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A battery of 26 tests was administered to professional soccer players (n = 31) at 3 points during the competitive season. Significant changes from start to mid-season were limited to improved muscular power, increased resting heart rate (fH), decreased force expiratory flow (FEF) and ankle mobility. No further changes with the duration of the competitive season were observed. Differences between squads were predominantly cardiac function measures at mid-season, and strength, muscular power and limb girths at the end of the season. The First Team squad had consistently less fat and poorer ankle mobility. It seems that mean fitness levels remain relatively stable during the playing season, though the measures significantly discriminating between the top and reserve squads fluctuate.
Thomas et al. (Sat,) studied this question.