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IntroductionPrior to the 2013/2014 season, the International Ski Federation (FIS) increased the helmet testing speed from 5.4 to 6.8 m/s for alpine downhill, super-G and giant slalom. Whether this increased testing speed reflects head impact velocities in real head injury situations on snow is unclear. We therefore investigated the injury mechanisms and gross head impact biomechanics in seven real head injury situations among World Cup (WC) alpine skiers.MethodsWe analysed nine head impacts from seven head injury videos from the FIS Injury Surveillance System, throughout nine WC seasons (2006–2015) in detail. We used commercial video-based motion analysis software to estimate head impact kinematics in two dimensions, including directly preimpact and postimpact, from broadcast video. The sagittal plane angular movement of the head was also measured using angle measurement software.ResultsIn seven of nine head impacts, the estimated normal to slope preimpact velocity was higher than the current FIS helmet rule of 6.8 m/s (mean 8.1 (±SD 0.6) m/s, range 1.9±0.8 to 12.1±0.4 m/s). The nine head impacts had a mean normal to slope velocity change of 9.3±1.0 m/s, range 5.2±1.1 to 13.5±1.3 m/s. There was a large change in sagittal plane angular velocity (mean 43.3±2.9 rad/s (range 21.2±1.5 to 64.2±3.0 rad/s)) during impact.ConclusionThe estimated normal to slope preimpact velocity was higher than the current FIS helmet rule of 6.8 m/s in seven of nine head impacts.

Background Research on balance measures as potential risk factors for ACL injury is limited. Objective To assess whether postural control was associated with an increased risk for ACL injuries in female elite handball and football players. Method Premier league players were tested in the preseason and followed prospectively for ACL injury risk from 2007 through 2015. At baseline, we recorded player demographics, playing experience, ACL and ankle injury history. We measured centre of pressure velocity in single-leg stabilisation tests and reach distances in the Star Excursion Balance Test. To examine the stability of postural control measures over time, we examined their short-term and long-term reproducibility. We generated logistic regression models, 1 for each of the proposed risk factors. Results A total of 55 (6.6%) out of 838 players (age 21±4 years; height 170±6 cm; body mass 66±8 kg) sustained a non-contact ACL injury after baseline testing (1.8±1.8 years). When comparing normalised balance measures between injured and uninjured players in univariate analyses, none of the variables were statistically associated with ACL injury risk. Short-term and long-term reproducibility of the selected variables was poor. Players with a previous ACL injury had a 3-fold higher risk of sustaining a new ACL injury compared with previously uninjured players (OR 2.9, CI 1.4 to 5.7). Conclusions None of postural control measures examined were associated with increased ACL injury risk among female elite handball and football players. Hence, as measured in the current investigation, the variables included cannot be used to predict ACL injury risk.

Background Sidestep cutting technique is essential in programmes to prevent anterior cruciate ligament (ACL) injury. A better understanding of how technique affects potentially harmful joint loading may improve prevention programmes. The purpose of this study was to investigate the effect of sidestep cutting technique on maximum knee abduction moments. Methods Cross-sectional study. Whole-body kinematics and knee joint kinetics were calculated in 123 female handball players (mean±SD, 22.5±7.0 years, 171±7 cm, 67±7 kg) performing sidestep cutting. Three cuts from each side were analysed. Linear regression was applied between selected technique factors and maximum knee abduction moment during the first 100 ms of the contact phase. Furthermore, we investigated to what degree the abduction moment originated from the magnitude of the ground reaction force (GRF) or the knee abduction moment arm of the GRF. Findings Technique factors explained 62% of the variance in knee abduction moments. Cut width, knee valgus, toe landing, approach speed and cutting angle were the most significant predictors. An increase in one of these factors of 1 SD increased the knee abduction moment from 12% to 19%. The effect of the moment arm of the GRF was more important than the force magnitude for maximum knee abduction moments. Interpretation Lower knee abduction loads during sidestep cutting may be achieved if cuts are performed as narrow cuts with low knee valgus and toe landings. These factors may be targeted in ACL injury prevention programmes.

