Robust Feature Representation Using Multi-Task Learning for Human Activity Recognition.

Learning underlying patterns from sensory data is crucial in the Human Activity Recognition (HAR) task to avoid poor generalization when coping with unseen data. A key solution to such an issue is representation learning, which becomes essential when input signals contain activities with similar patterns or when patterns generated by different subjects for the same activity vary. To address these issues, we seek a solution to increase generalization by learning the underlying factors of each sensor signal. We develop a novel multi-channel asymmetric auto-encoder to recreate input signals precisely and extract indicative unsupervised futures. Further, we investigate the role of various activation functions in signal reconstruction to ensure the model preserves the patterns of each activity in the output. Our main contribution is that we propose a multi-task learning model to enhance representation learning through shared layers between signal reconstruction and the HAR task to improve the robustness of the model in coping with users not included in the training phase. The proposed model learns shared features between different tasks that are indeed the underlying factors of each input signal. We validate our multi-task learning model using several publicly available HAR datasets, UCI-HAR, MHealth, PAMAP2, and USC-HAD, and an in-house alpine skiing dataset collected in the wild, where our model achieved 99%, 99%, 95%, 88%, and 92% accuracy. Our proposed method shows consistent performance and good generalization on all the datasets compared to the state of the art.

Ski-geometric parameters do not differ between ACL injury mechanisms in recreational alpine skiing.

PURPOSE: It is not known so far if ski-equipment-related factors differ between the ACL injury mechanisms, potentially influencing the circumstances and causes of falling, finally resulting in ACL injury. More specifically focusing on the injury mechanisms will provide a deeper understanding of injury causation. The aim of the study was to evaluate whether ACL injury mechanisms in recreational alpine skiing differ with regard to ski-geometric parameters, self-reported circumstances and causes of accident and injury severity. METHODS: Among a cohort of 392 ACL-injured (57.9% females) skiers, age, sex, height, weight, skill level, risk-taking behavior, circumstances and causes of accident, and ACL injury severity were collected by questionnaire. Additionally, patients had to recall their type of fall (ACL injury mechanism) by classifying forward and backward falls with and without body rotation. Ski length, side cut radius and widths of the tip, waist and tail were directly notated from the ski. RESULTS: The forward fall with body rotation was the most common reported ACL injury mechanism (63%). A riskier behavior was associated with forward falls without body rotation. Ski-geometric parameters did not significantly influence the type of ACL injury mechanism. Regarding accident characteristics, catching an edge of the ski was more frequent (p < 0.001) the cause for forward falls (75% and 67%) when compared to the backward falls (46 and 15%) and executing a turn was the most frequent action in all falls (39-68%). A complete rupture of the ACL (66-70%) was more commonly reported than a partial tear (30-34%) among all four non-contact ACL injury mechanisms (n.s.). CONCLUSION: In contrast to risk-taking behavior and accident characteristics, ski-geometric parameters and injury severity do not significantly differ between ACL injury mechanisms in recreational skiing. Thus, an individual skiing style seems to have more impact on ACL injury mechanisms than ski equipment. Future studies should evaluate potential effects of ski geometry on the incidence of ACL injury. LEVEL OF EVIDENCE: III.

Proposal of an Alpine Skiing Kinematic Analysis with the Aid of Miniaturized Monitoring Sensors, a Pilot Study.

The recent growth and spread of smart sensor technologies make these connected devices suitable for diagnostic and monitoring in different fields. In particular, these sensors are useful in diagnostics for control of diseases or during rehabilitation. They are also extensively used in the monitoring field, both by non-expert and expert users, to monitor health status and progress during a sports activity. For athletes, these devices could be used to control and enhance their performance. This development has led to the realization of miniaturized sensors that are wearable during different sporting activities without interfering with the movements of the athlete. The use of these sensors, during training or racing, opens new frontiers for the understanding of motions and causes of injuries. This pilot study introduced a motion analysis system to monitor Alpine ski activities during training sessions. Through five inertial measurement units (IMUs), placed on five points of the athletes, it is possible to compute the angle of each joint and evaluate the ski run. Comparing the IMU data, firstly, with a video and then proposing them to an expert coach, it is possible to observe from the data the same mistakes visible in the camera. The aim of this work is to find a tool to support ski coaches during training sessions. Since the evaluation of athletes is now mainly developed with the support of video, we evaluate the use of IMUs to support the evaluation of the coach with more precise data.

