Olympic Games during nationwide lockdown: sports injuries and illnesses, including COVID-19, at the Beijing 2022 Winter Olympics.

OBJECTIVE: The objective of this study is to describe the incidence of injuries and illnesses sustained during the Beijing Winter Olympic Games from 4 February 2022 to 20 February 2022. METHODS: We recorded the daily number of athlete injuries and illnesses (1) through the reporting of all National Olympic Committee (NOC) medical teams and (2) in the polyclinic and medical venues by the Beijing 2022 medical staff. RESULTS: In total, 2848 athletes (1276 women, 45%; 1572 men, 55%) from 91 NOCs were followed prospectively for the occurrence of injury and illness. NOC and Beijing 2022 medical staff reported 289 injuries and 109 illnesses, equalling 10.1 injuries and 3.8 illnesses per 100 athletes over the 17-day period. The injury incidence was highest in ski halfpipe (30%), ski big air (28%), snowboard slopestyle (23%) and ski slopestyle (22%), and lowest (1%-2%) in curling, alpine mixed team parallel slalom, Nordic combined and alpine super-G. The illness incidence was highest in ski aerials (10%), skeleton (8%), cross-country skiing (8%) and Nordic combined (7%). In the study period, COVID-19 affected 32 athletes, accounting for 29% of all illnesses affecting 1.1% of all athletes. CONCLUSION: Overall, 10% of the athletes incurred an injury and 4% an illness during the Beijing Winter Olympic Games. The incidence of illnesses overall, which was the lowest yet recorded in the Winter Olympic Games, and COVID-19 was mitigated through comprehensive countermeasures.

Kinematic Determination of the Aerial Phase in Ski Jumping.

The purpose of this study was to find a generic method to determine the aerial phase of ski jumping in which the athlete is in a steady gliding condition, commonly known as the 'stable flight' phase. The aerial phase of ski jumping was investigated from a physical point mass, rather than an athlete-action-centered perspective. An extensive data collection using a differential Global Navigation Satellite System (dGNSS) was carried out in four different hill sizes. A total of 93 jumps performed by 19 athletes of performance level, ranging from junior to World Cup, were measured. Based on our analysis, we propose a generic algorithm that identifies the stable flight based on steady glide aerodynamic conditions, independent of hill size and the performance level of the athletes. The steady gliding is defined as the condition in which the rate-of-change in the lift-to-drag-ratio (LD-ratio) varies within a narrow band-width described by a threshold τ. For this study using dGNSS, τ amounted to 0.01s-1, regardless of hill size and performance level. While the absolute value of τ may vary when measuring with other sensors, we argue that the methodology and algorithm proposed to find the start and end of a steady glide (stable flight) could be used in future studies as a generic definition and help clarify the communication of results and enable more precise comparisons between studies.

Differences in pacing pattern and sub-technique selection between young and adult competitive cross-country skiers.

The present study describes differences in pacing patterns and sub-technique selection in young compared to adult competitive cross-country skiers. Eleven young male skiers (YOS) (14.4 ± 0.5 years, V ˙ O2peak 63.9 ± 2.8 mL∙kg-1 ∙min-1 ) and eight adult male skiers (ADS) (22.6 ± 4.3 years, V ˙ O2peak 77.4 ± 4.4 mL∙kg-1 ∙min-1 ) performed a free technique rollerski time trial (TT) over a distance of 4.3 km (YOS) and 13.1 km (ADS) to simulate normal racing distances. A GNSS/IMU system was used to track position, speed, and classify sub-techniques. Skiing economy and V ˙ O2peak were measured on an additional day to calculate the relative oxygen demand ( V ˙ O2dem ) in 13 segments of the TT. YOS were slower than ADS in all types of terrain (mean speed difference of 13%), with differences for uphills of 19%, undulating terrain of 11% and downhills of 8% (all P < .05). The mean relative V ˙ O2dem tended to be higher for YOS compared to ADS (120% vs 112% of V ˙ O2peak , P = .09), and the difference was more pronounced in the initial four segments of the race (130% vs 110% of V ˙ O2peak , P < .01). YOS used more of the sub-technique Gear 2 (23 ± 7 vs 14 ± 4%), less Gear 3 (36 ± 7 vs 45 ± 5%), and had more frequent transitions between sub-techniques (18 ± 2 vs 15 ± 3 km-1 ) (all P < .05) than ADS. Over an age-related distance, young skiers tend to exhibit higher mean exercise intensity than adult elite skiers, with a more pronounced positive pacing pattern. Differences in physical ability affect speed and sub-technique selections, implying a need for differentiating technical training for different ages and levels.