Measuring joint moments at high speed is integral for sports-related research, but substantial artefacts can arise with certain computational methods. This is relevant for BJSM readers as measurement errors can lead to flawed conclusions. Potential settings where errors have the potential to creep in include anterior cruciate ligament (ACL) risk factor studies, running biomechanics analysis and other fields where biomechanists study fast movements. Our recent paper in Journal of Biomechanics demonstrated the effect of different filtering of force and marker data in joint moment calculations.1 This paper explains those findings for the clinical readership of BJSM and highlights the areas where it may be wise to ‘proceed with caution’ when interpreting previous studies that aim to identify key risk factors for injury. Estimates of joint moments are fundamental in the study of sporting motion. Joint moments calculated in standard motion analysis are net joint moments, the total effect of all structures that produce forces that work across the joint. Joint moments stem from muscle forces, articular contact and ligaments. We are interested in joint moments because, unlike these separate forces, they can be measured. A correct interpretation of joint moments may help us understand which muscles and ligaments are at risk of injury in certain settings. If biomechanics is not your thing, feel free to skip down to ‘Implications’ below. Joint moments are normally calculated through the process of inverse dynamics, where ground reaction forces (force plate data) and segment accelerations (marker data—the reflective markers attached to the participant) are combined. In all high-impact movements, such as running and sidestep cutting, it is normal to see an impact peak in ground reaction force that will result in high acceleration of body segments. Unfortunately, due to random noise in the measurement of marker positions, current motion analysis systems cannot reliably …

Published Online First 9 December 2010 Poor frontal plane knee control has been shown to be a risk factor for anterior cruciate ligament injuries in ball/team sports. It is necessary to develop simple tests that can identify players with poor knee control and among other factors be able to optimise preventive training programmes. The present study investigated the correlation between a two-dimensional (2D) video analysis and subjective assessment performed by one physiotherapist in evaluating knee control. We also tested the correlation between three simple clinical tests using both methods. A cohort of 186 female elite team handball players completed three tests: single-leg squat (SLS), single-leg vertical drop jump (SLVDJ) and two-leg vertical drop jump (VDJ). Receiver operating characteristic (ROC) analyses showed good to excellent agreement between 2D video analysis and subjective assessment for SLS and VDJ (area under the ROC curve (AUC) 0.83-0.89), but not for SLVDJ (AUC 0.65-0.76). Poor knee control was detected in 25-40% of the players using SLS and VDJ. In contrast, poor knee control was identified in less than 1% of the subjects based on the SLVDJ test. This indicates that SLVDJ may be inadequate for evaluating poor frontal plane knee control. The correlation between the SLS and VDJ tests was found to be low, suggesting that these tests identified different subjects with poor knee control. These results indicate that subjective assessment can be used to screen for poor knee control and that both SLS and VDJ tests should be used in screening athletes for poor knee control.

A number of different methodological approaches have been used to describe the inciting event for sports injuries. These include interviews of injured athletes, analysis of video recordings of actual injuries, clinical studies (clinical findings of joint damage are studied to understand the injury mechanism, mainly through plain radiography, magnetic resonance imaging, arthroscopy, and computed tomography scans), in vivo studies (ligament strain or forces are measured to understand ligament loading patterns), cadaver studies, mathematical modelling and simulation of injury situations, and measurement/estimation from "close to injury" situations. In rare cases, injuries have even occurred during biomechanical experiments. This review describes each research approach and assesses its strengths and weaknesses in contributing to the understanding and prevention of sports injuries.