Alpine Skiing Activity Recognition Using Smartphone's IMUs.

Many studies on alpine skiing are limited to a few gates or collected data in controlled conditions. In contrast, it is more functional to have a sensor setup and a fast algorithm that can work in any situation, collect data, and distinguish alpine skiing activities for further analysis. This study aims to detect alpine skiing activities via smartphone inertial measurement units (IMU) in an unsupervised manner that is feasible for daily use. Data of full skiing sessions from novice to expert skiers were collected in varied conditions using smartphone IMU. The recorded data is preprocessed and analyzed using unsupervised algorithms to distinguish skiing activities from the other possible activities during a day of skiing. We employed a windowing strategy to extract features from different combinations of window size and sliding rate. To reduce the dimensionality of extracted features, we used Principal Component Analysis. Three unsupervised techniques were examined and compared: KMeans, Ward's methods, and Gaussian Mixture Model. The results show that unsupervised learning can detect alpine skiing activities accurately independent of skiers' skill level in any condition. Among the studied methods and settings, the best model had 99.25% accuracy.

"Being" an Older Adult Skier: The Phenomenology of Masters Alpine Ski Racers.

This research examined the lived experience of older adult Alpine skiers in their continued participation in competitive ski racing. The aim was to gain an understanding of the meaning of the experiences for older adults as they continued to compete in Alpine ski racing. Masters skiers between the ages of 69 and 82 years participated in individual interviews. Data collection and analysis were iterative processes informed by phenomenological methods and visual mind mapping. The overarching theme identified in participants' accounts was the significance of the identity of "Being" a Ski Racer. Supporting subthemes were Enjoying the Feeling of Ski Racing, Adapting in the Sport, and Skiing as a Lifestyle. These insights offer the opportunity to encourage and support the holistic experiences of older adults who maintain their athletic identity as they age. This research highlights how inclusion of older adults' stories may foster critical reflexivity and challenge assumptions about aging.

The Influence of Trunk Impairment Level on the Kinematic Characteristics of Alpine Sit-Skiing: A Case Study of Paralympic Medalists.

This study aimed to examine the relationship between the trunk impairment level and the trunk kinematic characteristics during alpine sit-skiing from a classification perspective. Three Paralympic medalists in sitting classes (LW10-2, LW11, and LW12-2) participated in the present study. To simulate the racing conditions, giant slalom gates were set. To measure the kinematics of the skier and sit-ski during skiing, a motion capture method with inertial measurement units was used. The muscle activities of the trunk muscles were evaluated using electromyography. Chest lateral flexion, chest flexion, and hip flexion/extension angle during sit-skiing were reduced due to impairment. Additionally, the insufficient lateral flexion (angulation) caused a decrease in edging angle, and that the insufficient chest and hip flexion/extension caused a lower loading in the latter half of the turn through smaller vertical movement. Since edging angle and loading are key factors in ski control, the three joint motions could be measures of sport-specific activity limitation in sit-skiing classification. Between the LW10-2 and LW11 skiers, no distinct differences in trunk kinematics were found. Assuming the scaling factor of race time as a measure of skiing performance, one possible reason is that the difference in skiing performance the LW10-2 and LW11 skiers is considerably smaller relative to differences between the LW11 and LW12-2 skiers. There were no distinct differences among classes in the results of muscle activity, and therefore, this information appears to play a minimal role for classification.

Lower limb force-production capacities in alpine skiing disciplines.

