A new method for unconstrained measurement of knee joint angle and timing in alpine skiing : Comparison of crossover and crossunder turns

Timing is an important part in the analysis of alpine skiing technique. Key events extracted from dynamics and kinematics of ski were previously proposed, but timing of body segments kinematics is also required to understand the biomechanics and to assess the performance. In this study, we proposed a new method based on inertial sensors to measure timing parameters based on body segments kinematics. To show the efficacy of the method, angle and timing of knee joints during crossover and crossunder techniques were compared. Significant differences were obtained for the timing, but not for the amplitude of the knee angles. The proposed system reported a good repeatability, did not encumber the athletes and allowed the measurement of body movement during the whole run. So it could easily be used to assess performance in training conditions

Orientation of body segments in ski jumps using wearable sensors: Analysis of relevant parameters during stable flight phase

Biomechanics analysis of the ski jump is highly required. Some parameters and their interrelations have been reported in previous research studies limited to few athletes. The generalization of these parameters to athletes of various levels and under training conditions should be assessed, since they have the potential to be used for daily evaluation. This study proposed a new 3D approach based on inertial sensors to evaluate relevant kinematic and aerodynamic parameters of stable flight phase. The proposed wearable system can easily be used for daily training. Aerodynamic forces and body segments 3D angles were extracted during the stable flight phase of 86 jumps. Then, their correlations with respect to distance as well as their interrelations were analyzed. Their combination expressed 55% of the total distance variance

Three-Dimensional Body and Centre of Mass Kinematics in Alpine Ski Racing Using Differential GNSS and Inertial Sensors

A key point in human movement analysis is measuring the trajectory of a person’s center of mass (CoM). For outdoor applications, differential Global Navigation Satellite Systems (GNSS) can be used for tracking persons since they allow measuring the trajectory and speed of the GNSS antenna with centimeter accuracy. However, the antenna cannot be placed exactly at the person’s CoM, but rather on the head or upper back. Thus, a model is needed to relate the measured antenna trajectory to the CoM trajectory. In this paper we propose to estimate the person’s posture based on measurements obtained from inertial sensors. From this estimated posture the CoM is computed relative to the antenna position and finally fused with the GNSS trajectory information to obtain the absolute CoM trajectory. In a biomechanical field experiment, the method has been applied to alpine ski racing and validated against a camera-based stereo photogrammetric system. CoM position accuracy and precision was found to be 0.08 m and 0.04 m, respectively. CoM speed accuracy and precision was 0.04 m/s and 0.14 m/s, respectively. The observed accuracy and precision might be sufficient for measuring performance- or equipment-related trajectory differences in alpine ski racing. Moreover, the CoM estimation was not based on a movement-specific model and could be used for other skiing disciplines or sports as well

An effortless procedure to align the local frame of an inertial measurement unit to the local frame of another motion capture system

Inertial measurement units (IMUs) offer great opportunities to analyze segmental and joints kinematics. When combined with another motion capture system (MCS), for example, to validate new IMU-based applications or to develop mixed systems, it is necessary to align the local frame of the IMU sensors to the local frame of the MCS. Currently, all alignment methods use landmarks on the IMU's casing. Therefore, they can only be used with well-documented IMUs and they are prone to error when the IMU's casing is small. This study proposes an effortless procedure to align the local frame of any IMU to the local frame of any other MCS able to measure the orientation of its local frame. The general concept of this method is to derive the gyroscopic angles for both devices during an alignment movement, and then to use an optimization algorithm to calculate the alignment matrix between both local frames. The alignment movement consists of rotations around three more or less orthogonal axes and it can easily be performed by hands. To test the alignment procedure, an IMU and a magnetic marker were attached to a plate, and 20 alignment movements were recorded. The maximum errors of alignment (accuracy ± precision) were 1.02° ± 0.32° and simulations showed that the method was robust against noise that typically affect IMUs. In conclusion, this study describes an efficient alignment procedure that is quick and easy to perform, and that does not require any alignment device or any knowledge about the IMU casin

Determination of External Forces in Alpine Skiing Using a Differential Global Navigation Satellite System

In alpine ski racing the relationships between skier kinetics and kinematics and their effect on performance and injury-related aspects are not well understood. There is currently no validated system to determine all external forces simultaneously acting on skiers, particularly under race conditions and throughout entire races. To address the problem, this study proposes and assesses a method for determining skier kinetics with a single lightweight differential global navigation satellite system (dGNSS). The dGNSS kinetic method was compared to a reference system for six skiers and two turns each. The pattern differences obtained between the measurement systems (offset ± SD) were −26 ± 152 N for the ground reaction force, 1 ± 96 N for ski friction and −6 ± 6 N for the air drag force. The differences between turn means were small. The error pattern within the dGNSS kinetic method was highly repeatable and precision was therefore good (SD within system: 63 N ground reaction force, 42 N friction force and 7 N air drag force) allowing instantaneous relative comparisons and identification of discriminative meaningful changes. The method is therefore highly valid in assessing relative differences between skiers in the same turn, as well as turn means between different turns. The system is suitable to measure large capture volumes under race conditions