Dietary nitrate (NO3-) supplementation has been shown to speed reaction time and enhance linear sprint speed and power production in an unfatigued state. It may therefore be suggested that NO3- supplementation could be ergogenic during all-out sprint running and reactive agility tasks, as well as during explosive forms of exercise such as vertical jumping. NO3- supplementation may also attenuate the decline in such exercise performance following fatiguing exercise. The purpose of this thesis was firstly to determine the reliability of selected exercise tests for measuring running reactive agility (reactive agility test), planned agility (change of direction t-test), 15 m linear sprint and countermovement jump performance, and then to investigate the effect of NO3- supplementation on these parameters of team sports performance in an unfatigued state and following fatiguing exercise that mimics the high-intensity intermittent demands of team sport game-play. Chapter 3: Examined the reliability of selected exercise protocols for reactive agility, change of direction, sprint and vertical jump performance. The lowest coefficient of variation (COV) and highest intraclass correlation coefficient (ICC) was observed when the fastest 15 m linear sprint out of 5 attempts (COV: 1.0%; ICC: 0.98), and the highest countermovement jump out of 3 attempts (COV: 4.6%; ICC: 0.94) were assessed independently and when the mean of all 6 reactive agility (COV: 2.0%; ICC: 0.96) and all 3 change of direction t-test (COV: 2.9%; ICC: 0.87) attempts were assessed. This information was used to inform statistical analyses within chapter 4. Results from chapter 3 provided confidence in the use of the selected exercise protocols in an intervention study due to the low day-to-day variability in performance. Chapter 4: Five days of NO3- supplementation did not improve reactive agility (NIT: 2.64 ± 0.21 s vs PLA: 2.65 ± 0.17 s, P > 0.05), change of direction t-test (NIT: 7.12 ± 0.71 s vs PLA: 7.10 ± 0.76, P > 0.05), 15 m sprint (NIT: 3.204 ± 0.212 s vs. PLA: 3.215 ± 0.206 s, P > 0.05) or countermovement jump (NIT: 36.38 ± 6.58 cm vs PLA: 37.02 ± 6.83 cm, P > 0.05) performance in an unfatigued state. In a fatigued state, 15 m sprint (NIT: 3.27 ± 0.25 s vs PLA: 3.27 ± 0.25, P > 0.05) and countermovement jump (NIT: 36.7 ± 7.2 cm vs PLA: 36.5 ± 7.0 cm, P > 0.05) performance were also unaltered following NO3- supplementation. Performance declined in a fatigued compared to unfatigued state for 15 m sprint performance (P 0.05). NO3- supplementation did not attenuate the decline in fatigued 15 m sprint performance (P > 0.05). These findings suggest that NO3- supplementation does not alter agility, linear sprint or vertical jump performance. Overall, these findings provide an important contribution to the literature regarding the limits of the ergogenic effect of NO3- supplementation for particular determinants of team sports performance, specifically in male team sports players at a dose of 8 mmol · day-1 for 5 days
