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Injury situations in Freestyle Ski Cross (SX): a video analysis of 33 cases
  1. Stefan Randjelovic1,2,
  2. Stig Heir1,3,
  3. Lars Nordsletten1,2,4,
  4. Tone Bere1,
  5. Roald Bahr1
  1. 1Department of Sports Medicine, Oslo Sports Trauma Research Center, Norwegian School of Sport Sciences, Oslo, Norway
  2. 2Faculty of Medicine, University of Oslo, Oslo, Norway
  3. 3Martina Hansens Hospital, Baerum, Oslo, Norway
  4. 4Department of Orthopaedic, Oslo University Hospital, Ullevål, Oslo, Norway
  1. Correspondence to Stefan Randjelovic, Sports Trauma Research Center, Norwegian School of Sport Sciences, Oslo PB 4014 Ullevål Stadion, Oslo 0806, Norway; stefan.randjelovic{at}nih.no

Abstract

Background Although injury risk in Freestyle Ski Cross (SX) is high, little is known about the situations leading up to time-loss injuries.

Objective To describe the situations leading up to time-loss injuries in elite Freestyle SX.

Study design Descriptive video analysis.

Methods Thirty-three video recordings of SX injuries reported through the International Ski Federation Injury Surveillance System for four World Cup seasons (2006/2007 through 2010) were obtained. Five experts in the fields of sport medicine and SX analysed each case to describe in detail the situation leading up to the injury (skiing situation and skier behaviour).

Results Injuries occurred in four different skiing situations: jumping (n=16), turning (n=8), jumping and turning (n=7) and rollers (n=2). All injured skiers lost control before time of injury (n=33), due to skier–opponent contact (n=13), technical errors (n=8) or inappropriate strategy (n=8), which led to a fall (n=29). Contact occurred in 21 of 33 cases, usually unintentional at landing or take-off, caused by the opponent (n=11) or injured skier (n=8). The technical error cases (n=8) were dominated by bad jumping technique (n=6) and too much inside lean in turning situations (n=2), while inappropriate course line and bad timing at take off (n=7) dominated the inappropriate strategy cases (n=8).

Conclusions We identified four main injury situations in elite SX, dominated by jumping situations. The primary cause of injury was unintentional skier–opponent contact in jumping, bank turning and roller situations. Another common cause of injury was personal errors (inappropriate technique and strategy) at take-off and in turning situations.

  • Injury Prevention
  • Ski injuries
  • Olympics
  • Elite performance
  • Implementation

https://doi.org/10.1136/bjsports-2012-091999

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    Introduction

    Freestyle skiing is a relatively recent addition to the traditional Olympic winter sports—combining speed, showmanship and the ability to perform aerial manoeuvres while skiing.1 Today, the freestyle skiing events at the World Cup (WC) level includes six disciplines: moguls, dual moguls, aerials, half pipe, slope style and Ski Cross (SX).2

    A study from the 2010 Olympic Winter Games (OWG) reported SX as one of the highest risk disciplines.3 Recent data from the International Ski Federation (FIS) Injury Surveillance System (ISS), a prospective injury recording system established at the Oslo Sport Trauma Research Center (OSTRC) since 2006, show that the injury incidence among WC freestyle athletes is alarmingly high,4 with every third SX athlete suffering a time-loss injury during the 6-month competitive season (33.8 time-loss injuries/100 athletes/season in SX).5 Nearly half of these (14.9 injuries/100 athletes/season) were severe, defined as those leading to >28 days of absence from training/competition, mainly to the knee and head.5 In addition, the 2012 WC season even closed with a tragic death. Thus, attention needs to be directed at how to prevent SX injuries.

    An SX event is initiated by one timed individual run qualifying the competitors to race in heats of 4 (sometimes 6).6 The skiers manoeuvre head-to-head at high speed through several obstacles (eg, dozed turns, jumps, kickers and rollers). The first two (sometimes three) skiers to cross the finish line advance to the next round, until the final round settles the podium positions.2 In a recent study analysing injuries in snowboard cross (SBX), which has an identical competition format as SX,7 6 of 19 injuries resulted from interference between athletes during racing in heats.8 The combination of challenging courses, high speed and the possibility of interference between skiers may contribute to high-risk situations in SX as well. However, little is known about the situations leading up to injuries in SX.