Classification of Cross-Country Ski Skating Sub-Technique Can Be Automated Using Carrier-Phase Differential GNSS Measurements of the Head's Position.

Position-time tracking of athletes during a race can provide useful information about tactics and performance. However, carrier-phase differential global navigation satellite system (dGNSS)-based tracking, which is accurate to about 5 cm, might also allow for the extraction of variables reflecting an athlete's technique. Such variables include cycle length, cycle frequency, and choice of sub-technique. The aim of this study was to develop a dGNSS-based method for automated determination of sub-technique and cycle characteristics in cross-country ski skating. Sub-technique classification was achieved using a combination of hard decision rules and a neural network classifier (NNC) on position measurements from a head-mounted dGNSS antenna. The NNC was trained to classify the three main sub-techniques (G2-G4) using optical marker motion data of the head trajectory of six subjects during treadmill skiing. Hard decision rules, based on the head's sideways and vertical movement, were used to identify phases of turning, tucked position and G5 (skating without poles). Cycle length and duration were derived from the components of the head velocity vector. The classifier's performance was evaluated on two subjects during an in-field roller skiing test race by comparison with manual classification from video recordings. Classification accuracy was 92-97% for G2-G4, 32% for G5, 75% for turning, and 88% for tucked position. Cycle duration and cycle length had a root mean square (RMS) deviation of 2-3%, which was reduced to <1% when cycle duration and length were averaged over five cycles. In conclusion, accurate dGNSS measurements of the head's trajectory during cross-country skiing contain sufficient information to classify the three main skating sub-techniques and characterize cycle length and duration.

Performance Analysis in Ski Jumping with a Differential Global Navigation Satellite System and Video-Based Pose Estimation.

This study investigated the explanatory power of a sensor fusion of two complementary methods to explain performance and its underlying mechanisms in ski jumping. A differential Global Navigation Satellite System (dGNSS) and a markerless video-based pose estimation system (PosEst) were used to measure the kinematics and kinetics from the start of the in-run to the landing. The study had two aims; firstly, the agreement between the two methods was assessed using 16 jumps by athletes of national level from 5 m before the take-off to 20 m after, where the methods had spatial overlap. The comparison revealed a good agreement from 5 m after the take-off, within the uncertainty of the dGNSS (±0.05m). The second part of the study served as a proof of concept of the sensor fusion application, by showcasing the type of performance analysis the systems allows. Two ski jumps by the same ski jumper, with comparable external conditions, were chosen for the case study. The dGNSS was used to analyse the in-run and flight phase, while the PosEst system was used to analyse the take-off and the early flight phase. The proof-of-concept study showed that the methods are suitable to track the kinematic and kinetic characteristics that determine performance in ski jumping and their usability in both research and practice.

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.

Effect of ski geometry on aggressive ski behaviour and visual aesthetics: equipment designed to reduce risk of severe traumatic knee injuries in alpine giant slalom ski racing.

BACKGROUND/AIM: Aggressive ski-snow interaction is characterised by direct force transmission and difficulty of getting the ski off its edge once the ski is carving. This behaviour has been suggested to be a main contributor to severe knee injuries in giant slalom (GS). The aim of the current study was to provide a foundation for new equipment specifications in GS by considering two perspectives: Reducing the ski's aggressiveness for injury prevention and maintaining the external attractiveness of a ski racer's technique for spectators. METHODS: Three GS ski prototypes were defined based on theoretical considerations and were compared to a reference ski (Pref). Compared to Pref, all prototypes were constructed with reduced profile width and increased ski length. The construction radius (sidecut radius) of Pref was ≥ 27 m and was increased for the prototypes: 30 m (P30), 35 m (P35), and 40 m (P40). Seven World Cup level athletes performed GS runs on each of the three prototypes and Pref. Kinetic variables related to the ski-snow interaction were assessed to quantify the ski's aggressiveness. Additionally, 13 athletes evaluated their subjective perception of aggressiveness. 15 sports students rated several videotaped runs to assess external attractiveness. RESULTS: Kinetic variables quantifying the ski's aggressiveness showed decreased values for P35 and P40 compared to Pref and P30. Greater sidecut radius reduced subjectively perceived aggressiveness. External attractiveness was reduced for P40 only. CONCLUSIONS: This investigation revealed the following evaluation of the prototypes concerning injury prevention and external attractiveness: P30: no preventative gain, no loss in attractiveness; P35: substantial preventative gain, no significant loss in attractiveness; P40: highest preventative gain, significant loss in attractiveness.