Background Little research exists quantifying balance measures as potential risk factors for ACL injury. Objective To assess whether static and dynamic postural control were associated with an increased risk for ACL injury in female elite handball and football players. Design Prospective cohort study. Participants From 2007 through 2015, 838 female premier league players participated in pre-season testing and were prospectively followed for ACL injury risk. Assessment of risk factors and main outcome measures At baseline, we recorded player demographics, playing experience, ACL and ankle injury history. We measured the mean velocity of COP in medio-lateral (ML) and anterior-posterior (AP) directions (mm/s), as well as the area of the 95% confidence ellipse area (mm 2 ). We used the Star Excursion Balance Test to measure anteromedial, medial and posteromedial reach directions (cm). To examine the stability of postural control measures over time, we examined long-term and short-term reproducibility. We followed a pre-defined statistical protocol with logistic regression models, one for each of the proposed risk factors. Results A total of 55 (6.6%) players (age: 20±4 yrs; height: 171±7 cm; body mass: 68±9 kg) sustained a non-contact ACL injury after baseline testing (1.8±1.8 yrs). When comparing height and leg-length normalized balance measures between injured and uninjured players in univariate analyses, none of the variables were statistically associated with an increased risk of ACL rupture. Short- and long-term reproducibility of the selected variables was poor. Players with a previous ACL injury had a 3-fold higher risk of sustaining a new ACL injury compared to previously uninjured players (OR 2.9, CI 1.4 to 5.7). Conclusions None of the static and dynamic postural control measures examined were associated with increased ACL injury risk among female elite handball and football players. Hence, the variables included in the current investigation cannot be used to predict injury risk.

Background Stiff landings with less knee flexion and high vertical ground reaction force have shown to be associated with increased risk of ACL injuries (Leppanen et al. 2016). The association between other sagittal plane measures and the risk of ACL injuries is less studied in prospective study designs. Objective To investigate the relationship between selected sagittal plane hip, knee, and ankle biomechanics and the risk of ACL injuries in young female team sport players. Design Prospective cohort study. Setting Female youth team sports. Participants A total of 171 female basketball and floorball players (aged 12–21) participated in a VDJ test using 3D motion analysis. All new ACL injuries, as well as match and training exposure, were recorded for 1 to 3 years. Assessment of Risk Factors The following biomechanical variables were analysed: 1) peak knee extension moment, 2) peak hip flexion angle, 3) peak hip extension moment, and 4) peak ankle dorsiflexion angle. Cox regression models were used to calculate hazard ratios (HRs, adjusted for 10 unit change) and 95% confidence intervals (CIs). Main Outcome Measurements Non-contact ACL injuries. Results Subjects suffered a total of 15 ACL injuries during the follow-up (0.2 injuries per 1 000 player hours). Of the investigated variables, landing with decreased hip flexion was significantly associated with increased risk of ACL injury (HR 0.60; 95% CI 0.37 to 0.97, P=0.038). Peak knee extension moment (HR 1.13; 95% CI 0.79 to 1.62, P=0.490), peak hip extension moment (HR 0.88; 95% CI 0.77 to 1.01, P=0.065), and peak ankle dorsiflexion angle (HR 0.70; 95% CI 0.33 to 1.51, P=0.360) were not significantly associated with ACL injuries. Conclusions Landing with less hip flexion may increase the risk of ACL injury in young female team sport players. More studies with large study populations are needed to confirm these findings.

Background Single-leg squat (SLS) is a clinical tool used to assess knee control. Previous studies have proposed that knee valgus motion in the SLS could be related to injury risk, but this association has not been previously examined. Objective The objective of this study was to investigate the association between acute lower extremity injuries and frontal plane knee control in young athletes. Design This study is part of a prospective cohort study (PROFITS-study). As part of the baseline test battery, we measured frontal plane projection angles during SLS. We also measured the athlete9s height and weight, and the athletes filled out a questionnaire describing their time-loss injuries during the past 12 months. We registered new time-loss injuries in the following 12 months on a weekly basis. Only athletes free from lower extremity injuries during the 12 months prior to baseline were included in the analyses. Setting Research institute, youth sports. Participants Ten local basketball and ten local floorball teams were invited to the study. Out of these, 9 basketball and 9 floorball teams agreed to participate. Only athletes who were 21 years or younger and were free from injury during baseline were eligible. Complete data were obtained from 306 athletes. Assessment of Risk Factors A generalized linear mixed model was used to analyse the potential risk factors. Main Outcome Measurements Acute time loss lower extremity injuries sustained during the 12 month period after baseline. Results Displaying a frontal plane projection angle greater than 1 standard deviation above the mean (>23.8°) in the SLS was associated with a higher risk of lower extremity injuries (OR 5.57, 95% CI 1.61–19.32) and ankle injuries (OR 2.37, 95% CI 1.13–4.98). Conclusions Based on this study, athletes who display large frontal plane projection angles during the SLS have an elevated risk of lower extremity injuries.

