'temporary fatigue' is not apparent in elite youth soccer players

CHAPTER NUMBER 22

‘TEMPORARY FATIGUE’ IS NOT APPARENT IN ELITE YOUTH SOCCER PLAYERS 1. INTRODUCTION Recent time-motion analyses have identified and described the phenomenon of ‘temporary fatigue’ in elite-level soccer match-play (Mohr et al., 2003, 2008), whereby the players’ work-rate is temporarily decreased after intense period of play. The approach typically adopted to detect this phenomenon involves categorising time-motion data into pre-determined 5-min periods, with ‘temporary fatigue’ denoted as a marked reduction in the players’ high-speed running distance (≥15 km·h-1; HSR) in the 5-min period immediately subsequent to the 5-min period in which the most HSR is observed. ‘Temporary Fatigue’ is said to be evident where the HSR in the subsequent 5-min pre-determined period is lower than the average 5-min match period (Mohr et al., 2003, 2008). However, the use of velocity bands to characterise ‘temporary fatigue’ is a technique that might be insensitive to some high-intensity activities, which may cause transient fatigue through neuromuscular or metabolic pathways. Highintensity actions such as collisions, accelerations, decelerations, unorthodox running and turns often occur at velocities below 15 km·h-1, but these activities are metabolically taxing (Osgnach et al., 2010; Reilly and Bowen, 1984). The advancement of micro-sensor technology now enables practitioners in team sports settings to measure the frequency and magnitude of instantaneous accelerations in the anterior-posterior, medio-lateral, and longitudinal planes. The tri-axial accelerometer is typically used for estimates of physical activity and energy expenditure, therefore powerful movements and accelerations, which are absent from traditional time-motion techniques, can now be quantified during match-play in field-based settings. We postulated that the use of tri-axial accelerometer data might be a more sensitive tool to detect ‘temporary fatigue’. Whilst the ‘temporary fatigue’ phenomenon has been demonstrated in ‘topclass’ male (Mohr et al., 2003) and female players (Mohr et al., 2008), it is presently unclear if the same pattern is evident in youth players. There is a growing body of work in the literature researching the match-play demands of elite youth soccer (Buchheit et al., 2010a, 2010b; Harley et al., 2010) and players’ physical and physiological capacities (Buchheit et al., 2010b) in an attempt to improve talent identification and training processes. The demands of youth soccer

2 ‘Temporary fatigue’ in soccer match-play

match-play, in particular the distances covered in different locomotor categories have been reported between the ages of 12-18 (Buchheit et al., 2010b; Harley et al., 2010). However, in this population little is known about the changes in work-rate over the course of the match, which may provide information regarding the development of fatigue as a consequence of intense match periods. There is evidence to suggest that the fatiguing patterns may have different characteristics to those observed in the elite adult player, as adolescents are able to resist fatigue during repeated maximal exercise to a better extent that adults (Beneke et al., 2005). Therefore we hypothesised that ‘temporary fatigue’ as defined in the literature (Mohr et al., 2003, 2008) may not be evident in elite youth players. Therefore the aim of this study was to examine ‘temporary fatigue’ in elite youth players. We also aimed to compare methods for identifying this transient fatigue development, using both arbitrary velocity thresholds from GPS data, and a tri-axial accelerometer that resides inside the GPS unit. 2. METHODS 20 elite male youth soccer players (Age: 17 ± 1 yrs; VO2max: 61 ± 6 ml·kg-1·min-1) participated in this study, which attained a priori ethical approval and informed consent. Each player was post-adolescent with a mean of 3.2 (± 0.4) years after peak height velocity. Players trained on a ‘full-time’ professional basis for 13.5 hrs per week, which included specific technical and conditioning sessions, and one competitive fixture each week Players were harnessed with a 5 Hz GPS (MinimaxX, Catapult, Australia) unit during 21 competitive league fixtures (5 ± 3 matches per player) during the 2008/09 and 2009/10 seasons. Injury time was excluded in this study, as were any incidences where the player did not complete the full game or changed tactical position during match-play, resulting in 111 match observations. Locomotor activities in arbitrary velocity bands, and tri-axial accelerometer data (Player LoadTM - PL) were derived from the GPS system and classified into pre-defined 5min periods. High speed running was reported as the distance covered at ≥ 15 km·h-1. Peak HSR distance represented the greatest distance covered in a 5-min period specific to each match instance. The HSR performed in the subsequent 5min interval, and the mean of the remaining 5-min periods were compared as in previous research. However, we also ranked each 5-min epoch (1-18; where 1 = peak HSR period) to facilitate contextualisation of the ‘temporary fatigue’ phenomena in relation to other match-periods characterised by a reduced work-rate. The peak PL was compared to both the subsequent and mean values as described above. The PL in each 5-min epoch was also ranked (1-18; where 1 = peak PL period). The PL was reported as a vector magnitude, which sums the frequency and magnitude of accelerations in all axial planes and is the most common algorithm reported in studies using this technology. Whilst accelerometer measures in sport and exercise settings are in their infancy, the between-unit variability is low during team-sport activity (~2 % CV; Boyd et al., 2011) and they


may provide a more rigorous assessment of the players external load than the traditional time-motion metrics. Paired-samples T-tests were used for pairwise comparisons. Significance was accepted at p < 0.05. Data are presented as mean ± SD. 3. RESULTS High Speed Running 12.2 % of match observations were ineligible for this temporary fatigue analysis as the peak HSR was observed in the final 5 min periods of the first (4.7 %) and second halves (7.5 %). 60% of peak HSR incidences occurred during the first half, with 43% observed within the first 25 mins of match-play (see Figure 1). The mean rank of the subsequent 5-min period was 9.8 ± 4.4. The HSR distance covered in the peak 5-min periods was 178 ± 42 m (see Figure 2), with a 47 ± 23 % decrease observed in the subsequent interval (94 ± 46 m). However there was no significant difference between the HSR in the subsequent and mean (88 ± 25 m) 5 min epochs. The corresponding PL at the HSR peak (83.5 ± 20.2 au) was greater than the subsequent (70.9 ± 19.6 au) and mean (69.0 ± 15.0 au) periods, with no difference subsequent and mean values.

Figure 1. Distribution of peak HSR and PL according to the pre-determined match period in which they occurred. HSR = High speed running (≥ 15 km·h-1); PL = Player loadTM.

Player LoadTM


12.4 % of peak PL cases were ineligible the analysis as the peak was observed in the final 5 min periods of the first (4.8 %) and second halves (7.6 %). The 5-min periods were ranked for PL, and the mean rank of the subsequent 5-min period was 8.5 ± 4.5. The peak PL was 92.0 ± 18.7 au, with a 22 ± 12 % decrease observed in the subsequent interval (71.7 ± 17.4 au), and a significant difference between the HSR in the subsequent and mean (67.8 ± 13.6 au) 5 min epochs (See Figure 2).

Figure 2. Peak high-speed running and peak player load in a 5-min period, subsequent 5-min, final 5min (85-90 mins) and mean values of all other 5-min periods. * Denotes peak greater than all other 5min periods (P