Specific force capacities might be a limiting factor for alpine skiing performance, yet there is little consensus on the capabilities in question, and whether they differ between disciplines. We aimed to test discipline (speed and technical) and performance (event-specific world standing) effects on lower limb force-production qualities. National-level skiers (N = 31) performed loaded squat jumps and isometric mid-thigh pulls to detect dynamic force output at extremely low and high velocities and maximum isometric force and rate of force development, respectively. Discipline differences were assessed via a general linear model including performance and allowing for interaction effects, with performance associations further characterized via distinct Pearson's correlations. Jump height did not differentiate disciplines, with absolute power slightly higher in speed athletes (F(1,27)  = 4.42, P = .045, ω2  = 0.10), and neither variables were related to performance. Speed athletes possessed greater dynamic force at low velocities (F0 ; F(1,27)  = 13.8, P < .001, ω2  = 0.17), and greater relative and absolute maximum isometric force (F(1,25)  = 11.19-20.70, ω2  = 0.16-0.22, P < .003). Overall, higher ranked athletes possessed more force-dominant profiles (F(1,27)  = 16.28, ω2  = 0.34; r = 0.60 to 0.67, P < .001) and increased rate of force development characteristics (average and maximum, r = -0.50 to -0.82, P < .048). Very robust associations existed between maximum isometric force and speed performance (r = -0.88, P < .001), but only a trend for higher absolute isometric force in technical athletes (r = -0.49, P = .052). Alpine skiers display a preponderance for dynamic force output at low velocities, and isometric force for speed athletes, which highlights the interest in specific assessment and conditioning practices for ski athletes.

The effect of countermovement on force production capacity depends on extension velocity: A study of alpine skiers and sprinters.

In jumping, countermovement increases net propulsive force and improves performance. We aimed to test whether this countermovement effect is velocity specific and examine the degree to which this varies between athletes, sports or performance levels. Force-velocity profiles were compiled in high-level skiers (N= 23) and sprinters (N= 30), with their performance represented in their overall world ranking and season-best 100 m time, respectively. Different ratios between force-velocity variables were computed from squat and countermovement jumps (smaller = less effect): jump height (CRh), maximum power (CRP), force (CRF), and velocity (CRv). Countermovement effect differed per velocity (inverse relationship between CRF and CRv, rs = -0.74, p< .001), and variation force-velocity profiles with countermovement. Skiers exhibited smaller CRF (rrb = -0.675, p< .001), sprinters smaller CRv (rrb = 0.426, p= .008), and "moderate" velocity conditions did not differentiate groups (CRP or CRh, p> .05). 33% of the variance in skiers' performance level was explained by greater maximum force and a lower CRF (i.e., high explosiveness at low-velocities without countermovement), without an association for sprinters. Countermovement effect appears specific to movement velocity, sport and athlete level. Consequently, we advise sports-specific assessment, and potentially training to reduce the countermovement effect per the relevant velocity.

Exploration of the minimum visual disability criteria for Para nordic and Para alpine skiing using simulated vision impairments.

The International Paralympic Committee Classification Code requires sports to develop evidence-based, sports-specific classification systems. This project aimed to determine the minimum eligibility criteria for Para nordic and Para alpine skiing by simulating vision impairments and measuring the impact of the impairments on performance in twenty-two nordic (28.09 ± 9.68 years; 16 male) and eleven alpine (37.91 ± 18.9 years, 11 male) able-sighted skiers. Eight visual acuity (VA)/contrast sensitivity (CS) (Cambridge Simulation Glasses, University of Cambridge) and six visual field (VF) impairments (bespoke goggles; University of Waterloo) were simulated. VA, CS and VF were measured in each participant before they completed an on-snow session, skiing short competition-style courses with each of the 14 simulated impairments in a randomized order. Clear goggle (no impairment) trials were used as controls. Receiver Operating Characteristic (ROC) and decision tree analyses were conducted to determine the optimum VA, CS and VF cut-offs for classifying performance based on differences from baseline in real time. Moderate impairments in VA, CS and VF negatively affect skiing performance. The recommended cut-off criteria based on both analyses were VA ≥0.90 logMAR and ≤48% VF extent for nordic and VA of ≥0.60 logMAR and VF extent of ≤59.0% for alpine.

In recreational alpine skiing, the ACL is predominantly injured in all knee injuries needing hospitalisation.