    Understanding the mechanisms and situations associated with injuries is essential for their prevention,8–11 and videos of actual injuries may contain crucial information. Previous studies in SBX, alpine skiing and team sports have provided detailed descriptions of athlete behaviour, athlete–opponent interaction and the events leading to injury situations.8 ,12–15 In order to suggest effective injury prevention measures in SX, we believe it is essential to analyse the situations leading up to time-loss injuries. Thus, the aim of this study was to describe the racing situation and skier behaviour in WC and Olympic SX injuries based on systematic analyses of video recordings.8 ,16

    Methods

    Injury and video recording

    We collected injury videos from the 2006/2007 through the 2009/2010 seasons based on information from the FIS ISS.17 During the four seasons, 493 injury cases had been reported through the FIS ISS (figure 1). We excluded injuries recorded as resulting from ‘basic training on snow’, ‘training not on snow’, ‘other competitions’ and ‘other FIS competitions’. From the remaining 331 injuries from WC, World Championships (WSC) and OWG (including official training), we excluded all injuries not leading to time loss from training/competition, leaving 92 time-loss injuries (ie, leading to at least 1 day absence from training/competition) from SX competitions as potential cases for analysis.

    Figure 1
    Figure 1

    Identification of Ski Cross injuries and video procurement based on the  International Ski Federation Injury Surveillance System 2006–2010.

    We developed a document with information on gender, date, event, circumstances (official training, qualification, final), heat number and bib (start number) based on data from the FIS ISS and the FIS results database.18 Based on this document, we requested videos of whole runs from Infront Productions (Italy), the FIS TV broadcast rights holders.

    Video processing

    We received videos in BETA SP video format. Qualification heats were not available and of the 37 injuries during the finals available on tape (figure 1), we had to exclude 4 cases due to the quality of the video, leaving 33 SX injuries for analysis.

    We converted the videos to .mov files with Final Cut Pro V.7.0.3, anonymised and cropped them to a few seconds before and after the injury. The videos were further optimised for either Windows (720 Windows High Quality and H264 QuickTime) or MAC (PRORES files), and the material was analysed visually using QuickTime Player 7 (V.7.6.6 (1709), Apple Inc) because of its frame counting possibilities.

    Video analysis

    The injury videos were individually examined by five experts in the field of sports medicine and SX. We identified the index frame where the injury was thought to happen. Agreement on the index frame was said to have been reached if three or more experts reported identical answers. If not, the specific case was reviewed in a plenary session until a consensus on the index frame had been reached. If the injury was hidden behind a gate, opponent, jump or was outside the camera view, the last frame seen prior to the time of injury was used as reference for the analysis. Gate or security net contact was not reported if not observed on tape, even if it was likely to have occurred. The index frame was used in the subsequent analyses, based on a form created specifically for the SX project, from the one previously used and validated for video analyses of alpine skiing injuries.14 ,15 The analysis form had specific focus on environmental factors, skier situation, obstacles, course line, skier/opponent behaviour, time of injury and asked for a qualitative description of factors leading to injury. Emphasis was made to distinguish whether there was any direct contact between the injured skier and any opponent, and if so, how this contact took place—and what consequences it had.

    The injury situations were divided into four main categories: jumping, turning, jumping and turning and rollers. Further classification within the different categories relates to the different types of jumps and turns. We divided jumping injuries into take-off and landing, and classified the type of jump as a single jump (kicker), double and triple jump, spines and double spines, step up and step down jumps, and table top jump (figure 2). Turning injuries were classified as occurring in regular turns or dozed turns (bank turns) (figure 2). Jumping and turning injuries is a category where the skier is either jumping while entering a turn, recovering from a turn while getting into a jump, or a feature where the skier lands perpendicular to take-off (corner). Rollers are several waves in the terrain that can be either absorbed in the over-ride or jumped over one or two at a time (figure 2).

    Figure 2
    Figure 2

    Different obstacles in Ski Cross (SX) usually involve in elite SX injuries, with the single jump (kicker) being the most frequently implicated.

