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Effects of congested match periods on acceleration and deceleration profiles in professional soccer

The aim of the present study was to analyse the influence of congested periods of matches on the acceleration (Acc) and deceleration (Dec) profiles of elite soccer players. Twenty-three elite male professional soccer players participated in the study across 31 official matches. Assessed periods included: (i) congested periods (three to four days between games), and (ii) non-congested periods (more than four days between games). Physical activity during matches was recorded during games using a 10Hz global positioning system device, coupled with a 100 Hz accelerometer, and was analysed according to the periods. Maximal Acc- (73.2 ± 20.3 vs. 84.918.5 m), high Acc- (244.0 ± 49.5 vs. 267.0 ± 37.8 m), maximal Dec- (139.0 ± 44.8 vs. 152.039.3 m) and the total decelerating- distance (5132 ± 690 vs. 5245 ± 552 m) were lower in congested than in non-congested periods (p < 0.05, effect size 0.31–0.70). Neither a main effect of playing position nor a period*playing position interaction on Acc and Dec were observed (p > 0.05). It was concluded that Acc and Dec match activities were significantly affected during congested periods compared to non-congested highlighting a possible fatigue accumulation being responsible for the observed decrement in physical activity. Monitoring Acc and Dec metrics throughout particular periods of congested fixtures amongst professional soccer teams is advised and may be a way to assess physical and fatigue status.


INTRODUCTION
Competitive soccer match-play at the very elite level involves a high physical demand imposed on players to perform continued, explosive changes of direction (COD) coupled with a large amount of repeated high-speed actions [1]. Daily assessment and monitoring of player training and match demands have received much attention in recent times with global positioning systems (GPS) being one of the most common approaches of quantifying players' physical load. Total distance covered (TDC) and time spent at different speeds thresholds monitored via GPS are two of the most common reported metrics [2,3].
As a result of this, limiting the reported analysis to speed-based thresholds, and minimal reports surrounding few key movements involving high muscular and power demands (but with minimal or reduced speed involvement), are not taken into consideration.
According to research, the energy cost related to high-speed running (HSR) distance is underestimated based on the lack of Effects of congested match periods on acceleration and deceleration profiles in professional soccer AUTHORS: Léo Djaoui 1 , Adam Owen 1,2 , Matt Newton 3 , Pantelis Theodoros Nikolaidis 4 , Alexandre Dellal 1,5 , Karim Chamari 6

Experimental Approach to the Problem
To analyse the effect of a congested fixture period on Acc/Dec profiles, 32 official competitive matches were analysed across two consecutive seasons between March and May 2016 (n = 13 matches) and between July and December 2016 (n = 19 matches). Across the analysis, a total of seven matches (n = 7) were played in a congested period and twenty-five (n = 25) were played in a non-congested period. Three to four days between games separated the "congested matches", with the "non-congested matches" being characterized by at least five days between games. All the official matches included 30 competitive matches from a professional European country league, and 2 competitive match data sets taken from the Football Association's National cup competition.

Subjects
Twenty-four elite male professional soccer players (age: 23 ± 4.5 yearsrange between 19,5 and 28,5-; height: 178 ± 6.1 cm; weight: 73 ± 8.2 kg) from the same Swiss team participated to the study. to maintain usual nutritional and recovery protocols usually used in the club. All participants provided written consent before the beginning of the investigation. The present study was approved by the involved sport science department of the soccer club and by the local university ethics committee before the beginning of the assessments. The present study conforms with the ethical standards for research recommendation of the Helsinki Declaration.

Procedure
Throughout all matches, players' physical activity was assessed using a 10 Hz GPS coupled with a 100 Hz accelerometer (Viper, Statsport, Ireland) which has been reported as a valid and reliable GPS unit, especially when tracking team sport movements and accelerations [24,25]. The TDC and relative TDC per minute (RTDC) were measured. The distance covered and time spent at different speed thresholds were also measured and expressed in absolute distance over the game and relative distance over one minute: exertion [13]. It has also been shown that Acc and Dec activity was moderately correlated with HSR distances during elite soccer matches [14]. Utilising this information and suggested recommendations from the literature, the need to monitor soccer players' physical activity combining both speed and Acc/Dec indicators is of paramount importance [15].
Competitive and training demands imposed on elite level players is continually increasing based on the need to fulfil increased domestic, European and International fixtures. Such busy schedules impose short recovery bouts between matches. Literature suggests that during congested fixture periods, elite players can reproduce distances covered during fixtures through increasing match distance covered at low intensity [16,17], but without significantly changing the moderate-, high-and maximal-speed running activity [18]. Recently, it was also reported that congested fixture period increased the physiological stress and fatigue, especially increasing blood levels of C-reactive proteins, white-blood cells and creatine kinase (CK) [19].
Creatine kinase, as a marker of muscular damage, was the only indicator in this particular investigation that was significantly affected by a shorter recovery between matches [19]. Moreover, the CK increase observed 24hrs and 48hrs post-game has been related to a neuromuscular COD test impairment [20]. Further reports in this area highlighted how excessive competitive fixtures as a result of combined domestic and continental participation may induce significant end of season-accumulated haemolysis [21]. In particular, Owen et al. [21] revealed how relationships exist between lower mean cell volumes, mean cell haemoglobin, red-blood cell count and haematocrit values, highlighting the accumulative effects of seasonal training and congested match-play demands.
From the current literature, it might be hypothesised that the increased muscle damage marker observed during congested matches was a resultant of repetitive Acc and Dec movements performed during match-play, in accordance with a lack of sufficient between match recovery. Related to this, Arruda et al. [22] found that the number of Acc per minute (Acc/min) was significantly impacted by a very congested game schedule among youth soccer players. Furthermore, Acc/min were especially reduced during the 2nd and the 5th matches of a three-day tournament. Nevertheless, to date, and to the best of the authors' knowledge, no study has examined the effect of a congested calendar on Acc and Dec during official matches in professional senior soccer players. However, it should be taken into account that players perform different Acc and Dec profiles according to their positional role on the field [23]. As a result of the literature surrounding this topic, the aim of the present investigation was to examine the physical activity of professional players during congested and non-congested matches, with special reference to the Acc and Dec profiles, in terms of both distance covered and time spent. It was hypothesised that Acc and Dec scores would be negatively impacted across a congested fixture period and that this outcome would vary according to the playing positions.