PURPOSE: The knee joint still represents the most frequent anatomical injury location accounting for about one-third of all injuries in recreational alpine skiers. However, comprehensive information on current knee injury patterns in this populations is sparse. METHODS: During the winter seasons 2016/17 and 2019/20, this retrospective questionnaire-based study was conducted in an Austrian sportclinic situated in a large ski area. Among a cohort of 282 recreational skiers (51.8% females), all injuries were diagnosed by the use of magnetic resonance imaging. Additionally, data were recorded on anthropometric characteristics, the perceived speed at the moment of injury, type of fall, physical fitness, self-reported skill level and risk-taking behaviour. RESULTS: The anterior cruciate ligament (ACL) was injured in all knee injuries recorded. Of the total study sample, 64.5% (n = 182) were ACL injuries with concomitant injuries and about 35.5% (n = 100) were isolated ACL injuries, not involving any other structures of the knee joint. In general, most common concomitant injury diagnoses among ACL-injured recreational alpine skiers were injuries of the medial collateral ligament (MCL) (n = 92, 50.5%), medial meniscus (MM) (n = 73, 40.1%) and lateral collateral ligament (LCL) (n = 41, 22.5%). No significant differences regarding additionally recorded characteristics were found between ACL-injured individuals with concomitant injuries and those with isolated ACL injury. CONCLUSIONS: Whereas, before the introduction of carving skis, the MCL was reported being the most common injured part of the knee, currently, the majority of knee injuries are ACL injuries accompanied by injury of other knee joint structures, i.e. the MCL, MM and LCL. LEVEL OF EVIDENCE: Level III.

Development and Evaluation of a Low-Drift Inertial Sensor-Based System for Analysis of Alpine Skiing Performance.

In skiing it is important to know how the skier accelerates and inclines the skis during the turn to avoid injuries and improve technique. The purpose of this pilot study with three participants was to develop and evaluate a compact, wireless, and low-cost system for detecting the inclination and acceleration of skis in the field based on inertial measurement units (IMU). To that end, a commercial IMU board was placed on each ski behind the skier boot. With the use of an attitude and heading reference system algorithm included in the sensor board, the orientation and attitude data of the skis were obtained (roll, pitch, and yaw) by IMU sensor data fusion. Results demonstrate that the proposed IMU-based system can provide reliable low-drifted data up to 11 min of continuous usage in the worst case. Inertial angle data from the IMU-based system were compared with the data collected by a video-based 3D-kinematic reference system to evaluate its operation in terms of data correlation and system performance. Correlation coefficients between 0.889 (roll) and 0.991 (yaw) were obtained. Mean biases from -1.13° (roll) to 0.44° (yaw) and 95% limits of agreements from 2.87° (yaw) to 6.27° (roll) were calculated for the 1-min trials. Although low mean biases were achieved, some limitations arose in the system precision for pitch and roll estimations that could be due to the low sampling rate allowed by the sensor data fusion algorithm and the initial zeroing of the gyroscope.

Are Schatzker and AO classifications accurate enough to classify tibial plateau fractures in alpine skiers?

PURPOSE: The aims of this study were (1) to collect prospectively all tibial plateau fractures admitted to our department, over two ski seasons, and to classify them according to the Schatzker and AO classifications; (2) to assess if these classifications are accurate enough to include all types of fractures; and (3) to compare theses fractures with the series found in the literature, which included very few or no skiing accidents. METHODS: During the 2016-2017 and 2017-2018 ski seasons, we prospectively included 116 tibial plateau fractures caused by downhill skiing accidents. All patients underwent standard X-rays and 2D and 3D CT scans. The fractures were classified according to the AO and Schatzker revisited classifications. RESULTS: The full series consisted of 56 males (48%) and 60 females (52%), aged 49 ± 16 years (18-77). There were 60 type B (52%) and 56 type C fractures (48%) for AO classification and 45.5% types I, II and III and 54.5% types IV, V and VI for Schatzker classification. Thirty-five frontal fractures (30%) were not differentiated under the AO classification and, likewise, associated tibial spine fractures (28.5%) were not differentiated in the Schatzker classification. We were also unable to classify anterior tibial tuberosity fractures (14.5%) and fibula head fractures (8%). The anatomo-pathological types were not so different from road traffic accidents. CONCLUSION: Contrary to our hypothesis, the anatomical-pathological damage in tibial plateau fractures resulting from downhill skiing accidents was barely any different from those found in road traffic accidents. However, despite progress in classifications with the emergence of 3D CT scans, it is still not always possible to categorise all fractures within a given classification.

