APPLIED BIOMECHANICS IN ALPINE SKIING-PAST, PRESENT AND FUTURE ISSUES

Biomechanics in alpine skiing has always been studied from an applied point of view. Among others biomechanical studies in alpine skiing can be categorised into the groups (1) biomechanical description and functional aspects of alpine skiing, (2) biomechanical analysis and determination of parameters related to pertormance in alpine skiing, (3) biomechanical testing, training and imitation exercises, (4) development, optimisation and tuning of the equipment, (5) modelling and simulation and (6) loading on the musculo-skeletal system and injuries in alpine skiing. This paper provides a literature review on biomechanics in alpine skiing with respect to these categorie

Robust Algorithms for Pitch Detection and Parameter Estimation

In a lot of applications signals recorded via a measurement system are analyzed to deeply understand the underlying process and infer unknown parameters. Often, the recorded signal is periodic, and the period is the parameter of major interest. Two typical examples are condition monitoring for non-destructive testing and electrocardiogram signal processing for diagnosis of heart diseases. In all real-world applications, random noise is prevalent and unavoidable. The strength of the noise depends on the measurement system itself and on the environment. Especially in heavy industry applications, e.g., environmental influences cause large outlying events of high strength. If neglected, these outlier can have a massive impact on standard estimation procedures like the principle of least squares. In this thesis we consider a typical periodic signal together with measurement noise and with outliers as it might occur in heavy industry applications. The goal of this thesis is to estimate the period of such signals. Three approaches will be discussed, namely maximum likelihood estimation, the approximate Bayesian computation approach and last but not least a cross-correlation approach. For maximum likelihood estimation, in principle no prior knowledge of the signals shape necessary for processing. However it will be shown that it is highly beneficial to take prior knowledge regarding the signals shape into account. This leads to a major improvement with respect to the robustness against outliers. Prior knowledge of the signals shape turns out to be highly beneficial for the approximate Bayesian computation and the cross-correlation approach, too. We will also discuss two preprocessing techniques that remove the outliers before the actual estimator is applied. This enables a computationally efficient application of known standard estimators in the presence of outliers. To examine the effect of the preprocessing techniques and to finally compare all estimation techniques the results with and without preprocessing are compared for every estimation approach for different example scenarios.submitted by Lukas SchiefermüllerUniversität Linz, Masterarbeit, 2020(VLID)504116

Collecting Kinematic Data on a Ski Track with Optoelectronic Stereophotogrammetry: A Methodological Study Assessing the Feasibility of Bringing the Biomechanics Lab to the Field

<div><p>In the laboratory, optoelectronic stereophotogrammetry is one of the most commonly used motion capture systems; particularly, when position- or orientation-related analyses of human movements are intended. However, for many applied research questions, field experiments are indispensable, and it is not a priori clear whether optoelectronic stereophotogrammetric systems can be expected to perform similarly to in-lab experiments. This study aimed to assess the instrumental errors of kinematic data collected on a ski track using optoelectronic stereophotogrammetry, and to investigate the magnitudes of additional skiing-specific errors and soft tissue/suit artifacts. During a field experiment, the kinematic data of different static and dynamic tasks were captured by the use of 24 infrared-cameras. The distances between three passive markers attached to a rigid bar were stereophotogrammetrically reconstructed and, subsequently, were compared to the manufacturer-specified exact values. While at rest or skiing at low speed, the optoelectronic stereophotogrammetric system’s accuracy and precision for determining inter-marker distances were found to be comparable to those known for in-lab experiments (< 1 mm). However, when measuring a skier’s kinematics under “typical” skiing conditions (i.e., high speeds, inclined/angulated postures and moderate snow spraying), additional errors were found to occur for distances between equipment-fixed markers (total measurement errors: 2.3 ± 2.2 mm). Moreover, for distances between skin-fixed markers, such as the anterior hip markers, additional artifacts were observed (total measurement errors: 8.3 ± 7.1 mm). In summary, these values can be considered sufficient for the detection of meaningful position- or orientation-related differences in alpine skiing. However, it must be emphasized that the use of optoelectronic stereophotogrammetry on a ski track is seriously constrained by limited practical usability, small-sized capture volumes and the occurrence of extensive snow spraying (which results in marker obscuration). The latter limitation possibly might be overcome by the use of more sophisticated cluster-based marker sets.</p></div

Skier equipped with the <i>PLUG-IN-GAIT</i> marker-set and additional markers on the skis and poles.

<p>Top: body and pole marker placement. Middle: equipment used with ski marker placement. Bottom: reconstructed 3D-model.</p

Accuracy, precision, maximum instrumental error and coefficient of variation (CV) values for the measurement of the <i>VICON</i> standard wand marker distances a, b and c using an optoelectronic system.

<p>Accuracy, precision, maximum instrumental error and coefficient of variation (CV) values for the measurement of the <i>VICON</i> standard wand marker distances a, b and c using an optoelectronic system.</p

Mean, SD, maximum and coefficient of variation (CV) values of the absolute differences between the stereophotogrammetrically reconstructed, and directly measured distances between markers with equipment and skin fixation using an optoelectronic system.

<p>Mean, SD, maximum and coefficient of variation (CV) values of the absolute differences between the stereophotogrammetrically reconstructed, and directly measured distances between markers with equipment and skin fixation using an optoelectronic system.</p

Overview of the measurement-setup.

<p>Top: <i>VICON</i> camera set-setup. Bottom: schematic drawing of the on-hill measurement setup.</p