Physical activity in terms of speed threshold
The playing position effect for every analysed variables was presented in Table 2 (Table 3, Table 4, Figure 1). No interaction effects were observed for all the acceleration and deceleration variables (p > 0.05). CV obtained in Acc and Dec profiles ranged from 8 to 32% and Cronbach's alpha was 0.95.

DISCUSSION
The aim of the present study was to analyse the effects of acceleration and deceleration profiles of professional soccer players during official games across congested fixture periods in terms of distance covered and time spent in each Acc and Dec. The results highlighted the fact that both Acc and Dec profiles were significantly affected by congested periods of matches. It was observed that (i) the distance covered through maximal Acc, relative maximal Acc, high Acc, relative high Acc, relative maximal Dec and relative intermediate Dec during matches were lower in congested than in non- within congested periods, especially with a significant decrease in salivary testosterone, salivary immunoglobulin-A and an increase in muscle damage markers [19,31,32]. It is well known that COD, Acc and Dec, which are specific components of soccer player's movements [1], require a strong muscular involvement with both concentric and eccentric muscle contractions [6,7,8], especially when starting from low-speed running that imply very high Acc to reach great speed in short time [28,33].
No differences were specifically observed among playing positions for the effect of a congested calendar. Various authors observed that defensive players were negatively affected by a congested calendar, in terms of tactical synchronisation [34], defensive technical activity [31], and physical activity when comparing a microcycles of two matches within three-day vs. four-day [35]. Suggestions of why offensive players were less affected by the congested phases maybe due to fitness levels, potential benefits of more squad rotation aspects, or possible substitution strategies [34,35]. Indeed, it was suggested by Penedo-Jamardo et al. [35] that the observation that offensive players were less affected by the congested calendar was due to a better physical profile or because they beneficiated more from Biology of Sport, Vol. 39 No2, 2022 315 the investigated team, which might have highly involved the lateral players [41]. CB had the lowest overall activity, except in LSR, LAcc, TDec, LDec, RTDec and RLDec. These observations were in line with the current literature stating that CB is the playing position with the lowest physical demand, especially high intensity efforts [27,42].
However, their tactical position on the field, facing the game, make them see and anticipate the moves, especially realising more backward and lateral runs that cost more energy than classical forward running [43].
The analysis of distance covered and time spent at different speed threshold showed that the congested calendar did not affect the overall physical activity, except for distance covered per minute at ISR, which were negatively affected during congested matches (35.5 ± 7.66 vs. 37.9 ± 7.67, ES: 0.31). This result was not in accordance with the current literature. Some authors found a positive impact of congested calendar in running distance covered at speeds inferior to 12 km·h -1 [17] and distance per minute covered at speeds inferior to 11 km·h -1 [16]. It is possible that the different inclusion criteria related to time of play induced differences within the studies, as Djaoui et al. [17] included players who participated to the total duration of the match and only measured absolute distance covered, and Carling et al. [16] included every players including substitutes. We can also consider that if only data from players who participated in 100% of a match would have been retained for the present analysis, the results could have been different. Furthermore, players from the present study coverer ~66 m per min and players from comparable studies covered ~69-73 m per min [16] and ~7100-7400 m (which would be higher than 70m per min) [17], with similar differences in all speed categories, which might suggest a different physical involvement related to a different level of players. The potential different standard of play and physical fitness might therefore explain the different results observed. It might therefore be suggested that, even if they were professional soccer players, the potential lower physical fitness of players from the present study could have negatively impacted their physical activity in relative ISR distance per minute during the congested periods. However, the distance covered and the time spent at HSR and sprinting were unaffected by a congested calendar, which was in accordance with current literature on this topic [18]. It might also be suggested that reducing the physical activity at ISR would not have the same effect on overall match performance, since high and very high intensity actions are supposed to be the most decisive ones to succeed [36,44,45].
In the present study, physical activity was tracked using 10 Hz GPS units, which were reported to have a compromised accuracy when tracking Acc over 4 m·s -2 [46]. Nevertheless, even if caution is needed when interpreting MAcc and MDec assessed in the present investigation, 10 Hz units are, for now, and to the best of the author's knowledge, the best tool available when tracking Acc and Dec in team sport athletes with GPS [25].
To conclude, the present study is unique in its inception based on its analysis of the acceleration profiles of professional soccer playing rotation and substitution strategies than defensive players.