Dental trauma and tongue injuries in professional alpine ski racing-A worldwide survey.

BACKGROUND/AIM: Skiing is a sport with a medium risk of injuries, and injuries are increasingly common among professional alpine skiers. Examples of other medium-risk sports are for example handball, basketball, and karate. The aim of this study was to assess the frequency of dental trauma in professional alpine skiing and to determine whether the frequency of dental injuries is associated with a skier's performance level, alpine skiing discipline, and/or years of skiing experience. MATERIAL AND METHODS: A questionnaire consisting of 17 questions was distributed to professional male ski racers to gather information about their skiing discipline (category), performance level, and injuries, particularly dental trauma. RESULTS: A total of 161 out of 190 skiers returned the survey, with a response rate of 84.2%. Of these, 134 (83.2%) had suffered alpine ski racing-related injuries during their career and 38 (23.6%) reported skiing-related dental trauma-most commonly crown fractures (65.8%, n = 25), which mainly involved the maxillary or mandibular incisors (76.0%, n = 28). Ski racers with higher performance levels were more likely to experience dental injuries, but the difference in the frequency of dental trauma between participants in the speed versus technical category was not significant. Likewise, the number of years of ski racing experience did not impact the frequency of dental injuries. The participants rarely wore custom-made mouthguards (6.8%, n = 11). None of those who reported dental injuries were wearing a custom-made mouthguard when the injury occured. They preferred to wear chin guards, over-the-counter mouthguards, or no mouthguards. CONCLUSION: Professional alpine ski racing has a medium risk of dental trauma, which further increases with skier performance level. The participating skiers rarely wore custom-made mouthguards.

Muscle Activity and Morphology in Slalom Skiing by a Single-Leg Amputee Ski Racer: A Case Study of a Paralympic Athlete.

The aim of this study was to clarify the characteristics of skiing by a single-leg amputee ski racer from the viewpoints of muscle activity, morphology, and the relationship between both elements through comparisons with those of a non-disabled ski racer. One elite athlete, classified as LW2 (left thigh amputation), and one non-disabled athlete, as a control, participated in this study. The cross-sectional area of thigh muscles was measured through magnetic resonance imaging. Additionally, muscle activities and joint and segment kinematics during slalom skiing were measured using electromyography and inertial measurement units, respectively. The muscle activities and joint kinematics of the amputee racer in the turn in which he performed with the inside edge of the ski were similar to those of the outside leg of the non-disabled racer over a turn. In contrast, at the turn in which the amputee racer performed with the outside edge (more difficult side), the amputee racer largely activated the biceps femoris (BF) in the first half of the turn compared to the non-disabled racer. The reason could be to control the angular momentum of the trunk during the forward tilting motion. This is because a greater activity of the BF was observed during the period in which the forward tilt of the trunk was increased, and the mean activity of the BF was the greatest during the first half of the right turn in which the range of the motion of the forward tilt was the greatest. In terms of muscle morphology of the amputee racer, a significant hypertrophy of the BF and vastus lateralis was observed compared to the non-disabled racers. The well-developed BF was considered to be related to the large activity during the turn performed with the outside edge of the ski.

Acute on-snow severe injury events in elite alpine ski racing from 1997 to 2019: the Injury Surveillance System of the Austrian Ski Federation.