Background The most common movements that occur during anterior cruciate ligament (ACL) injuries in soccer are “pressing” movements. There are currently no accurate kinematic descriptions of joint kinematics in pressing movements in defensive situations. Objective To describe the knee and hip joint kinematics during pressing actions in soccer in order to identify kinematic patterns in high-risk situations for ACL injuries. Design Case study Setting Competitive level, division-1 amateur female soccer league in Japan. Participants Five video sequences involving 4 collegiate soccer players. Assessment of Risk Factors The high-risk pressing situations were determined by the large distance between trunk and foot placements in the sagittal plane. Main Outcome Measurements The knee and hip joint kinematics using model-based image-matching technique. Results The mean knee flexion angle at initial contact (IC) was 13°; it had increased by 11° (95% CI: 3°–14°) 40 ms after IC. As for knee adduction and rotation angles, the knee positions were close to neutral at IC, and only minor knee angular changes occurred later in the sequences. The mean hip flexion was 25° at IC, and had increased by 22° (95% CI: 11°–32°) 100 ms later. The hip was also externally rotated 7° at IC, and gradually rotated internally, reaching 10° of internal rotation 100 ms after IC. Conclusions In contrast with previously reported injury-causing situations, the present knee did not experience rapid knee valgus and increased internal rotation. The hip displayed a smooth transition after IC, which contrasted with previously reported hip kinematics of substantial internal rotation and static hip flexion in injury situations. This study suggests that the observed sudden changes in knee valgus and rotation, as well as the static hip flexion seen in ACL injury situations are unique to such situations, supporting the view that these characteristics likely contribute to the injury mechanism.

Background Poor knee alignment is associated with increased loading of the joints, ligaments and tendons, and may increase the risk of injury. The study purpose was to compare differences in knee kinematics between basketball and floorball players during a vertical drop jump (VDJ) task. We wanted to investigate whether basketball players, whose sport includes frequent jump-landings, exhibited better knee control compared with floorball players, whose sport involves less jumping. Methods Players (aged 12–21 years) were recruited from six basketball and floorball clubs of the Tampere City district, Finland. Complete data was obtained from 173 basketball and 141 floorball players. Peak knee valgus and flexion angles during the VDJ were analysed by 3 D motion analysis. Results Larger knee valgus angles were observed among basketball players (−3.2o, 95% CI: −4.5 to −2.0) compared with floorball players (−0.9o, 95% CI: −2.3 to 0.6) ( P = 0.022). Basketball players landed with a decreased peak knee flexion angle (83.1o, 95% CI: 81.4 to 84.8) compared with floorball players (86.5o, 95% CI: 84.6 to 88.4) ( P = 0.016). There were no significant differences in height, weight or BMI between basketball and floorball players. The female athletes exhibited significantly (P Conclusions This study revealed that proper knee control during jump-landing does not seem to develop in young athletes simply by playing the sport, despite the fact that jump-landings occur frequently in practice and games. Poor knee control was especially common among young female athletes. An important clinical implication of these findings is that young team sport athletes need to be taught a safer technique for landing and also need specific neuromuscular training in order to avoid potentially harmful movement patterns. (Int J Sports Med 2015, accepted for publication)

