Comparison of time-motion systems between central and external defenders in elite youth soccer

Time-motion systems are used to identify locomotive patterns for elite youth soccer players, providing information on movement intensities which is valuable for designing position specific training regimes. However, manual-based video systems are susceptible to human error and Global Positioning Systems (GPS) have underestimated the intensity of movements under controlled conditions. Therefore, further research is required to investigate time-motion systems for measuring locomotive patterns for different playing positions. The aim of this thesis was to compare the Bloomfield Movement Classification (BMC) (manual-based system) to GPS, for measuring locomotive patterns under controlled conditions and for central (CD) and external (ED) defenders in soccer match-play. CD (n = 10) and ED (n = 10) in a standardised 4-4-2 formation who competed in the U18s Barclays Premier League during season 2012/2013 were filmed and analysed. The BMC is a model applied with manual-based video software designed to measure locomotive patterns performed by soccer players, which was used for the manual- based method in this thesis. For the GPS method, the GPS devices worn by players produced velocity data which were categorised into the common locomotive patterns by applying absolute speed thresholds. Measurements of locomotive patterns between BMC and GPS were compared under controlled conditions using the Bangsbo Soccer Specific Test (BSST) and in soccer match-play between CD and ED. Study 1 illustrated ED walked and sprinted significantly more (p S 0.05). and performed more backwards movements (p s 0.05), while CD ranked shuffling and lateral directions higher as a more utilised movement. Study 2 demonstrated under controlled conditions GPS underestimated intensities during non-linear (zig-zag) and sideways patterns. Study 3 highlighted during soccer match-play BMC measured greater high-intensity activity; however, GPS applied with absolute speed thresholds measured greater low-intensity and sprinting (p < 0.001). GPS categorised 7.35 and 5.62 % of shuffle movements as standing still for CD and ED respectively. GPS method measured 33.26% and 25.89% of running as low-intensity for CD and ED respectively. To conclude, the BMC provided additional information on locomotive patterns performed between CD and ED in a 4-4-2 formation which had not been previously identified in soccer time-motion analysis research. GPS is susceptible to underestimate intensities in non-linear and sideways patterns, and can under- and over-estimate values if speed thresholds are set too high/low for an individual player's relative threshold. Moreover. GPS can measure a soccer player's physical performance with greater low-intensity activity in soccer match-play compared to a manual-based system, mainly because of the inaccurate interpretation of shuffle movements. To conclude, caution should be taken when GPS is used as a solitary method to measure locomotive patterns for elite youth soccer players. Further research is required to combine GPS and manual-based video analysis to develop a sound model which can provide an accurate representation of specific locomotive patterns measured. Understanding the true physical demands of a playing position can influence accurate position-specific training regimes which can enhance player development