We present force platform measurements and video tracking analysis of a kettlebell-trained international top athlete performing the kettlebell lifts long cycle and snatch. The ground reaction force measured with the force platform strongly varies during the kettlebell lift. Video analysis reveals the contributions of the kettlebells and the athlete’s body parts to the ground reaction force. The force platform measurements agree with the forces estimated from video tracking usually to within 30%. The presented data allows estimates of the energy and power required for kettlebell lifts, the mechanical efficiency (long cycle: 48%; snatch : 57%), and the forces on the athlete’s joints.
The present study investigated the dynamics and spatial magnitude of the center of pressure trajectories during stance tasks in elite sport rifle shooters and non-shooters. Thirteen shooters and eleven non-shooters completed 90 seconds of two-legged and single legged stance on a force platform. The dynamics of the center of pressure trajectory was assessed using sample entropy, correlation dimension and entropic half-life. Additionally, the body sway was quantified as the elliptical area of the trajectory. The shooters had lower sample entropy and tended to have longer entropic half-life in both directions during the single-legged stance. Across the two tasks, lower correlation dimension in the anterior-posterior direction and lower body sway in both directions was observed in the shooters when compared to the non-shooters. This suggests that extensive training in maintaining quiet upright stance is associated with altered postural control, especially during challenging single-legged stance and to a lesser extend during a simpler two-legged standing task. Thus, the sport rifle shooters solved the difficult single-legged standing task using a movement pattern with increased regularity and lower dimensionality and body sway when compared to non-shooters.
Standard university or high-school physics teaching material on projectile motion is usually based on Newton's second law in vacuum, neglecting aerodynamics. We present a low-cost experiment for teaching projectile motion using the students' cell phones and sports equipment, which allows the students to test theory and numerical simulation against experimental data in the real world. For a shot put, theoretical predictions assuming projectile motion in vacuum agree with experimentally obtained trajectories in air to within a few centimeters. However, for a table tennis ball, vacuum trajectories can be almost three times as long as experimentally obtained trajectories. An equation of motion including the aerodynamic drag force has no analytic solution, but it is straightforward to integrate numerically for high-school or first-year university students. Accounting for aerodynamic drag substantially improves the match with experimental data for any ball. In a second experiment, balls are shot with spin resulting in curveball trajectories. Numerical simulations including the Magnus force can give accurate predictions of 3D curveball trajectories, both curving according to the normal and the inverse Magnus effect. Balls shot with topspin and backspin are also accurately modelled. Finally, we model the bounce of an arbitrarily spinning ball using linear and angular impulse-momentum theorems and coefficients of restitution in vertical and horizontal directions. We find agreement with experimental data to within centimeters.
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