Mechanics and energetics of running at steady and non-steady speed (sprint and shuttles): the effects of muscle-tendon behaviour

When humans move, they could do it in steady or non-steady conditions. In the former case the speed of locomotion is constant (or with minimal oscillations) and occurs in a definite direction (e.g. walking or running along a linear path), whereas in the latter case the body accelerates, decelerates or moves in different directions. In both cases, the minimum work required to sustain locomotion is given by the product of the resistance offered by the environment and the distance covered. Finally, the efficiency of the locomotor apparatus may be expressed as the ratio between the work necessary to maintain motion and the chemical energy transformed by the muscles. However, whereas the energetics and mechanics of running at constant speed are well known, only few studies have investigated so far non-steady running conditions (e.g. accelerated or decelerated running as well as running with changes of direction). The role of muscles and tendons in determining the mechanical and physiological responses during human locomotion is another topic that needs to be further investigated, both in steady and unsteady conditions. As an example, when humans run at constant speed muscles and tendons stretch and recoil; into this succession of stretch-shortening cycles, tendons could play an important role as energy savers allowing this form of locomotion to be particularly efficient. Locomotion (apparent) efficiency during constant speed running can be, indeed, as high as 0.70 at high running speeds whereas in un-steady conditions (e.g. shuttle runs) the efficiency is much lower, approaching the values of muscle efficiency (0.25) when fast accelerations and decelerations are required; locomotion (apparent) efficiency is thus enhanced when tendon elastic recoil is maximized. Investigating the role of muscle and tendon behaviour during steady and non-steady state running could, therefore, provide important information about the underpinning mechanisms that determine the mechanical and energetic demands of human locomotion. For these reasons, this thesis focuses on two main topics (running at non-steady speeds and running at constant speed), each with its own specific aims.