Background: There is limited insight into the mechanisms of anterior cruciate ligament injuries in alpine skiing, particularly among professional ski racers. Purpose: This study was undertaken to qualitatively describe the mechanisms of anterior cruciate ligament injury in World Cup alpine skiing. Study Design: Case series; Level of evidence, 4. Methods: Twenty cases of anterior cruciate ligament injuries reported through the International Ski Federation Injury Surveillance System for 3 consecutive World Cup seasons (2006-2009) were obtained on video. Seven international experts in the field of skiing biomechanics and sports medicine related to alpine skiing performed visual analyses of each case to describe the injury mechanisms in detail (skiing situation, skier behavior, biomechanical characteristics). Results: Three main categories of injury mechanisms were identified: slip-catch, landing back-weighted, and dynamic snowplow. The slip-catch mechanism accounted for half of the cases (n = 10), and all these injuries occurred during turning, without or before falling. The skier lost pressure on the outer ski, and while extending the outer knee to regain grip, the inside edge of the outer ski caught abruptly in the snow, forcing the knee into internal rotation and valgus. The same loading pattern was observed for the dynamic snowplow (n = 3). The landing back-weighted category included cases (n = 4) where the skier was out of balance backward in flight after a jump and landed on the ski tails with nearly extended knees. The suggested loading mechanism was a combination of tibiofemoral compression, boot-induced anterior drawer, and quadriceps anterior drawer. Conclusion: Based on this video analysis of 20 injury situations, the main mechanism of anterior cruciate ligament injury in World Cup alpine skiing appeared to be a slip-catch situation where the outer ski catches the inside edge, forcing the outer knee into internal rotation and valgus. A similar loading pattern was observed for the dynamic snowplow. Injury prevention efforts should focus on the slip-catch mechanism and the dynamic snowplow.

Background Identifying individuals with a high risk for ACL injury is important for injury prevention. Current screening methods exist but rely on expensive and time consuming biomechanical analysis, limiting large scale application. Objective The aim of this experiment was to investigate observational ACL injury risk assessment performance. Design Individual raters viewed 25 frontal plane video clips of female high school athletes performing a drop vertical jump. Five video clips were repeated. The decision task was to rate the athletes risk for an ACL injury on a 10 point scale (1=very low, 10=very high). Concurrent 3D peak knee abduction moment (KAM) was calculated for the video-taped athletes and served as the criterion for ACL injury risk level. Setting Raters viewed the video clips and responded using a computer. Participants A convenience sample of 40 allied health professionals/students served as raters. Risk factor assessment In order to perform signal detection analysis on the judgments, ratings greater than 5 were considered high risk. Main outcome measurements To assess judgment performance, Spearman rho (ρ), sensitivity ( A9 ) and decision bias ( B D ) were calculated. Intra-rater consistency was assessed by calculating typical error between the 5 repeated video clips. Rater performance was also compared to the ACL Nomogram (KAM prediction model). Results Correlation between continuous KAM and subjective ratings ranged from −0.16 to 0.39 (Nomogram ρ=0.64). However, the best performer (M.S. Physiotherapist) displayed a sensitivity value of 0.85 ( B D =0.19) which was similar to the Nomogram measure of 0.97 ( B D =−0.48). In general, intra-rater consistency was sufficient with 83% of raters displaying typical error values less than 1. Conclusions Superior performance during visual assessment of ACL injury risk appears to be difficult but achievable. Research describing expertise mechanisms is warranted.

Background Poor frontal plane knee control is suggested to be a risk factor for anterior cruciate ligament injuries. Both 3D motion analysis and less complex methods, such as 2D video analysis and subjective assessments, have been used to assess frontal plane knee motion. However, the agreement between all three methods has not been investigated. Objective To determine the agreement between 3D and 2D motion analysis and real-time observational assessment of frontal plane knee control in a vertical drop jump (VDJ) landing. Design Observational study. Setting Research laboratory. Participants 119 Norwegian female elite handball players. Risk factor assessment Players performed a VDJ during which movement was recorded with a 3D motion analysis system, a digital video camera, and real-time observational screening in frontal plane. Main outcome measurements 3D and 2D knee-to-ankle separation ratios were calculated to assess frontal plane knee control. A physiotherapist independently scored the knee control as “good”, “reduced” or “poor”. All variables were assessed at peak knee flexion. Results Correlations between 3D and 2D knee-to-ankle-separation ratios and 2D and 3D versus real-time observation ranged from 0.63 to 0.75. Mean 3D and 2D knee-to-ankle-separation ratios were significantly lower for players being scored with “poor” compared to those with “reduced” or “good” frontal knee control. Conclusion Compared with the “gold standard” (3D analysis), both 2D motion analysis and real-time observations seem to be feasible tests to assess frontal knee control. The injury predictive value of the VDJ screening test remains to be answered.