Effect of ski geometry and standing height on kinetic energy: equipment designed to reduce risk of severe traumatic injuries in alpine downhill ski racing.

BACKGROUND: Injuries in downhill (DH) are often related to high speed and, therefore, to high energy and forces which are involved in injury situations. Yet to date, no study has investigated the effect of ski geometry and standing height on kinetic energy (EKIN) in DH. This knowledge would be essential to define appropriate equipment rules that have the potential to protect the athletes' health. METHODS: During a field experiment on an official World Cup DH course, 2 recently retired world class skiers skied on 5 different pairs of skis varying in width, length and standing height. Course characteristics, terrain and the skiers' centre of mass position were captured by a differential Global Navigational Satellite System-based methodology. EKIN, speed, ski-snow friction force (FF), ground reaction force (FGRF) and ski-snow friction coefficient (CoeffF) were calculated and analysed in dependency of the used skis. RESULTS: In the steep terrain, longer skis with reduced width and standing height significantly decreased average EKIN by ∼ 3%. Locally, even larger reductions of EKIN were observed (up to 7%). These local decreases in EKIN were mainly explainable by higher FF. Moreover, CoeffF differences seem of greater importance for explaining local FF differences than the differences in FGRF. CONCLUSIONS: Knowing that increased speed and EKIN likely lead to increased forces in fall/crash situations, the observed equipment-induced reduction in EKIN can be considered a reasonable measure to improve athlete safety, even though the achieved preventative gains are rather small and limited to steep terrain.

Sidecut radius and kinetic energy: equipment designed to reduce risk of severe traumatic knee injuries in alpine giant slalom ski racing.

BACKGROUND: Kinetic energy (Ekin) increases with speed by the power of 2 and is considered a major risk factor for injuries in alpine ski racing. There is no empirical knowledge about the effect of ski geometry on Ekin. Consequently, the aim of this study was to investigate the influence of sidecut radius on the progress of Ekin while skiing through a multigate section in giant slalom (GS). METHODS: 5 European-Cup level athletes skied on three different pairs of GS skis varying in sidecut radii (30, 35 and 40 m). Each athlete's position over time within a six gate section (including flat and steep terrain) was captured by the use of a differential Global Navigational Satellite System. Ekin, speed, time and path length were analysed for each pair of skis used. RESULTS: When using skis with greater sidecut radius, average Ekin was significantly lower over the entire six gate section, but not locally at every turn cycle. Particular decreases of Ekin were observed for both turns on the flat terrain, as well as for the turn at the terrain transition and the first turn on the steep terrain. The observed decreases in Ekin were found to be primarily explainable by increases in turn time. CONCLUSIONS: With respect to typical sport mechanisms that cause severe knee injuries, using skis with greater sidecut radius potentially provides additional injury preventative gain, particularly in specific areas within a run. However, this injury preventative gain during falls in GS should not be overestimated.

Sidecut radius and the mechanics of turning-equipment designed to reduce risk of severe traumatic knee injuries in alpine giant slalom ski racing.