OBJECTIVES: The study aimed to (1) determine the incidence and gender-specific risk ratio of acute on-snow severe injury events (SIE) in elite alpine ski racing, (2) examine the development of SIE over 22 seasons, and (3) analyse SIE with respect to the severely injured body parts and structures. METHODS: Data recorded in the Austrian Ski Federation's Injury Surveillance System over 22 seasons were analysed. The Austrian Ski Team consists of four groups: Team National (n=477), Team A (n=444), Team B (n=696) and Team C (n=608). Team National and Team A comprised World Cup, Team B European Cup and Team C junior alpine ski racers. Simple and multiple Poisson regressions were calculated. RESULTS: The SIE incidence was 15.7 (95% CI 14.2 to 17.5) per 100 skier seasons. ACL injury events accounted for 70.8% of severe knee injury events and 48.6% of SIE. The incidence of severe ACL injury events was 7.6 (95% CI 6.6 to 8.9). Female World Cup alpine ski racers had a 1.65 times (95% CI 1.02 to 2.69) higher risk of severe ACL injury events than their male counterparts. CONCLUSION: The incidence of acute on-snow SIE in World Cup alpine ski racing was higher than previously reported. Despite various prevention efforts, the average seasonal incidence of SIE in World and European Cup alpine ski racers has grown from approximately 11 in 1997 to 23 in 2019; thus with roughly one more injured athlete every second season.

Preventing injuries in alpine skiing giant slalom by shortening the vertical distance between the gates rather than increasing the horizontal gate offset to control speed.

BACKGROUND/AIM: To set a safe giant slalom course, speed needs to be controlled in certain sections. Speed may be reduced by adjusting how the gates are set on a course. We studied the effect of elements of course-setting, entrance speed and terrain incline on the mechanics of turning (ie, turn speed, turn radius, and ground reaction force and impulse). METHODS: During seven World Cup alpine giant slalom competitions, the course and terrain characteristics of the official racetracks and the mechanics of a professional-level athlete skiing the course immediately prior to competition were analysed with differential global navigation satellite system technology. Data were analysed using a linear mixed-effects model. RESULTS: Course-setting geometry (vertical gate distance and horizontal gate offset), entrance speed and terrain incline modulated the injury-relevant factor turn speed. Depending on the terrain, the speed throughout a turn can be reduced by 0.5 m/s either by shortening the vertical gate distance by 4.9-6.9 m (from -20% to -29%) or by increasing the horizontal gate offset by 2.8-3.2 m (from +33% to +55%). However, increasing the horizontal gate offset causes the skier to turn with a smaller minimal turn radius, increase maximal ground reaction force and also increase impulse. DISCUSSION: To reduce speed, we recommend decreasing the vertical gate distance rather than increasing the horizontal gate offset. Increasing horizontal gate offset would require the skiers to sharpen and prolong their turns (reducing turn radius), and this increases the acting ground reaction force and impulse and thus the athlete's fatigue.

Acute Effects of Winter Sports and Indoor Cycling on Arterial Stiffness.

Sedentary lifestyle predisposes to endothelial dysfunction, increased arterial stiffness and cardiovascular diseases, all of which can be positively modified by regular physical exercise training. A decrease in physical activity during winter months coincides with higher rates of cardiovascular events. In order to identify winter sports suitable to overcome this seasonal exercise deficit and thus contribute to cardiovascular health, it was the aim of this study to compare immediate effects of cross-country skiing (XCS) and alpine skiing (AS) on arterial stiffness as an alternative to indoor cycling (IC). After baseline assessment, eighteen healthy subjects performed one session of XCS, AS, and IC in randomized order. Pulse wave analysis was conducted (Mobil-o-Graph®) before and 10-min after exercise. Parameters of arterial stiffness and wave reflection were reduced after XCS and IC, but not after AS: central systolic blood pressure (IC: -8.0 ± 5.4 mmHg; p < 0.001), amplitude of the backward pressure wave (IC: -1.4 ± 2.7 mmHg; p < 0.05), reflection coefficient (XCS: -6.0 ± 7.8%; IC: -5.7 ± 8.1%; both p < 0.1), and pulse wave velocity (IC by -0.19 ± 0.27 m/s; p < 0.01). Higher exercise intensities correlated with greater reductions of arterial stiffness (all p < 0.05). Single sessions of XCS, IC but not AS led to comparable improvement in arterial stiffness, which was even more pronounced during higher exercise intensities. With regard to arterial stiffness, IC and XCS emerge as more effective to counteract the winter exercise deficit and thus the deleterious cardiovascular effects of a sedentary lifestyle.

Is ski boot sole abrasion a potential ACL injury risk factor for male and female recreational skiers?

