Abstract
This study uses inertial sensors to measure ski jumper kinematics and joint dynamics, which was until now only a part of simulation studies. For subsequent calculation of dynamics in the joints, a link-segment model was developed. The model relies on the recursive Newton–Euler inverse dynamics. This approach allowed the calculation of the ground reaction force at take-off. For the model validation, four ski jumpers from the National Nordic center performed a simulated jump in a laboratory environment on a force platform; in total, 20 jumps were recorded. The results fit well to the reference system, presenting small errors in the mean and standard deviation and small root-mean-square errors. The error is under 12% of the reference value. For field tests, six jumpers participated in the study; in total, 28 jumps were recorded. All of the measured forces and moments were within the range of prior simulated studies. The proposed system was able to indirectly provide the values of forces and moments in the joints of the ski-jumpers' body segments, as well as the ground reaction force during the in-run and take-off phases in comparison to the force platform installed on the table. Kinematics assessment and estimation of dynamics parameters can be applied to jumps from any ski jumping hill.
Highlights
Ski jumping consists of a complex sequence of movements and can be divided into five main phases according to the biomechanical analysis [1,2]: in-run (IR), take-off (TO), early-flight (EF), stable-flight and landing
This paper presents a method to estimate the forces and moments in the ankle, knee and hip of a ski jumper during the IR and TO phases, as well as a method to estimate the ground reaction force (GRF) value during the TO phase
Graphs a–c in Figure 4 show a comparison of the calculated forces in the joints, normalized to body weight (BW), according to the bottom-up and top-down-up methods
Summary
Ski jumping consists of a complex sequence of movements and can be divided into five main phases according to the biomechanical analysis [1,2]: in-run (IR), take-off (TO), early-flight (EF), stable-flight and landing. Most researchers consider the action during the TO phase as the most important [1,2,3,4]. The IR phase, which consists of a curved path, is considered important, because the athletes actions during the IR determine the initial velocity of TO, the rotating moment and the position of the jumper-ski system during the TO phase [5]. There are two physically-demanding sections of the IR: the curve entrance and the curve exit. In the curve entrance section, the ski jumper maintains an aerodynamic squat position by withstanding the increasing ground reaction force (GRF) and a sudden demand for rotation during the curved part of the hill. The curve exit section requires the opposite effort and is important because it is near the beginning of the TO phase [5], which is the most critical
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