    We also developed an exact topographical sketch of the relevant course section for all cases involving a jump or a turn (n=27). Different features and profiles in the terrain were clearly marked and the analysts were asked to outline the injured skier's course line (dotted), as well as the ideal line (solid line, figure 3F).

    Figure 3
    Figure 3

    Jumping injury. The figure shows a typical case caused by opponent contact from behind. (A) Skiers are positioning before take-off and opponent (Blue bib, #4 in heat) is gaining from behind. (B) Opponent (Blue bib) has an inappropriate course line, causing contact in air from behind. (C) Injured skier (Green bib, now #4 in heat) comes out of balance to the right (sagittal plane) and has an uncontrolled flight and one-legged landing. (D) During landing, skier is out of balance forward and to the right, suffering a tibia fracture (>28 days absence) at landing. (E) Injured skier (last in heat) tumbles and eventually gets caught in the security net. (F) A sketch of the course with injured skier's actual course line (dotted line, x=injury point) versus ideal line (solid line). Note that both the injured skier and opponent mistimes the jump and overshoots the ideal flight line considerably.

    Results

    Injury characteristics

    As shown in table 1, the injury pattern of the 92 time-loss injuries suffered in SX competitions during the 2006–2010 seasons resembles that of the 33 cases captured on videos from the finals, dominated by knee (n=9), head (n=9) and shoulder injuries (n=7).

    View this table:
    Table 1

    Distribution of injuries by body part in the total material of time-loss injuries (n=92) and the video material (n=33)

    Skiing situation and skier behaviour

    The majority of injuries occurred in jumping situations (n=16), followed by turning situations (n=8), a combination of jumping and turning (n=7) or while skiing on rollers (n=2). The main causes of injury are characterised in table 2.

    View this table:
    Table 2

    Expert assessment of the main causes of injury

    Contact with an opponent occurred in 21 of 33 cases, in 13 jumping injuries, 4 turning injuries, 2 jumping and turning and 2 on rollers. In three cases, the analysers were unable to determine whether there was contact or not, one when jumping and two when jumping and turning. The contact was mainly unintentional, but influenced on skier control in 17 of 21 cases, and contributed to the injury in 13 of these (table 3). One case, originally classified as a technical error, was changed to unintentional contact caused by opponent (view blocked on tape due to a roller) based on information provided by the injured athlete.

    View this table:
    Table 3

    Characteristics of skier-to-skier contact situations (n=21)

    Jumping situations

    Of the 16 jumping situations, 10 cases resulted from an incident at take-off, followed by 6 when landing. A similar distribution was observed in the jumping and turning situations (n=7), with four injuries at take off and three when landing.

    Taken together, of the two main categories involving jumping (n=23, including jumping and turning), a single jump (kicker) was the type of jump implicated most often (n=11; 8 at take-off, 3 when landing), followed by a step-down jump (n=3; 2 at take-off, 1 when landing), step-up jump (n=3; 2 at take-off, 1 when landing), corner (n=3; 1 at take-off, 2 when landing), double jump (n=2; both when landing) and table top jump (n=1; at take-off).

    During the 16 jumping incidents, there was contact between skiers in 13 cases, ski–ski contact (n=8) (figure 3), arm–arm (n=4) or ski–trunk (n=1). Contact was seen during two of the seven jumping and turning incidents, one ski–ski (with arm-arm) and one ski–trunk. Further characteristics of the contact situations across the main categories are shown in table 3, and the videos are available to view as online supplementary files.

    Turning situations

    Of the eight injuries occurring in turning situations, five occurred in a regular turn and three in a bank turn. Only one of the five regular turn cases involved contact (ski–ski boot), in contrast to the bank turning cases, which all involved contact. No particular turning phase or contact situation dominated.

    Skiing on rollers

    Two injuries occurred while skiing on rollers. Both the injured skiers lost control due to ski-ski contact with the opponent when landing, resulting in a fall and injury.

    Skier control

    The detailed characteristics of skier control are shown in table 4. The injured skier lost control and came out of balance before the time of injury in all cases (n=33), leading to a fall in 29 cases. The main causes of loss of control were skier–opponent contact (n=13), technical errors (n=8) and inappropriate strategy (n=8; table 4).

