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Stretching is primarily used to improve flexibility, decrease stiffness of the muscle- tendon unit or reduce risk of injury. However, previous animal studies from 1970 to 1990 showed significant hypertrophy effects in skeletal muscle in response to chronic stretching intervention with stretching durations of 30 minutes to 24 hours per day. However, no study in humans was performed using comparatively long stretching durations of more than 30 minutes per day with a daily frequency. The present cumulative dissertation includes six studies aiming to investigate the effects of long-lasting static stretching training on maximum strength capacity, hypertrophy and flexibility in the skeletal muscle. Before starting own experimental studies, a meta-analysis of available animal research was conducted to analyze the potential of long-lasting stretching interventions on muscle mass and maximum strength. To induce long-lasting stretching on the plantar flexors and to improve standardization of the stretch training by quantifying the angle in the ankle joint while stretching, a calf muscle stretching orthosis was developed. In the following experimental studies, the orthosis was used to induce daily long- lasting static stretching stimuli with different stretching durations and intensities in the plantar flexors to assess different morphological and functional parameters. For this, a total of 311 participants were included in the studies and, dependent on the investigation, the effects of daily stretching for 10-120 minutes for six weeks were analyzed. Therefore, effects on maximal isometric and dynamic strength as well as flexibility of the plantar flexors were investigated with extended and flexed knee joint. The investigation of morphological parameters of the calf muscle was performed by determining the muscle thickness and the pennation angle by using sonographic imaging and the muscle cross-sectional area by using a 3 Tesla magnetic resonance imaging measurement. In animals, the included systematic review with meta-analysis revealed increases in muscle mass with large effect size (d = 8.51, p < 0.001), muscle cross-sectional area (d = 7.91, p < 0.001), fiber cross-sectional area (d = 5.81, p < 0.001), fiber length (d = 7.86, p < 0.001) and
fiber number (d = 4.62, p < 0.001). The thereafter performed experimental studies from our laboratory showed a range of trivial to large increases in maximum strength of 4.84% to 22.9% with d = 0.2 to 1.17 and ROM of 6.07% to 27.3% with d = 0.16 to 0.87 dependent on stretching time, training level and testing procedure. Furthermore, significant moderate to large magnitude hypertrophy effects of 7.29 to 15.3% with d = 0.53 to 0.84 in muscle thickness and trivial to small increases of 5.68% and 8.82% (d = 0.16 to 0.3) in muscle crosssectional area were demonstrated.
The results are discussed based on physiological parameters from animal studies and in the front of knowledge in resistance training, suggesting mechanical tension to be one important factor to induce muscle hypertrophy and maximal strength increases. Further explanatory approaches such as hypoxia and changes in the muscle tendon unit are debated in the following. Since these studies are the first investigations on long-lasting stretch-mediated hypertrophy in humans, further research is needed to explore the underlying mechanisms and confirm the results in different populations to enhance the practical applicability for example in clinical populations when, e.g. counteracting muscular imbalances or sarcopenia in the elderly.
