18 taekwondo athletes will participate in this double-blind randomized clinical trial with crossover design. Before intervention every person's background information will be recorded, including age, height, weight, BMI and dominant leg. The information related to the intended exercise tests (isokinetic and balance tests), blood pressure and heart rate will be also collected before and after intervention. Subjects will be randomly allocated into two groups of nine supplement and placebo. In phase I of the study, intervention group will receive concentrated nitrate-rich beetroot juice (2x 70 ml/day of concentrated beetroot juice with 8 mmol nitrate content) and placebo group will receive black currant juice (2x 70 ml/day of black currant juice with negligible nitrate content) for six days. After a 10-day washout period, phase II of the study will be initiated and then intervention and placebo groups will be interchanged. The measurement of isokinetic parameters and balance test will be conducted on day 6 of the study. isokinetic parameters such as peak torque, peak power and total work during knee extension for both legs will be measured using the Biodex isokinetic dynamometer system at angular velocities of 60, 90, 180, 270 and 360 °/s. subsequently, the subjects performed a 50 contraction fatigue test at angular velocity of 180°/s. The balance will be assessed using athlete single leg protocol on the the Biodex balance system. Then the effect of concentrated nitrate-rich beetroot juice on the isokinetic parameters and balance will be determined.