Interestingly, in the present investigation, 49% of the substitutions made by the team across the investigated periods were for WF and CF, while only 21% were for CB and FB, as reported by the data selection. Offensive players did not benefit from this substitution strategy to maintain the level of Acc and Dec during the congested periods. Anyway, no difference was made between the microcycles that includes two consecutive games within three to four days. It could therefore be interesting to analyse the differences between these two conditions in order to detect any position-related impact.
Duration spent within the TAcc and in the LAcc were increased during the congested match periods assessed. Since the present study was the first to analyse Acc profiles during congested periods of elite professional soccer match-play, comparisons with similar literature was not viable. However, similar positive effects of a congested fixture period have been found concerning the relative total distance covered, and the relative low-speed running (< 11 km·h -1 ) bouts per minute of play amongst professional players [16]. Authors of this particular investigation suggested that these increased values may be linked to tactical factors such as team formation, playing style, specific physical and technical demands and different levels of opposition [16]. Increases in the absolute and relative TAcc and LAcc were accompanied by decreases in absolute and relative MAcc and HAcc, during the congested matches in the current study. Therefore, it can be suggested that the significant increase of TAcc and LAcc may be seen as an involuntary coping strategy that players perform subconsciously in order to compensate the reduction in MAcc and HAcc. Furthermore, these may be performed or caused by an accumulated fatigue across the congested periods, with the aim of trying to maintain performance levels. Faude et al. [36] have shown that soccer performance is more dependent on high speed runs and sprints than on low intensity efforts highlighting that soccer goals are very often preceded by high intensity straight sprints. Aerobic capacity is a key-factor to reproduce high and very high intensity running, accelerating and decelerating efforts over a long period of time like in a soccer game [1]. Moreover, developing aerobic capacity might be important to reduce injury risks and improve match performance [37].
The report from playing position differences (  [38] with similar profiles and positional demand in term of attacking and defensive involvement [17,39,40]. The difference observed in the total time spent in Acc and Dec might be explained by the fact that FB is positioned deeper on the field, therefore FB would need more time to reach offensive zones when his team gains ball possession and, in the same way, to get back to his defensive position when his team has lost the ball.
In the present study, FB and WF had the highest overall physical implication, which might be related to the playing system used by players within congested periods of matches. Results have revealed that congested fixture periods negatively affected maximal acceleration and deceleration of professional players in addition with a reduction of the total distance covered and time spent performing maximal and high accelerations. Defensive players like CB, FB and CM were mostly impacted through decrements during congested periods, however furthermore, the maximal-and HSR activity remained stable across the period of analysis. Subsequently, it may therefore be suggested that the accumulated fatigue imposed as a result of congested game periods affected the acceleration activity of the players.
As a result, the findings of the present study may not be generalised across level of play and other teams or players at the professional level due the data being obtained from one playing squad, which is the main limitation of the present study. Professional soccer teams should be advised to monitor acceleration and deceleration metrics throughout particular periods of congested fixtures, like those imposed by the COVID-19 pandemic delays in competitions, as a way of monitoring and assessing physical player status and possible recovery. However, additional research is required and requested to analyse key relationships between acceleration profile, subjective fatigue, physiological indicators and injury rates during periods of fixture congestion.

PRACTICAL APPLICATION
Based on the fact that elite players are continually exposed to greater number of fixtures year upon year with limited recovery periods between games, this novel investigation highlighting the effects of acceleration and deceleration profiles and the potential fatiguing effects of fixture congestion is of great practical relevance. The analysis in the present study highlights that professional soccer players' acceleration and deceleration profiles were significantly affected across congested playing periods. It is therefore recommended that soccer teams pay particular attention to the individual-related fatigue while monitoring Acc and Dec. The use of individual strategies as such as squad rotation, timing of substitutions and/or recovery strategies utilised to potentially eradicate such decreased physical activity and maintain performance in congested periods is highly suggested.

KEY POINTS
-A reduction in high and maximal accelerations was observed during congested compared to non-congested periods.
-A reduction in high and maximal decelerations was observed during congested compared to non-congested periods.
-The role of congested period on accelerations and decelerations did not vary by playing position.