The significant improvement of 5.7% in estimated from OFF to PRE phase is largely due to both the players’ relatively low level of fitness at the start of PRE phase as well as the traditional focus on aerobic conditioning training methods during this period (Reilly and Thomas, 1977; White et al., 1988). The maintenance of aerobic endurance fitness during the rest of the league season is supported by some studies (Thomas and Reilly, 1979; Heller et al., 1992; Casajus, 2001; Dunbar, 2002). Interestingly however, some investigators have found that players significantly reduced their aerobic fitness towards the end of the season (Mohr et al., 2002) while in contrast, others have reported that players were able to improve their aerobic endurance continuously throughout the league season (Brewer, 1990; Rebello and Soares, 1997; Edwards et al., 2003). The latter is obviously favourable for the teams concerned and was due mainly to the optimal training stimulus (i.e., intensity and frequency) and recovery from the weekly games
imposed on the players throughout the season (Heller et al., 1992; Bangsbo, 1994). It must also be pointed out that in the studies cited above, the investigators have used various criteria as indicators of aerobic endurance fitness. Both maximal exertion tests like the intermittent field run, incremental treadmill running with gas measurements and progressively faster shuttle runs to exhaustion as well as sub-maximal type tests like lactate profile curve, lactate response at specific running speeds, speed or gas values at lactate and ventilatory thresholds, and the Harvard step test, have been utilized. The nonsoccer specific protocol and consequently, the lack of sensitivity of these tests to detect the subtle changes in aerobic training status of soccer players throughout the season could be possible reasons for the contrasting findings among the studies (Edwards et al., 2003).