BACKGROUND: There is limited empirical knowledge about the effect of ski geometry, particularly in the context of injury prevention in alpine ski racing. We investigated the effect of sidecut radius on biomechanical variables related to the mechanics of turning. METHODS: During a field experiment, six European Cup level athletes skied on three different pairs of giant slalom (GS) skis varying in sidecut radii (30 m, 35 m and 40 m). Using a video-based three-dimensional (3D) kinematic system, a 22-point body segment model of the athletes was reconstructed in 3D, and the variables ground reaction force, centre of mass (COM) speed, COM turn radius, ski turn radius, edge angle, fore/aft position and skid angle were calculated. RESULTS: While steering out of the fall line after gate passage, ground reaction force significantly differed between the 30 m and 40 m skis and between the 35 m and 40 m skis. These differences were mainly explainable by larger COM turn radii when skiing on the 40 m ski. During the same turn phase, significant differences in ski turn radius also were found, but there were no differences in edge angle, fore/aft position and skid angle. SUMMARY: The sidecut-induced reduction in ground reaction force and the sidecut-induced increase in centre of mass and ski turn radius observed in this study provides indirect evidence of reduced self-steering of the ski. Self-steering plays a central role in the mechanism of anterior cruciate ligament rupture in alpine ski racing.

Determination of the centre of mass kinematics in alpine skiing using differential global navigation satellite systems.

In the sport of alpine skiing, knowledge about the centre of mass (CoM) kinematics (i.e. position, velocity and acceleration) is essential to better understand both performance and injury. This study proposes a global navigation satellite system (GNSS)-based method to measure CoM kinematics without restriction of capture volume and with reasonable set-up and processing requirements. It combines the GNSS antenna position, terrain data and the accelerations acting on the skier in order to approximate the CoM location, velocity and acceleration. The validity of the method was assessed against a reference system (video-based 3D kinematics) over 12 turn cycles on a giant slalom skiing course. The mean (± s) position, velocity and acceleration differences between the CoM obtained from the GNSS and the reference system were 9 ± 12 cm, 0.08 ± 0.19 m · s(-1) and 0.22 ± 1.28 m · s(-2), respectively. The velocity and acceleration differences obtained were smaller than typical differences between the measures of several skiers on the same course observed in the literature, while the position differences were slightly larger than its discriminative meaningful change. The proposed method can therefore be interpreted to be technically valid and adequate for a variety of biomechanical research questions in the field of alpine skiing with certain limitations regarding position.

Mechanics of turning and jumping and skier speed are associated with injury risk in men's World Cup alpine skiing: a comparison between the competition disciplines.

BACKGROUND/AIM: In alpine ski racing, there is limited information about skiers' mechanical characteristics and their relation to injury risk, in particular for World Cup (WC) competitions. Hence, current findings from epidemiological and qualitative research cannot be linked to skiers' mechanics. This study was undertaken to investigate whether recently reported differences in numbers of injuries per 1000 runs for competition disciplines can be explained by differences in the skiers' mechanics. METHODS: During seven giant slalom, four super-G and five downhill WC competitions, mechanical characteristics of a forerunner were captured using differential global navigation satellite technology and a precise terrain surface model. Finally, the discipline-specific skiers' mechanics were compared with the respective number of injuries per hour skiing. RESULTS: While the number of injuries per hour skiing was approximately equal for all disciplines, kinetic energy, impulse, run time, turn radius and turn speed were significantly different and increased from giant slalom to super-G and downhill. Turn ground reaction forces were largest for giant slalom, followed by super-G and downhill. The number of jumps was doubled from super-G to downhill. CONCLUSIONS: Associating the number of injuries per hour in WC skiing with skiers' mechanical characteristics, injuries in super-G and downhill seem to be related to increased speed and jumps, while injuries in giant slalom may be related to high loads in turning. The reported differences in the number of injuries per 1000 runs might be explained by a bias in total exposure time per run and thus potentially by emerged fatigue.

The effect of different Global Navigation Satellite System methods on positioning accuracy in elite alpine skiing.

