Comprehensive mechanical power analysis in sprint running acceleration.

Sprint running is a common feature of many sport activities. The ability of an athlete to cover a distance in the shortest time relies on his/her power production. The aim of this study was to provide an exhaustive description of the mechanical determinants of power output in sprint running acceleration and to check whether a predictive equation for internal power designed for steady locomotion is applicable to sprint running acceleration. Eighteen subjects performed two 20 m sprints in a gym. A 35-camera motion capture system recorded the 3D motion of the body segments and the body center of mass (BCoM) trajectory was computed. The mechanical power to accelerate and rise BCoM (external power, Pext ) and to accelerate the segments with respect to BCoM (internal power, Pint ) was calculated. In a 20 m sprint, the power to accelerate the body forward accounts for 50% of total power; Pint accounts for 41% and the power to rise BCoM accounts for 9% of total power. All the components of total mechanical power increase linearly with mean sprint velocity. A published equation for Pint prediction in steady locomotion has been adapted (the compound factor q accounting for the limbs' inertia decreases as a function of the distance within the sprint, differently from steady locomotion) and is still able to predict experimental Pint in a 20 m sprint with a bias of 0.70 ± 0.93 W kg-1 . This equation can be used to include Pint also in other methods that estimate external horizontal power only.

Mechanical work in shuttle running as a function of speed and distance: Implications for power and efficiency.

Biomechanics (and energetics) of human locomotion are generally studied at constant, linear, speed whereas less is known about running mechanics when velocity changes (because of accelerations, decelerations or changes of direction). The aim of this study was to calculate mechanical work and power and to estimate mechanical efficiency in shuttle runs (as an example of non-steady locomotion) executed at different speeds and over different distances. A motion capture system was utilised to record the movements of the body segments while 20 athletes performed shuttle runs (with a 180° change of direction) at three paces (slow, moderate and maximal) and over four distances (5, 10, 15 and 20 m). Based on these data the internal, external and total work of shuttle running were calculated as well as mechanical power; mechanical efficiency was then estimated based on values of energy cost reported in the literature. Total mechanical work was larger the faster the velocity and the shorter the distance covered (range: 2.3-3.7 J m-1 kg-1) whereas mechanical efficiency showed an opposite trend (range: 0.20-0.50). At maximal speed, over all distances, braking/negative power (about 21 W kg-1) was twice the positive power. Present results highlight that running humans can exert a larger negative than positive power, in agreement with the fundamental proprieties of skeletal muscles in vivo. A greater relative importance of the constant speed phase, associated to a better exploitation of the elastic energy saving mechanism, is likely responsible of the higher efficiency at the longer shuttle distances.

Effects of Joint Kinetics on Energy Cost during Repeated Vertical Jumping.

PURPOSE: The present study was designed to investigate the effects of lower limb joint kinetics on energy cost during jumping. METHODS: Eight male middle and long-distance runners volunteered for the study. The subjects were asked to repeat vertical jumps at a frequency of 2 Hz for 3 min on a force platform in three different surface inclination conditions: Incline (+8°), Level (0°), and Decline (-8°). Sagittal plane kinematics were obtained using a high-speed video camera. Simultaneously, ground reaction forces and EMG of the lower limb muscles were recorded. Energy cost was calculated using steady-state oxygen uptake, respiratory ratio, and vertical distance of the body. RESULTS: In all conditions, energy cost correlated positively with total mechanical work of the knee joint (r = 0.636, P < 0.01), but negatively with total mechanical work of the ankle joint (r = -0.584, P < 0.01). The muscle-tendon complex length of the gastrocnemius and soleus muscles were significantly longer in incline than in level and decline. The gastrocnemius muscle showed different activity pattern in decline as compared with the incline and level conditions. CONCLUSIONS: The present study revealed that the ankle and knee joint kinematics and, therefore muscles' coordination are associated with energy cost during repeated vertical jumping. The lower limb joints contributed different efficiencies to generate the same total mechanical work in repeated vertical jumping on different surface inclinations. Energy cost was smaller when mechanical work was mainly done by ankle joint. Whereas, when the ankle joint did less mechanical work, the knee and/or hip joints compensated for the lack of mechanical work of the ankle joint and energy cost was increased.