Energetic Analysis of Running Demands in Australian Football using Global Positioning Systems Technology

Global positioning system (GPS) technology is used in Australian football and other field based team sports to assess player movement. The overarching aims of the studies in this thesis were to evaluate the accuracy and reliability of GPS technology for use in field based team sports and to derive greater physiological meaning from GPS technology used to describe the demands of Australian football competition. Study One assessed the effects of movement intensity and path linearity on the validity and reliability of GPS distance data. One participant wore eight 1 Hz GPS receivers while walking, jogging, running and sprinting over linear and non-linear 200 m courses. Five trials were performed at each movement intensity on each 200 m course. The results showed the mean (± SD) and percent bias of the GPS distance values on the 200 m linear course were 205.8 ± 2.4 m (2.8%), 201.8 ± 2.8 m (0.8%), 203.1 ± 2.2 m (1.5%) and 205.2 ± 4 m (2.5%) for the walk, jog, run and sprint trials respectively. Walk and sprint distances were significantly different from jogging and running distances (p 1/3 of the total energy cost during short, intense running bouts. Study Four used GPS technology to model the energy cost and metabolic power of Australian football competition. The aim was to describe the metabolic demands of different playing positions in elite Australian football competition; and to evaluate the utility of energy expenditure as a monitoring tool. The model introduced in Study Three was used to estimate energy expenditure and metabolic demands of 24 elite Australian football players tracked during the 2010 AFL season using 5 Hz GPS receivers. Players were allocated to 4 positional groups: midfielders (n= 74 files), ruckmen (n= 24 files), small forwards/backs (n= 78 files) and key forwards/backs (n= 76 files). Total energy expenditure ranged from ~46-63 kJ·kg-1 equivalent to running 11.5- 15.7 km at a steady aerobic pace. Time on-field, distance travelled, total energy expenditure and the rates of energy expenditure varied between playing positions. Midfielders were found to expend the most energy between moderate aerobic and light anaerobic metabolic rates (10- 35 W⋅kg-1). Ruckmen expended the least energy overall, and ranked lower on energetic indices of running intensity. Key and small forwards/backs had more intermittent running profiles. Small forwards/backs expended the most energy at high exercise intensities (>55 W⋅kg-1) followed by key forwards/backs. Key forwards/backs expended the most energy at low intensities (<10 W⋅kg-1). Changing forward running velocity accounted for ~24%, of the total energy expenditure for all positions. Collectively the four studies presented in this thesis provide new insight into the application of GPS technology in Australian football and other field based team sports. Study One identified that path linearity and movement intensity influence the accuracy of GPS distance measurements. These were attributed to inherent positioning errors within the GPS system and update rate. The reliability of GPS distance measures decreases with movement intensity. Variations in movement intensity and path linearity were also found to influence the accuracy of GPS velocity measurement in both 1 and 5 Hz non-differential GPS receivers. An increased update rate resulted in only marginal improvements in velocity accuracy during non linear movement. Users of this technology must consider these inherent limitations of the GPS system when interpreting GPS data collected during field based team sports. The findings in Study Three suggest the proposed energetic model provides reasonable estimates of energy expenditure at changing and constant running velocities. When evaluating intermittent running profiles, metabolic power derived using GPS modelling provides a useful index of running intensity as it reflects the interaction between absolute running velocity and the rate of change in velocity. The energy cost of changing forward velocity contributes largely to the total energy cost of brief intense running efforts and should be considered when evaluating the running demands of field based team sports. Study Four demonstrated that the energy cost of Australian football is similar to that found for soccer, and the demands are position specific. The routine use of energy expenditure and energetically derived indices of running intensity appears a valid approach to match analysis as trends identified using these methods revealed findings consistent with recent analyses of Australian football match play.