In sport science, Global Navigation Satellite Systems (GNSS) are frequently applied to capture athletes' position, velocity and acceleration. Application of GNSS includes a large range of different GNSS technologies and methods. To date no study has comprehensively compared the different GNSS methods applied. Therefore, the aim of the current study was to investigate the effect of differential and non-differential solutions, different satellite systems and different GNSS signal frequencies on position accuracy. Twelve alpine ski racers were equipped with high-end GNSS devices while performing runs on a giant slalom course. The skiers' GNSS antenna positions were calculated in three satellite signal obstruction conditions using five different GNSS methods. The GNSS antenna positions were compared to a video-based photogrammetric reference system over one turn and against the most valid GNSS method over the entire run. Furthermore, the time for acquisitioning differential GNSS solutions was assessed for four differential methods. The only GNSS method that consistently yielded sub-decimetre position accuracy in typical alpine skiing conditions was a differential method using American (GPS) and Russian (GLONASS) satellite systems and the satellite signal frequencies L1 and L2. Under conditions of minimal satellite signal obstruction, valid results were also achieved when either the satellite system GLONASS or the frequency L2 was dropped from the best configuration. All other methods failed to fulfill the accuracy requirements needed to detect relevant differences in the kinematics of alpine skiers, even in conditions favorable for GNSS measurements. The methods with good positioning accuracy had also the shortest times to compute differential solutions. This paper highlights the importance to choose appropriate methods to meet the accuracy requirements for sport applications.

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.

Methods to assess validity of positioning systems in team sports: can we do better?

Objectives: There is previously reported a large variety of criterion measures and reference systems applied to validate position tracking systems in sports. This study aims to investigate the effect of different criterion measures and reference systems on the outcome of accuracy assessments of tracking systems in sports. Methods: Data from a commercially available standalone global navigation satellite system (GNSS) were compared with two different reference systems: a high-end differential GNSS and a tape measure. Differences in accuracy outcomes of position (static and dynamic), distance and speed (mean and instantaneous) were investigated in team sport imitation courses. Results: The mean horizontal position error was larger when athletes were in motion (dynamic position; 1.53±0.82 m) compared with static measurements (1.10±0.60 m). Measured distances of the courses were markedly different (+6% to -17%) between the two reference systems, causing differences in error. Differences in error were also found between mean speed and instantaneous speed (0.10 vs 0.28 m). Errors in mean speed were highly affected by the time over which speed was averaged. Conclusion: Choice of criterion measure and reference system has a substantial impact on the accuracy assessments of tracking systems. Specifically, assessing static position is not a substitute for dynamic position, and mean speed is not a substitute for instantaneous speed. Therefore, the outcomes of validation studies should always be interpreted in light of the reference methods that were used.

Investigation of individual strategies in the aerial phase in ski jumping.

The purpose of this investigation was to examine the performance strategy of three ski jumpers during the steady glide phase and explain how different strategical solutions can lead to jumps of roughly the same length. In this study, a total of 24 jumps performed by two World Cup (WC) athletes and one Continental Cup (COC) athlete were measured with a differential Global Navigation Satellite System (dGNSS) on a large ski jumping hill. For each athlete, the continuous position data, velocity, aerodynamic forces and lift-to-drag ratio (LD-ratio) were averaged and compared for the steady glide phase to examine individual jump strategies. The dGNSS yielded accurate measurements of trajectory, velocity and aerodynamic forces, revealing clear differences between the athletes. The largest differences were found between the WC athletes and the COC athlete. The WC athletes focused on maximizing horizontal velocity while the COC athlete minimized vertical velocity. This difference may be explained by the different hill sizes the athletes normally compete on. One of the WC athletes consistently increased their horizontal velocity during the end of the steady glide phase by maintaining a high LD-ratio, which highlights the effect of aerodynamics on the resulting velocity, trajectory and jump length.

Assessment of the steady glide phase in ski jumping.

The purpose of this investigation was to compare how key variables of the steady glide phase relate to performance in the two hill sizes used in World Cup and Olympic competitions, i.e, normal and large hills. In this study, 38 and 33 jumps of elite ski jumpers were measured with a differential global navigation satellite system (dGNSS) on a normal (HS106) and large hill (HS140), respectively. For the steady glide phase, the average aerodynamic forces, lift-to-drag-ratio (LD-ratio), vertical and horizontal acceleration and velocity were measured and related to the jump distance as a performance outcome. The aerial time difference between the two hill sizes was 1.1s, explained by the time spent in the steady glide phase. The results for HS106 were in line with the assumptions in recent literature, which propose that the performance is largely determined by the take-off and glide preparation. Hence for normal hills, skiers should aim to reduce vertical acceleration through high aerodynamic forces during the glide phase. Also, no correlation was observed between the LD-ratio and jump length. The data from the large hill indicate that the performance during the steady glide is very important for performance; hence clear differences were found compared to the normal hill. On a large hill, the aim should be to minimize the horizontal deceleration by reducing the aerodynamic drag. A high LD-ratio was correlated to jump length for HS140 and seen to be one of the most important performance factors.

