Josef Kröll

Course setting and selected biomechanical variables related to injury risk in alpine ski racing: an explorative case study

Background Course setting has often been discussed as a potential preventative measure in the World Cup ski-racing community. However, there is limited understanding of how it is related to injury risk. Objective This study was undertaken to investigate the effect of increased horizontal gate distance on energy-related and injury mechanism-related variables. Methods During a video-based three-dimensional (3D)-kinematic field measurement, a top world-class racer performed giant slalom runs at two course settings with different horizontal gate distances. A full-body segment model was reconstructed in 3D and selected biomechanical parameters were calculated. Results For the analysed turn, no significant differences were found in turn speed for increased horizontal gate distance. However, a large effect size was observed for speed reduction towards the end of the turn. Turn forces were by tendency higher at the beginning and significantly higher towards the end of the turn. Additionally, significant differences were found in higher inward leaning, and large effect sizes were observed for a decreased fore/aft position after gate passage. Conclusions On the basis of the data of this study, no final conclusion can be made about whether, for a section of consecutive turns, increasing horizontal gate distance is an effective tool for speed reduction. However, this study pointed out two major drawbacks of this course setting modification: (1) it may increase fatigue as a consequence of loading forces acting over a longer duration; (2) it may increase the risk of out-of-balance situations by forcing the athlete to exhaust his backward and inward leaning spectrum.

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.

Perceived key injury risk factors in World Cup alpine ski racing—an explorative qualitative study with expert stakeholders

Background There is limited knowledge about key injury risk factors in alpine ski racing, particularly for World Cup (WC) athletes. Objective This study was undertaken to compile and explore perceived intrinsic and extrinsic risk factors for severe injuries in WC alpine ski racing. Methods Qualitative study. Interviews were conducted with 61 expert stakeholders of the WC ski racing community. Experts’ statements were collected, paraphrased and loaded into a database with inductively derived risk factor categories (Risk Factor Analysis). At the end of the interviews, experts were asked to name those risk factors they believed to have a high potential impact on injury risk and to rank them according to their priority of impact (Risk Factor Rating). Results In total, 32 perceived risk factors categories were derived from the interviews within the basic categories Athlete, Course, Equipment and Snow. Regarding their perceived impact on injury risk, the experts’ top five categories were: system ski, binding, plate and boot; changing snow conditions; physical aspects of the athletes; speed and course setting aspects and speed in general. Conclusions Severe injuries in WC alpine ski racing can have various causes. This study compiled a list of perceived intrinsic and extrinsic risk factors and explored those factors with the highest believed impact on injury risk. Hence, by using more detailed hypotheses derived from this explorative study, further studies should verify the plausibility of these factors as true risk factors for severe injuries in WC alpine ski racing.

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.

Turn Characteristics of a Top World Class Athlete in Giant Slalom: A Case Study Assessing Current Performance Prediction Concepts

Recently, four concepts explaining time differences in alpine ski racing have been suggested. Since the demands on a “well performed” turn are contradicting among these concepts, it is unclear which turn characteristics a skier should aim for in a specific giant slalom situation. During a video-based 3D-kinematic field measurement, single repetitive runs of a world class athlete were compared regarding section times over one turn and variables explaining time differences. None of the existing concepts was able to entirely explain time differences between different performed turns. However, it was found that the skiers line and timing played an important role for time over short sections. Hence, for both science and coaching, there is a need for more comprehensive approaches that include all variables influencing performance in one concept. In coaching, one such approach could be the training of implicit adaptation mechanisms in terms of situation-dependent line and/or timing strategies.

Quadriceps Muscle Function during Recreational Alpine Skiing.

PURPOSE Since the introduction of carving skis, muscle activity has been investigated primarily on expert-level skiers with respect to EMG intensities. The three-part aim of this recreational skiing study was to analyze functional differences within the quadriceps muscle, to analyze the topographical influence, and to apply a time-frequency analysis of the EMG intensities using wavelets. METHODS Seven female subjects performed two runs through a standardized corridor on a slope with different inclinations (13 degrees , 29 degrees , and 21 degrees ). Knee angle and EMG of vastus lateralis (VL) and rectus femoris (RF) of the right leg were measured during the runs. The recorded EMG signal was resolved with a set of 10 wavelets (11-432 Hz) into a time-frequency space. Subsequently, the EMG intensity and mean frequency (MF) were calculated for different time windows (inside leg; outside leg). RESULT For RF, a significantly higher MF (+15.5%, P = 0.009) but similar EMG intensities were detected in the inside leg compared with the outside leg. For VL, the MF (-9.6%, P = 0.053) and EMG intensities (-54.3%, P = 0.010) were lower in the inside leg compared with the outside leg. Both muscles responded with higher EMG intensities on increasing slope inclination (VL = 90.8%, P = 0.022; RF = 115%, P = 0.01). MF is not directly related to inclination. CONCLUSIONS Contrary to previously suggested coloading of the inside leg while carving, our results do not support this hypothesis for VL. However, the functional demand for RF in the inside leg is very high when skiing recreationally. The ability of a situation-dependent loading (RF as knee extensor) and unloading (RF as hip flexor) of the inside leg seems to be a crucial point with respect to specific fatigue during a skiing day.