Background Ranking the athletes according to their knee valgus angle contributes to the development of screening tools for the risk of knee injury. However, studies used different thigh marker clusters to calculate knee valgus angle in marker-based motion analysis system. There is a methodological concern on the thigh marker cluster design which relates to the reliability of measurement. Objective To quantify the differences between six different thigh marker clusters in terms of the calculated knee valgus angles and the ranking of athletes according to the maximal knee valgus angle and knee valgus angle at initial contact. Design A methodological study. Setting A marker-based motion analysis system (Oqus, Qualisys, Gothenburg, Sweden) with 480 Hz frame rate was used. Six different marker clusters was put on the thigh of subjects. Subjects performed sidestep cutting and vertical drop jumping motions while being filmed with a skin marker-based motion capture system. The knee valgus angles from six different thigh marker clusters were calculated based on the dynamic trials. Participants 19 elite female handball players and 22 elite female football players (n=41). Main outcome measurements The calculated knee valgus angles and the resulting ranking of athletes from six different thigh marker clusters. Results There was a discrepancy in knee valgus angles between the six marker sets up to 13.9° of peak valgus angle. The rank correlation coefficient varied from 0.505 to 0.974. Conclusions The present study demonstrated that the choice of thigh marker clusters in analyses of drop jumps and sidestep cutting affected the magnitude of knee valgus angle, as well as the ranking of athletes. A standardized thigh marker cluster setup would facilitate the reliability of measurement, especially for a study on risk factor.

Background A link has been suggested between increased dynamic knee valgus and the risk of anterior cruciate ligament (ACL) injuries in female athletes. However, these findings are based on complex time-consuming 3D motion analysis. There is a need for simple screening tools to identify athletes at increased risk of an ACL rupture. Objective To assess whether an experienced clinician can identify athletes with increased valgus angles and moments during a vertical drop jump using a simple screening test. Design Cross-sectional study. Participants Elite female football players. Methods Players in the Norwegian elite league (12 teams) were screened for potential risk factors for ACL injuries. Knee kinematics and kinetics in the landing phase of a 30 cm vertical drop jump were calculated using 3D motion analysis. Using a similar jump task, a physiotherapist assessed how well players stabilised their knees, and scored their knee control as ‘good’, ‘reduced’ or ‘poor’. Results Of the 136 players included, 38% were classified as ‘good’, 34% had ‘reduced’ control and 28% were scored as ‘poor’. Players with good control had significantly lower maximal valgus angles compared to players with poor control (0.5° vs 6.8°, p≤0.001). There were no differences in valgus moments (0.28 Nm/kg vs 0.28 Nm/kg, p=1.0). One-fifth of all players (17.6%) displayed valgus angles more than one SD above the mean in the 3D analyses (3.1° ±4.5°). The screening test captured 87.5% of these players. 22 (16.2%) players were classified in the ‘poor’ group without displaying high valgus angles. Conclusion The screening test identified 87.5% of players with increased valgus angles. No correlation was found between valgus angles and moments, and the test could not identify players with high valgus moments. Further investigations are needed to determine the cut-off value for defining high- versus low-risk groups, as well as the predictive value of the test.

