Objectives: This study aimed to (i) establish the concurrent validity and intra-unit reliability of a foot-mounted inertial measurement unit for monitoring soccer technical actions, (ii) quantify the within-microcycle inter-positional differences in the technical actions of professional soccer training, and (iii) determine the influence of drill category on the technical actions of professional soccer training. Methods: Twenty-one professional soccer players` technical performance data (ball touches, releases, ball touches per minute, releases per minute), collected during training sessions throughout twenty-four weekly microcycles, were analysed using general linear modelling. Results: The inertial measurement unit exhibited good concurrent validity (PA = 95.1% - 100.0%) and intra-unit reliability (PA = 95.9% - 96.9%, CV = 1.4% - 2.9%) when compared with retrospective video analyses. The most ball touches (X = 218.0) and releases (X = 110.8) were observed on MD - 1, with MD - 5 eliciting the highest frequency of ball touches (X = 3.8) and releases (X = 1.7) per minute. Central midfielders performed the most ball touches (X = 221.9), releases (X = 108.3), ball touches per minute (X = 3.4) and releases per minute (X = 1.6). Small-sided games evoked more ball touches (X_diff = 1.5) and releases per minute (X_diff = 0.1) than previously reported in match-play. The fewest ball touches (X = 1.2) and releases per minute (X = 0.5) were observed during tactical drills. Conclusion: The results of this study provide a novel understanding of the within-microcycle, inter-positional and drill category differences in the technical actions performed by professional players during training.
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