    View this table:
    Table 4

    Characteristics of skier control and choice of course line across the four main injury categories

    Skier strategy and course line

    The injured skier had an inappropriate course line related to take-off when jumping in 5 of 33 cases (table 4); 4 of these involved skier–opponent contact and 1 gate contact.

    Seven of the eight cases classified as ‘caused by an inappropriate strategy’ were due to strategic errors at take-off (five jumping, two jumping and turning), such as inappropriate course line (four cases, see example in figure 4), bad timing of take-off point (two cases, see example in figure 5), and bad positioning (one case). The final case occurred during turning, the skier having difficulties with recovering from a previous obstacle when attempting to handle a compression before the next turn.

    Figure 4
    Figure 4

    Jumping injury (inappropriate strategy and course line). (A) Injured skier (yellow bib) has an inappropriate strategy and bad line at take-off, (B) is out of balance backwards and has an uncontrolled flight and landing. (C) Skier jumps too far, straight into the turn, (D) causing ski–ski contact with an opponent. (E) Left ski catches an edge while unbalanced in the compression, skier suffering an anterior cruciate ligament injury. (F) A sketch of the course showing injured skier overshooting the ideal flight line (dotted line, x=injury point) versus ideal line (solid line).

    Figure 5
    Figure 5

    Jumping injury (inappropriate strategy; bad timing). (A) Injured skier makes timing mistake at take-off. (B) Hesitating and back-weighted, (C) over the jump to avoid contact/crash with opponent in front, thus never gaining control in the air (D) and landing unbalanced, (E) resulting in a concussion. (F) A sketch of the course with injured skier's substantially higher and longer flight line (dotted line) versus ideal line (solid line). Small x=injury point as seen in frame E. Large X=point of landing after injury and uncontrolled flight.

    Skier technique

    Of the eight injury cases that were due to technical errors (table 2), six involved jumping (three jumping, all at take-off, and three jumping and turning, one at take-off, two before) and two turning.

    In four of these six jumping cases, the skier made a technical error at take-off due to bad jumping technique, being back weighted and unbalanced, resulting in an uncontrolled flight. The other two jumping cases were due to a technical error that forced the skier out of balance before take-off, catching an inside edge or because of an error through the previous obstacle, leading to an uncontrolled flight.

    In both of the turning cases, the skier was out of balance because of a technical error with too much inside lean (figure 6).

    Figure 6
    Figure 6

    Turning (technical error) (A) skiers coming out of previous turn, fighting for positions. (B) Skiers have high speed into the turn and (C) injured skier (yellow bib) catches an inside edge and gets out of balance forward and to the right, (D) flipping skier in air. (E) Skier falls, sustaining a concussion (8-28d). (F) A sketch of the course with skier's actual course line (dotted line, x=injury point) versus ideal line (solid line).

    Course influence on contact

    The course design was described to have influenced 7 of the 21 cases involving skier–opponent contact. In two cases (jumping), the course was considered to have a strong influence because of too little space between obstacles and a too narrow course with no other options for the injured skier. In the remaining five cases (three jumping, one jumping and turning, one turning), the course design had a slight influence as the obstacles were placed too closely, but with the possibility for the skier to choose another option.

    Gate and security net contact

    Five cases involved gate contact, influencing skier control in one case, while the skier already had lost control in the remaining four cases. Nine cases involved contact with the security net. The security net functioned adequately in six cases, but in two cases the net was of the wrong type (see figure 7) and one net was misplaced (one case could not be assessed).

    Figure 7
    Figure 7

    Turning—wrong type of net. (A) Injured skier (last in heat) gets cut off on the inside by skier coming from behind, (B) opponent drifts out, (C) and wipes skis away from injured skier. Injured skier catches an edge and (D) has an uncontrolled flight into the net. (E) Skier gets thrown back into the piste. (F) Skier sustains an injury instead of getting caught by the net attenuating the landing impact.

    Discussion

    This is the first study to analyse the events leading to time-loss injuries in WC freestyle SX, based on systematic video analyses. The study reveals that skiers lose control and become unbalanced before the time of injury, thus sustaining an injury when landing/falling after the previous obstacle, usually a jump or a turn. The main causes of loss of control were contact with an opponent and personal mistakes at take-off, inappropriate technique and strategy (mistimed take-off, bad line).