Background Excessive dynamic knee valgus may leave athletes prone to serious knee injuries. The Star Excursion Balance Test (StarReach) is an objective clinical test, well applicable for non-clinicians, to identify athletes with ankle instability, but it9s potential to detect athletes with reduced knee control has not been examined. Objective To compare StarReach test performance with subjective assessment of dynamic knee valgus during a single-leg squat in female athletes. Design Controlled, laboratory study. Setting Norwegian female elite team handball and football. Participants This study is part of an ongoing cohort study aimed at investigating risk factors for non-contact anterior cruciate ligament injuries. Since 2007, a total of 321 handball and 246 football players (22±4 years) have been included. Main outcome measures A physiotherapist assessed how well players stabilised their knees while performing a single-leg squat, and scored their performance as ‘good’, ‘reduced’ or ‘poor’. Maximum reach distances in the StarReach test were measured on both feet in the antero-, medio- and posterolateral directions. A total of 440 players were tested. Results StarReach test performance (reach distance adjusted for leg length) was significantly better for those players who were scored with ‘good’ versus ‘poor’ knee stability (p Conclusion Poor relationships between StarReach-test performance and subjective scores illustrate why these two tests cannot be used interchangeably to assess knee stability. Further investigations are needed to determine whether any of these tests can predict increased knee injury risk.

Background Ankle sprain is among the most frequent sports injuries and a significant contributor to time lost from sports participation. The injury mechanism is traditionally described as an inversion trauma, but a detailed description including joint kinetics is lacking. Objective To provide an accurate description of an ankle sprain. Design Case study. Setting Sidestep cutting in 240 Hz motion analysis lab. Patient Elite female team handball player (1.73 m, 63.7 kg, 22 years). Accidental grade 1 ankle sprain confirmed by orthopaedic surgeon. Assessment and outcome Ankle kinetics and kinematics and ground reaction forces were compared between two normal trials and the injury trial. Results In the injury trial we observed a sudden increase in ankle inversion (15° vs 4.9° and 4.8°) and ankle internal rotation (6.9° vs −1.2° and −3.6°) the first 50 ms. The injury trial centre of pressure had an increased lateral excursion after 50 ms (8.4 cm vs 3.3 and 3.0 cm). From 80 ms there was an increasing ankle inversion moment, reaching a peak of 72 Nm at 138 ms. At this time the ankle inversion angle had reached 33.1°, the ankle internal rotation angle was 53.7° and the ankle dorsiflexion was 26.5°. In the same period there appeared to be an attempted unloading of the foot, with a dorsiflexion of the ankle and reduced ground reaction force. The control trials displayed mainly eversion moment of the ankle throughout the stance phase, and joint angular deflections less than 6°. Conclusion This study provides the most accurate and detailed description of ankle sprain dynamics to date. The injury involved inversion and internal rotation combined with an unloading of the foot that was initiated after approx 80 ms. The inversion moment likely caused the injury in the time period between 120 and 150 ms.

Background We have identified that a slip-catch situation is the main mechanism for anterior cruciate ligament (ACL) injuries in World Cup alpine skiing by simple visual inspection of injury videos. This occurs when the skier loses pressure on the outer ski during turning, and while extending the outer knee to regain grip, the inside edge of the outer ski catches abruptly, forcing the knee into valgus and internal rotation. To develop injury prevention methods based on this mechanism, a more precise description is needed. Objective To describe the detailed joint kinematics in a slip-catch situation using a model-based image-matching (MBIM) technique. Design Case study. Setting Men9s World Cup alpine skiing. Methods Video of a slip-catch injury situation in slalom captured with two nearly perpendicular camera views was analyzed using the MBIM method to obtain the time course of knee and hip joint kinematics. Results Knee valgus and internal rotation angle increased by 13° and 13°, respectively, in the last 60 ms before the time of ACL rupture, which was estimated to occur 100 ms after the catching the edge. Knee flexion angle increased by 32° in the last 20 ms from 22° at snow contact to a final position. Hip flexion and adduction angle increased by 18° and 13°, respectively, in the last 20 ms and the hip stayed at approximately 30° of internal rotation in the last 60 ms. Conclusion Abrupt compression, valgus and internal rotation loading seems to be the main mechanism of slip-catch injury situations in ACL injuries in World Cup alpine skiing. The knee loading pattern described from the previous simple visual inspection study matched well with the more detailed analysis using the MBIM technique. Prevention efforts should focus on avoiding valgus and internal rotation loading on the knee.