OBJECTIVES: To evaluate the potential impact of ski boot sole abrasion on the ACL injury risk of recreational skiers. METHODS: During the past two winter seasons 2016/17 and 2017/18, this retrospective case-control study was conducted in one Austrian ski area. Among a cohort of 148 ACL-injured (51.4% females) and 455 uninjured recreational skiers (43.3% females), age, sex, height, weight, and self-reported skill level were collected by questionnaire, ski length and sidecut radius were notated and sole abrasion of the toe and heel piece of the ski boot was measured using a digital caliper. RESULTS: ACL-injured skiers showed a higher proportion of female (51.4% vs 43.3%, P < 0.001) and less skilled skiers (48.6% vs 20.9%, P < 0.001), and ski length to height ratio was higher (94.7 ± 3.7 vs 93.8 ± 5.0%, P = 0.019) compared to uninjured skiers. ACL-injured skiers used ski boots of greater abrasion at the toe (4.8 ± 1.8 vs 2.4 ± 2.5 mm, P < 0.001) and heel piece (5.4 ± 1.8 vs 3.3 ± 2.3 mm, P < 0.001) compared to controls. Multivariate regression analysis revealed, beside female sex (OR 6.0, 95% CI, 3.1-11.5, P < 0.001), lower skill level (OR 3.2, 95% CI, 1.9-5.4, P < 0.001) and ski length to height ratio (OR 1.1, 95% CI, 1.0-1.2, P < 0.001), sole abrasion at the toe (OR 1.8, 95% CI, 1.5-2.1, P < 0.001) and heel piece (OR 1.4, 95% CI, 1.2-1.6, P < 0.001) to be independently associated with an ACL injury among recreational alpine skiers. CONCLUSIONS: Based on the underlying findings, ski boot sole abrasion was found to be an independent risk factor and may contribute to an increased ACL injury risk.

Development of a New Embedded Dynamometer for the Measurement of Forces and Torques at the Ski-Binding Interface.

In alpine skiing, understanding the interaction between skiers and snow is of primary importance for both injury prevention as well as performance analysis. Risk of injuries is directly linked to constraints undergone by the skier. A force platform placed as an interface between the ski and the skier should allow a better understanding of these constraints to be obtained to thereby develop a more reliable release system of binding. It should also provide useful information to allow for better physical condition training of athletes and non-professional skiers to reduce the risk of injury. Force and torque measurements also allow for a better understanding of the skiers' technique (i.e., load evolution during turns, force distribution between left and right leg…). Therefore, the aim of this project was to develop a new embedded force platform that could be placed between the ski boot and the binding. First, the physical specifications of the dynamometer are listed as well as the measurement scope. Then, several iterations were performed on parametric 3D modeling and finite element analysis to obtain an optimal design. Two platforms were then machined and equipped with strain gauges. Finally, the calibration was performed on a dedicated test bench. The accuracy of the system was between 1.3 and 12.8% of the applied load. These results show a very good linearity of the system, which indicate a great outcome of the design. Field tests also highlighted the ease of use and reliability. This new dynamometer will allow skiers to wear their own equipment while measuring force and torque in real skiing conditions.

A Magnet-Based Timing Tystem to Detect Gate Crossings in Alpine Ski Racing.

In alpine skiing, intermediate times are usually measured with photocells. However, for practical reasons, the number of intermediate cells is limited to three⁻four, making a detailed timing analysis difficult. In this paper, we propose and validate a magnet-based timing system allowing for the measurement of intermediate times at each gate. Specially designed magnets were placed at each gate and the athletes wore small magnetometers on their lower back to measure the instantaneous magnetic field. The athlete's gate crossings caused peaks in the measured signal which could then be related to the precise instants of gate crossings. The system was validated against photocells placed at four gates of a slalom skiing course. Eight athletes skied the course twice and one run per athlete was included in the validation study. The 95% error intervals for gate-to-gate timing and section times were below 0.025 s. Each athlete's gate-to-gate times were compared to the group's average gate-to-gate times, revealing small performance differences that would otherwise be difficult to measure with a traditional photocell-based system. The system could be used to identify the effect of tactical choices and athlete specific skiing skills on performance and could allow a more efficient and athlete-specific performance analysis and feedback.