Footballers' performance levels rely on many simultaneous factors, including athletic, technical, tactical, mental, and physiological abilities. Performance in football has been described in the literature using the athletic factor (Anderson et al., 2008; Bradley et al., 2009; Di Salvo et al., 2009; Krustrup et al., 2003; Mohr et al., 2003; Rampinini E et al., 2007) or, more recently, based on actions performed with the ball (Wisloff et al., 1998; Hugues et al., 2005). We believe that those two components must be considered together in order to get a complete performance overview of a football game. The duration of a football game being 90 minutes, it is essentially based on aerobic metabolism. Bangsbo et al. (2008) and Krustrup et al. (2006a) show that high demands are made on the aerobic metabolism (average heart rate and peak respectively 85% and 98% maximum heart rate (HRmax). Footballers' aerobic potential is of paramount importance to their performance since it is the main factor determining speed of movement in the sport (Bangsbo et al., 2006) and also appears as a key factor in recovery during the repetition of efforts at very high intensity (Girard et al., 2011a; 2011b). Similarly, Bangsbo et al. (2006), Drust et al. (2000) and Stolen et al. (2005) have shown the positive effects of a high level of aerobic fitness on player performance in a game. Wisloff et al. (1998) reported a significant difference (P
Given the importance of the aerobic system in football performance, as described above, evaluating this feature is therefore crucial when developing physical training programs. Physical tests must reproduce, as much as possible, the specific patterns of the sport they are tailored to, whether under laboratory conditions or on the field. The aim of the present study was to investigate the validity and reproducibility of the Footeval Test.
Presentation of "Footeval"
Footeval is an incremental, intermittent test based on the spatial organization of Leger's "20m shuttle run" test (Leger et al., 1982) in order to include direction changes (180[degrees]). The purpose of this test is to determine a global index of football players, providing a clear idea of their level, including their physical and technical skills. Notation takes into account players' aerobic power and technical capacities in real football conditions (MASS). This test allows V[O.sub.2]max to be measured in specific conditions and will be influenced by many factors such as running economy, muscular abilities, or technical skills with the ball. It differentiates football players according to their level (Ziogas et al., 2011).
Footeval is an intermittent incremental test with thirty seconds of passive recovery between each step. The first step is designed as a warm-up and lasts two minutes (Figure 1). All other steps last one minute followed by a recovery phase of thirty seconds. The protocol was designed to reproduce the requirements of football as much as possible. Several studies have demonstrated that most recovery periods during a football game last less than thirty seconds (Bloomfield et al., 2007; Spencer et al., 2005; Vigne et al., 2010). For each step in this test, the intensity is set by speed in km/h. The first level starts at 6.5km/h and intensity is increased by 0.5 km/h between each workout phase. To ensure players accurately follow intensity increases, a specific soundtrack is played to provide audio feedback to players, allowing them to adjust their speed at each extremity of the test area (20m). A single beep signals the beginning or intermediate positions in the step, while a double beep indicates the end of the workout.
The test is over when the subject is no longer able to maintain the intensity indicated by the audio beep. If the player is more than 3 meters from the line, and not able to reduce the gap, we consider that the test is over. The test is also considered to have ended when the player is not able to restart a workout session after the recovery phase. Furthermore, the player is stopped if he makes more than 2 technical errors within a step. We considered bad passes, bad shots, or bad ball control to be technical errors.
The intermittent structure of the Footeval test is based on analysis of football as a sport and is designed to induce a substantial turnover of aerobic and anaerobic pathways. The duration of the different steps (two minutes for the first and one minute for the others), interspersed with a recovery period of 30", is justified by the works by Astrand et al. (2003). They have highlighted that respiratory and heart rate adjustments during the first two minutes of activity result in a deficiency of oxygen. During this period, the body relies on its anaerobic system (depending on the intensity of the exercise). Thus, the first two-minute level of Footeval was designed to obtain a stable cardiovascular level and to ensure a gradual warm up. After several different experiments, we decided to adjust the duration of each subsequent step to 1 minute, because a shorter duration might lead to overestimating the MASS and link the final result to anaerobic pathway. This balance between steps and recovery duration also allows for gradually improving fatigue and leads to an expected duration of the test that is between 12 and 18 minutes. This duration allows the player to adapt his own capacity to different physiological demands. Reilly et al. (1984; 2005) show an increase (8%) in V[O.sub.2] when the player runs with the ball compared to running at the same speed without a ball. Similarly, Stolen et al. (2005) point out that a change in speed of 1 km x [h.sup.-1] generates a V[O.sub.2] increase of 5 ml x [min.sup.-1] [kg.sup.-1] and a 10-beat HR increase.
A passive recovery time of 30 seconds corresponds to the recovery times generally observed during official games. Balsom et al. (1992) have shown that V[O.sub.2] is directly influenced by recovery time: the shorter the recovery time, the more oxygen uptake will increase. Balsom et al. (1992) found respectively a 52, 57 and 66 ml/min/kg oxygen concentration for the same exercise (15x40m) with intermittent recovery times of 120 seconds, 60 seconds and 30 seconds. Recovery periods are short in football, less than 30 seconds according to Spencer et al. (2005), so the importance of the kinetics of the V[O.sub.2] is primary (Dupont et al., 2005; 2010). This refers to the concept of the metabolic efficiency of the aerobic system. 30 seconds is not sufficient time to allow complete recovery and maintains a high level of aerobic pathway for the start of the next level. The speed increase between each step during the test is of 0.5 km/h and the first step starts at 6.5 km/h.' The goal of this increment is to delay substantial demands on the anaerobic pathway, as this will strongly impact assessment of the subject's aerobic potential. A higher increment of speed between each level could cause fast blood lactate accumulation and lead to the test ending too soon. Muscular acidosis increases results in a pH decrease, which leads to the reduction of muscular contractility (Westerblad et al., 2002) and glycolytic activity inhibition (Hollidge-Horvat et al., 1999). The oxidative potential that we want to assess relies on oxygen delivery and V[O.sub.2] (Buchheit et al., 2010; Glaister et al., 2005).
Figure 2 shows the spatial lay out of the "Footeval". To comply with the distance of 20m, as in Leger's "20m shuttle run test", we drew an optimal trajectory in the slalom using plastic strips (0.5m long and 1cm thick) located 0.5m from the center of the poles, which did not interfere with the player or the ball. The route of this optimal trajectory aims to help the player to respect the tempo beeps. We placed a marker at 0.4m from the first pole, so that the player would start his slalom at this point and not before. The last step completed by the player provides the Maximal Aerobic Speed Specific (MASS).
During each level, the subject must be at one end or the other, i.e. in the shooting zone or at line A. At the first beep, the player has to:
* Run with the ball for a distance of 2.6 m
* Slalom between poles separated by 2.5m
* After the last cone, the player has to run with the ball for a distance of 2.5 m
* Then the player has to make a pass on the plastic board, located on the right or the left, and then control the ball. After controlling the ball, the player has to run with the ball for a distance of 4m up to line A, located 19m from the start.
* At the second beep, the player has to stop the ball on line A with the sole of his foot and do a U-turn.
If the player loses the ball in the first part of the route, he must finish it without the ball and pick up a new one at line A.
* For the return, the...