Several studies (Ekstrand et al. 1983, Arnason 96, Inklaar 96, Dvorak 2000) have found a high incidence of injuries in elite male soccer and higher in games than in practice. The ethiology of the soccer injuries is less well understood. No data on the relationship between playing style and injuries are available in the literature. However, match analysis has been widely used in Norway by national team coaches (Olsen & Larsen 1997). This project is an attempt to take a similar approach to injury risk analysis using the experience and methods of the coaching staff.
The aim of the present study is to design and validate a method where video analysis is used to obtain data on situations leading up to an injury or a high - risk incident. Videotapes from 35 (46%) of 76 official Norwegian U-21 matches played in the period from February 1994 to June 1998 were traced. Of the 35 videotapes, 30 covered the match in full. The total duration of matches was 3017 min. The videotapes were analysed by two experienced doctors (TEA/LE) and all situations where the match had to be stopped because a player appeared to be injured on the pitch were noted. An incident was recorded if the match was interrupted by the referee, and a player on the Norwegian team lay down on the pitch for more than 15 s, and the player appeared to be in pain. Two soccer experts with long experience in match analysis and the team doctor (TEA) reviewed the master videotape.
Match analysis is defined as analysis of the team events leading up to the incidents, e.g. attacking, defending, and aspects related to the effectiveness of a match play on the field. The playing field was divided into five zones. Classification of playing positions was based on static playing formations. The Norwegian U-21 soccer team used a 4-5-1 formation in all matches. Incidents were classified as occurring in two playing phases, the attacking or defending phase. Several other variables, including defending team balance, types of duels e.g. heading, running, tackling and dribbling duels, awareness of the duel, movement intensity, were also recorded.
Information on injuries was obtained by retrospective review of team medical records by the team physician. Each incident identified on the videotapes was cross-referenced with the medical records and classified as an injury if the player had been unable to participate in training or match play at least for one day following the incident. Injuries were classified as minor when the player could not practice soccer normally or play matches for 1-7 days, moderate if absent for 8-21 days, and serious if absent for more than 21 days (Lewin 89).
Incidents and injuries
During the 35 matches available for video analysis, 52 incidents were recorded for the Norwegian team, i.e. 1.5 incidents per team per match or 94 incidents per 1000 player hours. Of the 52 incidents, 16 (31%) led to traumatic injuries, i.e. 0.5 injuries per match or 29 injuries per 1000 player hours. Injuries and incidents were distributed evenly throughout the six quarters of the game (chi square, p=0.50). Of the 16 injuries, seven were classified as serious, three as moderate and six as minor.
The Kappa analysis showed that reliability was high and within acceptable limits for all the variables used. The inter-rater agreement was good (0.61-0.80) for ten variables and very good (>0.81) for nine variables. The intra-rater agreement was very good for 18 variables and good for one variable.
Of the 52 incidents recorded, 28 occurred when the team was in the attacking phase (8 injuries) and 24 in the defending phase (8 injuries). Most of the incidents during defence occurred in the defensive zone or midfield zone 1, whereas most of the incidents during offence took place in midfield zone 2 and the attacking zone. Midfielders, i.e. central midfielder, inside left/right midfielder, and wing midfielder, were exposed in 67% of the incidents.
In 17 cases a short pass was the last team event before an offensive incident, while there were only five incidents after long forward passes. In 17 cases a short pass was the last opponent team event before an defensive incident, and there were three incidents after a long forward pass.
Most incidents (70%) were the result of tackling duels. Of the 21 offensive incidents resulting from tackling duels, in 19 cases the exposed player was unaware of the opposing player or engaged in another activity (passive duellist). In eight incidents the exposed player was tackled from the front, in seven from the side, and in four incidents from the rear. Of the 15 defensive incidents resulting from tackling duels, the exposed player was the active duellist in seven cases and the passive in eight cases. Of the seven active tackling duels, the exposed player was tackling from the front in five cases and from the side in two cases.
In 27 (52%) of the incidents no free kick was awarded by the referee, fourteen led to a free kick for the exposed player and one led to a free kick against. Eight incidents resulted in a yellow card, whereas no red card was called. In two incidents the decision of the referee was unknown.
Conclusion: The main outcome of this study was that football incident analysis (FIA) has been developed as a reliable tool to analyse and describe video recordings of incidents and injuries in football-specific terms. Although soccer is a complex game where it is difficult to classify the various playing actions and player interactions, the inter- and intra-observer reproducibility for the majority of variables developed during pilot testing of FIA was high. This study shows that video analysis of incidents is a potentially valuable tool to understand the events leading up to injuries in football.