    Skiing situation

    The injuries in SX occurred while jumping, turning, jumping and turning and skiing on rollers, very similar to the pattern seen in SBX.8 As suggested in previous epidemiological studies examining the injury incidence in SX,3 ,5 jumping was involved in more than half of the injury cases. The features/jumps in competitive SX are specifically designed to provide speed, minimise air time andare incorporated in a smooth race line,2 in contrast to jumps in terrain parks. However, half of the jumping cases and the majority of the jumping and turning cases were due to personal mistakes at take-off. The other half were due to skier–opponent contact, usually at take-off or landing. The skier typically used an inappropriate strategy (bad timing, overshooting the jump or choosing a bad line), causing contact with an opponent at take-off or landing and becoming unbalanced, or making a technical error (bad jumping technique), thus becoming back-weighted and having an uncontrolled flight. Not surprisingly, this mirrors findings from SBX,8 where the racers compete in the same type of course, sometimes even identical as in the Vancouver 2010 OWG.

    Interestingly, all bank turning cases occurred because of skier–opponent contact caused by the opponent drifting out and wiping the skis away from the injured skier, just as described previously in SBX.8 The regular turning situations were mainly due to technical errors at the previous obstacle, such as too much inside lean, reflecting findings from the WC alpine skiing.15 In contrast to SBX, where technical errors were thought to dominate,8 SX injuries in roller situations resulted from ski–ski contact with the opponent in landings.

    Skier behaviour

    In all cases, we found that skiers had already lost control before the injury happened, mainly due to technical/tactical errors and/or unintentional contact with an opponent, usually ski–ski and occasionally arm–arm contact between two skiers. These patterns were also observed in the study of Bakken et al.8 One notable difference is that the opponent causing the contact usually came from behind (8 of 13) in SX, while in SBX the responsible athlete was in front. The current study supports the suggestion of Engebretsen et al3 that ‘accidental ski and body contact or within the rules of the sport might force the skier to have an unanticipated reaction, loss of control and thus probably leading to high-risk situations.’3 ,8 It is clear that skier–opponent contact regularly causes loss of control and high-risk situations and injuries.

    Methodological considerations

    It is important to keep in mind that all the videos we were able to obtain for this study are from final runs, where the skiers race in groups of four, as opposed to the single qualification runs. Thus, we do not know whether injury mechanisms in SX differ between finals and qualifications. However, from our findings, it seems likely that skier–opponent interaction in heats is not only a source of direct contact, but also represents an additional stress factor possibly explaining the personal mistakes observed.3 On the other hand, Engebretsen et al3 reported a surprisingly high injury proportion during a single run official training (three of every four injuries recorded in the 2010 OWG). In the current 4 year material of 92 SX injuries, a majority of injuries were from the finals (61%) versus qualifications (39%). Interestingly, a recent epidemiological study based on the same material, but including accurate exposure data, revealed no difference in the injury rate between qualifications versus final runs in SX. In contrast, in SBX, the incidence was threefold higher in heats compared with single-rider qualification runs among men, but not among women. The apparent discrepancy between studies warrants further research with a larger sample, which includes video material from qualification single runs.

    Video quality and available camera views represent a challenge not only when determining the moment of injury but also when assessing the mechanisms, but it should be noted that the assessment was quite consistent across analysts.

    One advantage with research at the elite level is that not only do the athletes represent a well-defined cohort of subjects that can be followed prospectively, but the TV coverage of major events also enables the systematic collection of injury videos. Previous studies8 ,12 –15 from the Oslo Sports Trauma Research Center have shown that critical information can be learnt from analyses of such injury tapes, which can then be applied to other groups.

