This dissertation examined the application of a monitoring system in football (soccer). An observational design was adopted to analyse training load (TL) and physical fitness measurements of two professional male football teams during their regular training and match routines. During the competitive period, a higher training volume was observed before and after playing against top-level opponents (1st to 5th in the league rank), and after losing a match. Additionally, the amount of high-intensity activity performed during training was higher when preparing a game against a top-level opponent. Regarding TL-induced effects on physical fitness, training volume was associated to a reduction of heart rate response during the submaximal Yo-Yo Intermittent Recovery Test - level 1, indicating improved cardiorespiratory fitness. Regarding the proper choice of external TL quantification method, the use of arbitrary or individualised speedbased intensity zones adjusted to player`s physical fitness (maximal aerobic speed and maximal sprinting speed) showed similar sensitivity in the estimation of external TL magnitude (based on correlation) but differed at their absolute level (based on measurement bias). Notwithstanding, when external TL was adjusted to individual physical fitness, it revealed slightly stronger associations with perceptual responses to training, rather than when calculated using arbitrary intensity zones. Additionally, reporting external TL values as percentage values of distance does not inform about player`s perceptual responses to training. The present findings can be considered by coaching and medical departments, and anyone involved with fitness testing and TL monitoring in football players.
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