The aim of this paper was to analyze the internal load of footballers, comparing the metabolic power (MP) approach by Osgnach and Di Prampero, derived from the external load detected using GPS or video tracking system, with the real metabolic effort gained with a portable metabolimeter used as a gold standard. Two different protocols were used. First protocol was performed on 20 amateur footballers from 7th Italian League (Age: 27,3 ± 3,3, eight: 1,77 ± 3,4 and weight: 80 ± 4,5). Subjects were running 10 minutes with two different velocities, 8 km/h and at 16 km/h (5 minutes for both) and have a 5 minutes pause between them. The type of running was circular asking the players to maintain the velocity without any changes of speed or direction; during the test, the players were analyzed usinga 50Hz GPS (K-Sport Universal, Italia) and a portable metabolimeter, Cosmed K4 b2. The second protocol was performed by 20 semi-professional football players (Age: 26,3 ± 3,6, eight: 1,78 ± 4,5 and weight 77 ± 4,3) from a football team of 4th Italian Championship league. The change of the sample was important in order to investigate the possible differences that can be recorded between amateur and semi-professional athletes. The test protocolincluded 6 parts (18 minutes) inside which 2 recovery periods, one at the beginning of the test in order to evaluate the individual basal metabolism and one at the end of the test to determine the recovery time. Osgnach’s MP approach results to be an invalid method for deriving theinternal load of the athletes because individual physiological differences makeit impossible to accurately estimate the real energy expenditure of each individual with the simple use of a pre-built formula.Another critical point of Osgnach’s method is based on the formulas of Minetti et al. (2002) which were derived from a study carried out on cross-country athletes. In order to detect the true energy expenditure provided by the aerobic mechanism duringtraining or competitions, will be necessary to integrate a respiratory rate sensor into the GPS shirts. Analysisof the player performance model (PPM) during training and match, result to be crucial to determinate the performance of teams and individual players, to create specific training.
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