To determine the reliability of a new method to measure the stiffness of the knee extensors and thereafter establish the relationship between knee extensor stiffness and, jump and running performance.A professional rugby union football club and a university laboratory.Thirty semi-elite male rugby union players.The stiffness of the knee extensors across loads ranging from 30 to 70% maximal voluntary isometric contraction, 20m sprint, maximal squat strength, countermovement-jump and drop-jump performance was assessed.Within trial variation (coefficient of variation—range 5.41–7.45%) for all five loads (30, 40, 50, 60 and 70%) and test–retest reliability (intra-class correlation coefficients—range 0.92–0.96) were deemed acceptable. Knee extensor stiffness was found to have no significant relationship (P>0.05) to any of the functional performance measures (r=−0.160–0.172).It was concluded that the new protocol to assess the stiffness of the knee extensors was reliable, however the relationship between knee extensor stiffness and measures of strength, power and speed was weak and non-significant.