high volume (30 minutes/week) throughout several weeks can decrease muscle stiffness, increase fascicle lengths as well as increase muscle thickness and muscle strength. Additionally, we investigated further research questions such as the impact of high-volume static stretch training on the upper limbs. Besides the increase in range of motion, we have seen an increase in maximum strength. Additionally, chronic foam rolling and static stretching intervention of the plantar foot sole had no impact on the non-treated tissue such as the calf muscle function and structure. Finally, we have confirmed that there are no chronic changes in non-local heterologous muscles/joints in range of motion following stretch training. However, we have seen non-local homologous changes in range of motion following stretch training. Considering the acute effects, stretching and foam rolling seems to be similarly effective in increasing flexibility. However, considering muscle performance, foam rolling shows some small advantages when compared to stretching. Various myofascial techniques, such as stretching, foam rolling, flossing, massage gun application, or heat application, can increase flexibility and affect muscle performance (e.g., jump performance) acutely. However, the mechanism for the changes, especially in flexibility, depends on the myofascial technique. In addition, the acute effects of stretching and foam rolling on muscle function are sustained for approximately 40 minutes.