Potential Mechanisms Leading to Overuse Injuries of the Back in Alpine Ski Racing: A Descriptive Biomechanical Study

Background: Overuse injuries of the back are a common complaint among top athletes and of competitive alpine skiers in particular. However, there is limited understanding about the sport-specific causes of these injuries that is essential for their prevention. Purpose/Hypothesis: This study was undertaken to describe the sport-specific, overall trunk kinematics and skiers’ loading during giant slalom turns and to assess the plausibility of the hypothesis that a combination of frontal bending, lateral bending, and/or torsion in the loaded trunk might be a potential mechanism leading to overuse injuries of the back in alpine ski racing. Study Design: Descriptive laboratory study. Methods: Eight European Cup–level athletes performed giant slalom runs with 2 different pairs of skis (varying in length, width, and sidecut). They were analyzed with respect to selected kinematic variables related to spinal disc loading. The overall trunk movement components (frontal bending, lateral bending, and torsion) were measured using 2 inertial measurement units fixed on the sacrum and sternum. Total ground-reaction forces were measured by pressure insoles. Results: During the turn phase in which the total ground-reaction forces were the greatest (up to 2.89 times the body weight), the highest average values of frontal bending (38.7°), lateral bending (14.7°), and torsion (7.7°) in the trunk occurred. Similar magnitudes were observed when skiing on longer, giant slalom skis with less width and sidecut. Conclusion: The typical loading patterns of the back in alpine ski racing include a combined occurrence of frontal bending, lateral bending, and torsion in the loaded trunk. Because these factors are known to be related to high spinal disc loading, they may be considered important components of mechanisms leading to overuse injuries of the back in alpine ski racing. Clinical Relevance: Prevention measures should aim to control and/or reduce the magnitude of frontal bending, lateral bending, and torsion in the trunk, as well as the peak loads, while skiing.

Characterization of Course and Terrain and Their Effect on Skier Speed in World Cup Alpine Ski Racing

World Cup (WC) alpine ski racing consists of four main competition disciplines (slalom, giant slalom, super-G and downhill), each with specific course and terrain characteristics. The International Ski Federation (FIS) has regulated course length, altitude drop from start to finish and course setting in order to specify the characteristics of the respective competition disciplines and to control performance and injury-related aspects. However to date, no detailed data on course setting and its adaptation to terrain is available. It is also unknown how course and terrain characteristics influence skier speed. Therefore, the aim of the study was to characterize course setting, terrain geomorphology and their relationship to speed in male WC giant slalom, super-G and downhill. The study revealed that terrain was flatter in downhill compared to the other disciplines. In all disciplines, variability in horizontal gate distance (gate offset) was larger than in gate distance (linear distance from gate to gate). In giant slalom the horizontal gate distance increased with terrain inclination, while super-G and downhill did not show such a connection. In giant slalom and super-G, there was a slight trend towards shorter gate distances as the steepness of the terrain increased. Gates were usually set close to terrain transitions in all three disciplines. Downhill had a larger proportion of extreme terrain inclination changes along the skier trajectory per unit time skiing than the other disciplines. Skier speed decreased with increasing steepness of terrain in all disciplines except for downhill. In steep terrain, speed was found to be controllable by increased horizontal gate distances in giant slalom and by shorter gate distances in giant slalom and super-G. Across the disciplines skier speed was largely explained by course setting and terrain inclination in a multiple linear model.

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

Abstract 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.

The Relationship of Heart Rate and Lactate to Cumulative Muscle Fatigue During Recreational Alpine Skiing

Seifert, J, Kröll, J, and Müller, E. The relationship of heart rate and lactate to cumulative muscle fatigue during recreational alpine skiing. J Strength Cond Res 23(3): 698-704, 2009-Common indices of fatigue may not respond similarly between downhill skiing and other activities because of the influence of factors such as snow conditions, changing terrain, and skiing style. The purpose of this study was to investigate the relationship and predictors of common fatigue indices during downhill skiing. Ten healthy female recreational skiers skied for 3 hours under standardized conditions. Feedback on heart rate (HR) and finishing time were given to each skier at the end of each run to maintain a relatively stable load. A chronic stress score (Cstress) was calculated from creatine kinase (CK), cortisol, and isometric endurance. Finishing times and HR from runs 2, 12, and 24 were similar. Heart rate averaged 82% of HRmax. Heart rate was an insignificant predictor (p = .65) and was poorly correlated (r = 0.16) to Cstress. Blood lactate (LA) was a significant predictor of the Cstress (p = 0.05; r = 0.62). Pre- to postskiing peak forces were not different (p = 0.62), but skiers experienced a significant decrease in isometric endurance from 106.1 ± 29.6 to 93.2 ± 24.0 seconds. Endurance decreased by 13%, whereas cortisol and CK increased by 16 and 42%, respectively. Isometric contraction endurance and blood LA were significant predictors of overall stress. Individual compensation mechanisms and skiing style contributed to highly variable responses during skiing. Whereas HR may indicate stress within a given run, it is not a significant indicator of Cstress and fatigue during recreational alpine skiing. However, the cumulative stress variables and LA can be used in field testing of skiers. It is suggested that LA is a practical on-hill marker of chronic stress.