TENNIS GROUND STROKES FROM A BIRD’S EYE VIEW - A ESTIMATE OF ANGULAR MOMENTUM ABOUT THE LONGITUDINAL BODY AXIS

In this paper, a simple 2D video method will be outlined to estimate the angular momentum about the longitudinal body axis in tennis ground strokes from the base line. From a bird’s eye view, ground strokes of 19 young male experienced tennis players with two different skill levels were analyzed when returning balls released from a ball machine with three different ball frequencies. The angle between the shoulder axis and the base line was used as an estimate for the angular momentum about the longitudinal body axis. A validation procedure with a fully automated 3D motion capture system as adopted to evaluate the error involved in the 2D motion analysis. The results of this study show that for forehand and backhand strokes advanced young tennis players show consistently larger shoulder-baseline angles across all ball frequencies than players with a lower skill level

KINEMATIC STRATEGIES TO KEEP AN UNCHANGED MARGIN OF STABILITY DURING TREADMILL RUNNING ON AN EVEN AND UNEVEN SURFACE

Understanding how to control stability when running, particularly when being exposed to uneven terrain, is vital to prevent falls and to get an insight into compensatory strategies while running on uneven terrain. The purpose of this study was to assess surface related differences of the margin of stability, kinematics of hip and knee and upper body acceleration which may affect the control of running stability. Eighteen healthy younger adults ran on an even and an uneven surfaced treadmill for two minutes at fixed speeds of 2.0 m/s (female) and 2.2 m/s (male), respectively. Results showed an unchanged margin of stability in both conditions. Further, lower limb kinematics, step width variability and upper body acceleration increased on the uneven surface meaningfully to keep the extrapolated centre of mass within the base of support

Bewegungspriming

Le mouvement comme mode de réaction alternatif et implicite en jeu et dans les sports d'opposition, aux représentations visuelles quand celles ci ne sont pas suffisante

A LABORATORY TEST FOR THE EXAMINATION OF ALACTIC RUNNING PERFORMANCE

A new testing procedure is introduced to evaluate the alactic running performance in a 10s sprint task with near-maximal movement velocity. The test is performed on a motor-equipped treadmill with inverted polarity that increases mechanical resistance instead of driving the treadmill belt. As a result, a horizontal force has to be exerted against the treadmill surface in order to overcome the resistant force of the engine and to move the surface in a backward direction. For this task, subjects lean with their hands towards the front safety barrier of the treadmill railing with a slightly inclined body posture. The required skill resembles the pushing movement of bobsleigh pilots at the start of a race. Subjects are asked to overcome this mechanical resistance and to cover as much distance as possible within a time period of 10 seconds. Fifteen male students (age: 27.7 ± 4.1 years, body height: 1.82 ± 0.46 m, body mass: 78.3 ± 6.7 kg) participated in a study. As the resistance force was set to 134 N, subjects ran 35.4 ± 2.6 m on the average corresponding to a mean running velocity of 3.52 ± 0.25 m·s-1. The validity of the new test was examined by statistical inference with various measures related to alactic performance including a metabolic equivalent to estimate alactic capacity (2892 ± 525 mL O2), an estimate for the oxygen debt (2662 ± 315 ml), the step test by Margaria to estimate alactic energy flow (1691 ± 171 W), and a test to measure the maximal strength in the leg extensor muscles (2304 ± 351 N). The statistical evaluation showed that the new test is in good agreement with the theoretical assumptions for alactic performance. Significant correlation coefficients were found between the test criteria and the measures for alactic capacity (r = 0.79, p < 0.01) as well as alactic power (r = 0.77, p < 0.01). The testing procedure is easy to administer and it is best suited to evaluate the alactic capacity for bobsleigh pilots as well as for any other running discipline

Challenging human locomotion

The need to move over uneven terrain is a daily challenge. In order to face unexpected perturbations due to changes in the morphology of the terrain, the central nervous system must flexibly modify its control strategies. We analysed the local dynamic stability and the modular organisation of muscle activation (muscle synergies) during walking and running on an even- and an uneven-surface treadmill. We hypothesized a reduced stability during uneven-surface locomotion and a reorganisation of the modular control. We found a decreased stability when switching from even- to uneven-surface locomotion (p < 0.001 in walking, p = 0.001 in running). Moreover, we observed a substantial modification of the time-dependent muscle activation patterns (motor primitives) despite a general conservation of the time-independent coefficients (motor modules). The motor primitives were considerably wider in the uneven-surface condition. Specifically, the widening was significant in both the early (+40.5%, p < 0.001) and late swing (+7.7%, p = 0.040) phase in walking and in the weight acceptance (+13.6%, p = 0.006) and propulsion (+6.0%, p = 0.041) phase in running. This widening highlighted an increased motor output’s robustness (i.e. ability to cope with errors) when dealing with the unexpected perturbations. Our results confirmed the hypothesis that humans adjust their motor control strategies’ timing to deal with unsteady locomotion.Peer Reviewe