Ranking surgical skills using an attention-enhanced Siamese network with piecewise aggregated kinematic data.

PURPOSE: Surgical skill assessment using computerized methods is considered to be a promising direction in objective performance evaluation and expert training. In a typical architecture for computerized skill assessment, a classification system is asked to assign a query action to a predefined category that determines the surgical skill level. Since such systems are still trained by manual, potentially inconsistent annotations, an attempt to categorize the skill level can be biased by potentially scarce or skew training data. METHODS: We approach the skill assessment problem as a pairwise ranking task where we compare two input actions to identify better surgical performance. We propose a model that takes two kinematic motion data acquired from robot-assisted surgery sensors and report the probability of a query sample having a better skill than a reference one. The model is an attention-enhanced Siamese Long Short-Term Memory Network fed by piecewise aggregate approximation of kinematic data. RESULTS: The proposed model can achieve higher accuracy than existing models for pairwise ranking in a common dataset. It can also outperform existing regression models when applied in their experimental setup. The model is further shown to be accurate in individual progress monitoring with a new dataset, which will serve as a strong baseline. CONCLUSION: This relative assessment approach may overcome the limitations of having consistent annotations to define skill levels and provide a more interpretable means for objective skill assessment. Moreover, the model allows monitoring the skill development of individuals by comparing two activities at different time points.

Is there a contextual interference effect for sub-elite alpine ski racers learning complex skills?

Scientific understanding of the contextual interference effect stems mainly from studies on unskilled participants learning artificial laboratory tasks. Although one goal of such studies is to extrapolate the findings to include real-world learning situations such as sports, this generalization is not straightforward. This study tested the contextual interference effect with 66 sub-elite, competitive alpine ski racers who learned a new movement pattern-the pumping technique to increase velocity in slalom-by practicing this skill in three different slalom courses over a 3-day training period. The interleaved group practiced all three courses each day in a semi-random order. In contrast, the blocked group practiced only one course each day, which was randomized and counterbalanced across the participants in this group. A retention test was delivered 72 h after the last practice day. In contrast to our hypothesis, the interleaved group did not display significantly better retention than the blocked group. The interleaved group's performance was also not significantly attenuated during skill learning compared to the blocked group. Our results underscore the importance of conducting motor learning experiments in natural environments to understand the conditions that facilitate learning beyond the laboratory environment.

Towards more valid simulations of slopestyle and big air jumps: Aerodynamics during in-run and flight phase.

OBJECTIVES: This study aimed to investigate air drag and lift during the in-run and flight phase of ski and snowboard slopestyle and big air, to allow more valid modeling of jumps and hence reduce injury risk. DESIGN: We present an experimental, multiple single athlete study based on wind tunnel measurements of 4 skiers and 3 snowboarders. METHODS: Measurements were carried out in a closed loop wind tunnel, measuring airflow speed and 3D forces acting on the athletes. Athletes performed trials in typical postures at 35, 60 and 85 km/h wearing slim-, regular- and wide fit apparel. Drag and lift area (cDA; cLA) were calculated and analyzed using linear and multiple regression to describe their dependencies on posture, apparel and speed. RESULTS: cDA values were higher than earlier assumed and ranged from 0.3 to 0.95 m2 for skiers and from 0.35 to 0.55 m2 for snowboarders, primarily dominated by posture, and followed by apparel. cLA ranged from -0.1 to 0.45 m2 for skiers and from 0.04 to 0.17 m2 for snowboarders. To facilitate more valid jump modeling posture- and apparel-dependent formulations for air drag coefficients were provided and the consequences of sport specific differences on modeling were highlighted. CONCLUSIONS: Applying the air drag coefficients and relationships determined in this study will help to improve validity of jump modeling in big air and slopestyle. The variability in aerodynamic forces in slopestyle and big air is caused by differences between sports, posture and apparel.