    Perspectives

    This study revealed that jumping was the most challenging obstacle, especially the single jump-kicker, where personal mistakes at take-off and contact caused an uncontrolled flight, unbalanced landings and subsequent injury. Bank turn injuries were caused by skier–opponent contact, while personal mistakes caused injuries in regular turning situations. Thus, it appears that preventive measures should focus on course building as well as skier technique and strategy. According to the FIS WC SX rules, there are no standards for course and obstacle building in terms of height, number of different features, etc, except for the distance from the start to the first bank and the type of obstacles allowed.2 The current findings suggest that reducing the number of narrow parts and increasing the space between obstacles (especially jumps into compressions, turns into jumps, etc) to a minimum standard determined by FIS could be beneficial, even though the majority of the jumping-related injuries were caused by a personal mistake at take-off or skier–opponent contact, and not by the design of the jump itself. Such an intervention will provide the skiers with valuable extra time enabling a more appropriate strategy and handling of the obstacles. Nevertheless, it could be beneficial to reduce the energy involved at landing, as suggested previously.8 Also, attention should be directed at the jumping and landing areas in relation to the jump profile, course width at take-off and, most importantly, the landing area. In addition, a reduction of the number of skiers in each heat could potentially reduce some stress factors that possibly contribute to the personal mistakes frequently observed, but this would represent a radical change to the nature of the sport. Thus, an increased focus on awareness training, inspired by other team sports (eg, soccer), and emphasis on anticipating critical situations may be beneficial. Furthermore, it is essential that the right types of safety nets be used and that their placement be correct to maximise their protective effect in case of a fall. In the major WC downhill skiing venues, fixed safety nets have been installed. Although the cost is high, this may be a necessary next step in SX venues as well.

    Conclusions

    We identified four main injury situations in elite SX, with jumping situations being the most frequent. The primary cause of injury was unintentional skier–opponent contact in jumping, bank turning and roller situations. Another common cause of injury was personal errors (inappropriate technique and strategy) at take-off and in turning situations.

    What are the new findings?

    • This is the first study to describe injury situations in elite Freestyle Ski Cross (SX), and we identified four main injury situations in elite SX, with jumping situations being the most frequent.

    • The primary cause of injury was unintentional skier–opponent contact in jumping, bank turning and roller situations.

    • Another common cause of injury was personal errors (inappropriate technique and strategy) at take-off and in turning situations.

    • We also document a few cases of inappropriate use of security nets, either misplaced nets or using the wrong type of net.

    How might it impact on clinical practice in the near future?

    • The International Ski Federation (FIS) will take our results into consideration and might implement our preventive suggestions through strict rule enforcement, as well as a standardisation of course building with a focus on jumping and landing areas, as well as course width.

    • Owing to the high injury risk and last season's tragic death, there is an increased focus on prevention in Ski Cross (SX), both within the FIS and among the national team coaches, which is crucial in order to improve the technical and strategic errors among the racers, especially at take-off and turning.

    • Although the cost of installing fixed safety nets is high, this may be a necessary next step in SX venues, as in major World Cup downhill skiing venues, in order to maximise their protective effect in case of a fall.

    Acknowledgments

    The authors would like to thank the International Ski Federation staff and officials for all practical support in collecting the injury data, as well as Anna Banach and Infront for the help with video capturing and editing. A special thanks to Audun Grønvold and Morten Skarpaas for help with the video analyses and to Kjetil Bakken-Engelsen for the topographical sketches and figure 2.

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    Supplementary materials

    Footnotes

    • Contributors SR, SH, TB, LN and RB contributed to the study conception, design and methodology for the video analyses. SR coordinated the study and managed all aspects of the video analysis process, including video processing and organisation of the video analyses. SR wrote the first draft of the paper, and all authors contributed to the final manuscript. SR and RB are the guarantors.

    • Funding The Oslo Sports Trauma Research Center has been established at the Norwegian School of Sport Sciences through generous grants from the Royal Norwegian Ministry of Culture, the South-Eastern Norway Regional Health Authority, the International Olympic Committee, the Norwegian Olympic Committee & Confederation of Sport, and the Norsk Tipping AS. The FIS Injury Surveillance System is supported by the International Ski Federation and has been established through a generous grant from Don Joy Orthotics (DJO). SR was funded by a grant from the medical student research programme at the Faculty of Medicine, University of Oslo.

    • Competing interests None.

    • Ethics approval The study was reviewed by the Regional Committee for Medical Research Ethics, South-Eastern Norway Regional Health Authority, Norway.

    • Provenance and peer review Not commissioned; externally peer reviewed.