Over the past two decades, fatigue development in football has become one of the primary research areas in the broader field of football physiology (Bangsbo et al., 1994; Mohr et al., 2003; 2010; Reilly, 1997; Bradley et al., 2013; Waldron and Highton, 2014). Among mental and tactical factors encompassing fatigue, particular attention was paid to the element of physical fatigue (Paul et al., 2015), leading to a wealth of time-motion analyses pertaining to match running performance in top-level football (Bradley et al., 2009; Di Mascio and Bradley, 2013; Di Salvo et al., 2009; Rampinini et al., 2007; Weston et al., 2011). To evaluate physical fatigue in terms of a decline in players' match running performance, the approach taken by most studies has been to compare temporal running patterns in the early and later stages of a match. Using such segmentation methods, studies have demonstrated a decline in running performance between the first and second halves of football matches, particularly in total distance covered (TD), time spent in high intensity running (HIR) and number of sprints (Di Salvo et al., 2009; Mohr et al., 2003; Rampinini et al., 2009). Others have examined match-running performance across 15-min periods, reporting significant reductions of HIR and acceleration efforts over the course of a match (Akenhead et al., 2013; Bradley et al., 2010). A more detailed analysis on minute-by-minute observations revealed that by eight minutes into the second half the median distance per minute had already substantially decreased in comparison to the corresponding median distance in the first half (Barros et al., 2007).
To gain practice-oriented recommendations from such observations, sports scientists need to consider the work rate-specific characteristic of the sport. Football is an intermittent sport that involves frequent but brief periods of high-intensity movement, interspersed with lower intensity running (Bangsbo, 1994) and matches are usually composed of a series of play periods randomly interspersed with game stoppages, such as when the referee has called an infringement, or the ball is off the playing field (Wallace and Norton, 2014). Accordingly, a study of game interruptions in elite football showed that matches are halted on average for 38% of the total match time (Siegle and Lames, 2012). More importantly, evaluations of the time the ball is in play over predefined match periods have indicated that the duration of game interruptions increases towards the end of a match (Carling and Dupont, 2011), indicating that an increase in game interruptions towards the end of a match could have an impact on match running performance. However, only one study has analyzed football-specific match running performance while considering the effective playing time ([T.sub.eff]), and this was based only on an entire match without consideration of fatigue development (Castellano et al., 2011). Thus, to the best of our knowledge, no study has evaluated the contribution of game interruptions to the decline in match running performance in professional football.
Therefore, the aims of this study were twofold: (1) to test whether a verifiable decrease in effective playing times occurs over the course of a match, and (2) to quantify the contribution of game interruptions to the decline in match running performance as the game progresses.
Participants and sample size
The sample comprised positional data from 51 matches of the German Bundesliga across the 2012/2013 (n = 21) and 2013/2014 (n = 30) season. Analysis included data for outfield players (goalkeepers excluded) who completed the full duration of the match, excluding substitutes, as their performance differs significantly from the performance of the players they replace (Carling et al., 2010). In addition, only matches with a narrow end result (one-goal difference or tie games) were considered for analysis. Based on these criteria, the match running performances of 792 players (single observations) were examined. It was a condition of players' employment that data such as those used in this study could be obtained for routine assessment of their performance during the competitive Bundesliga season. Hence, the usual ethics committee approval was not required (Winter and Maughan, 2009). All subject identifiers were removed to ensure confidentiality. This study conformed to the recommendations of the Declaration of Helsinki.
Analysis included the official match running performance data of the Deutsche Fuhball Liga GmbH, which were assessed by a computerized multiple-camera tracking system (TRAcAb[R], Stockholm, Sweden) operating at 25 Hz. This tracking system semi-automatically assesses the match running performance data of all players, the position of the ball, and corresponding match events (such as game stoppages), allowing the quantification of effective playing time ([T.sub.eff]) as the total playing time ([T.sub.tot]) minus all game stoppages such as for fouls, goals, free kicks, substitutions and injuries, i.e. the total time during the match that the ball is in play (Castellano et al., 2011).
Using a football-specific criterion measure approach pioneered by Reilly and Thomas (1976) (for a review, see Carling et al., 2005; Reilly, 2003), an independent study on the validity and reliability of the TRACAB[R] tracking system reported average measurement errors of 2% for measures of distance covered.
To investigate temporal patterns in match running performance, data were divided into six pre-defined 15-minute match periods (three per half). Periods of extra time at the end of the first and second halves were excluded from the analysis. The categorical independent variables were (i) the match period, and (ii) the method of time registration ([T.sub.eff] vs. [T.sub.tot]). The continuous dependent variable was the match running performance data. In accordance with previous...