Quantifying technical actions in professional soccer using foot-mounted inertial measurement units

This thesis aimed to (i) establish the concurrent validity and intra-unit reliability of a foot-mounted inertial measurement unit (IMU), for measuring the frequency of technical actions performed during soccer training activities, and (ii) to quantify the within-microcycle, inter-positional, and between-drill differences in the technical actions of professional soccer training using foot-mounted IMUs.Twelve male amateur soccer players collectively performed 8,640 ball touches and 5,760 releases, throughout a series of technical soccer tasks, repeated over two pre-determined distances. Concurrent validity was determined by calculating the proportion of agreement (PA) between the IMU and retrospective video analyses. Intra-unit reliability was established using the same method, supplemented by a percentage coefficient of variation (CV). Intra-operator reliability of the reference performance analyst, who conducted all analyses, was established by manually coding three randomly selected repetitions of each soccer task three times (PA = 100.0%). The IMU exhibited good concurrent validity (PA = 95.1% - 100.0%) and intra-unit reliability (PA = 95.9% - 96.9%, CV = 1.4% - 2.9%) for measuring ball touches and releases throughout all experimental conditions.Twenty-one male professional soccer players’ technical performance data (ball touches, releases, ball touches per minute, releases per minute), collected during training sessions throughout 24 weekly microcycles (i.e., match day [MD] minus day number [MD - n]), was subsequently analysed using general linear modelling. 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 per minute (X = 3.8) and releases per minute (X = 1.7). 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 per minute (Xdiff = 1.5), and releases per minute (Xdiff = 0.1), than previously reported in match-play. The fewest ball touches per minute (X = 1.2) and releases per minute (X = 0.5) were observed during tactical drills. The results of this thesis indicate that the foot-mounted IMU displayed promising capacity as a valid and reliable method of quantifying technical actions in soccer, as well as providing a novel understanding of the within-microcycle, inter-positional, and between-drill differences in the technical actions performed by professional players during training