Umverteilung der muskulären Antriebe bei bewegungsinduzierter ermüdender Beanspruchung

For over a century, scientists have studied the fatigue caused by physical activity. Any energy-consuming activity eventually depletes the body’s energy reserves, leading to fatigue of biological structures. Fatigue is classified into two main types: central (neuronal) and peripheral (muscular). Studies on central fatigue typically focus on isometric contractions in isolated muscle groups using the twitch interpolation technique. Whereas recent research has also examined peripheral fatigue induced by complex movements such as jumping and running. Despite a generally good understanding about both central and peripheral fatigue, there is still limited knowledge about the fatigue-induced joint-specific changes in mechanical power and work generation. Therefore, the current thesis, consisting of three experimental studies, aimed to enlighten our understanding of the mechanical and physiological effects of prolonged high-intensity movements as vertical jumping and running.

The aim of the first study was to investigate the effect of 300 intermittent CMJs on the mechanical power distribution at the joints of the lower limbs and the influence of the upper body to explain vertical jump performance. The findings demonstrate that the jump-induced fatigue protocol led to a reduction in vertical jump performance and resulted in a decreased (P < .010) knee and ankle joint power. Although a decrease in internal muscle force and an increase in relative active muscle volume of the hip extensors was observed, no changes in the hip joint torque or power were detected. In addition, a significant higher angular momentum at the center of mass was detected during the braking and propulsion phase. This is possibly due to additional torque created by the trunk extensor muscles, which increased the angular momentum and maximal vertical velocity of the upper body segment. We suggest that a targeted exercise to enhance trunk and leg-extensor muscles, especially knee extensor and ankle plantar flexor muscles, would improve vertical jump and athletics performance in sports requiring constant ability to jump. The mechanical influence of the trunk extensor muscles should not be neglected in vertical jump performance, especially under fatigue conditions.

The aim of the second study was to investigate the joint-specific contributions to the total lower-extremity joint work during a prolonged fatiguing run (10 km). A decrease (P < 0.05) of positive ankle joint work as well as an increase of positive knee and hip joint work was found. These findings were associated with a redistribution of the individual contributions to total lower-extremity work away from the ankle toward the knee and hip joint which was more distinctive in the recreational runner group than in the competitive runner group. The redistribution of joint work from the ankle to more proximal joints might be a biomechanical mechanism that could partly explain the decreased running economy in a prolonged fatiguing run. This might be because muscle–tendon units crossing proximal joints are less equipped for energy storage and return compared with ankle plantar flexors and require greater muscle volume activation for a given force. To improve running performance, long-distance runners may benefit from an exercise-induced enhancement of ankle plantar flexor muscle–tendon unit capacities.

The aim of the third study was to examine the effects of a racing flat and cushioned running shoe on the joint-specific contributions to lower extremity joint work during a prolonged fatiguing run (10 km). For both shoes, we found a redistribution of positive joint work from the ankle to the knee and the hip throughout the entire run. Negative ankle joint work was higher (P < 0.01) with the racing flat compared with the cushioned shoe. Initial differences in foot strike patterns between shoes disappeared after 2 km of running distance. Irrespective of the shoe design, alterations in the running mechanics occurred in the first 2 km of the run, which might be attributed to the existence of a habituation rather than fatigue effect. Although we did not find a difference between shoes in the fatigue-related redistribution of joint work from distal to more proximal joints, more systematical studies are needed to explore the effects of specific footwear design features.

In summary, this work offers explanations for muscular fatigue and its mechanical and physiological effects during prolonged, high-intensity movements. Depending on the kind of the movement, there is a joint-specific redistribution of the muscular work in order to perform the mechanical work in the body's center of gravity. The findings underline the importance for sports performance, training practice and the development of footwear. If it is possible in the future to systematically determine the training condition and fatigue resistance of the lower extremities and to develop individually adapted training protocols and running shoes (mass, midsole stiffness and midsole material) based on this, an individual increase in running performance is conceivable. This could possibly also make it possible to break the 2-hour barrier in the marathon.