Flexibility is widely accepted as an important component of fitness, yet flexibility training can be detrimental to muscle performance particularly where a high number of stretch cycles are performed. The purpose of this study was to investigate whether chronic PNF stretch training could successfully improve the knee flexion range of motion without having a detrimental effect on the peak isokinetic torque of the quadriceps. The minimum knee angle in flexion and the peak isokinetic quadriceps torque was measured at 120 and 270 degrees per second. Subjects then participated in a four week quadriceps flexibility training program consisting of three cycles of PNF stretching performed three times a week. The range of motion was recorded before and after the first stretching session of each week. At the end of the four week period the peak isokinetic quadriceps torque and flexibility was again measured. The mean (standard error) improvement in the knee flexion range of motion over the whole programme was 9.2 (1.45) degrees, typical gains following a single stretching session were around three degrees. Post-hoc analysis showed that the pre training session range of motion was significantly improved in Week 4 compared to the pre training session range of motion in Weeks 1 and 2 (p<0.05). There was no change (p=0.9635) in the peak isokinetic torque produced at 120 degrees per second (Week 1: 121.9 (4.6) N.m; Week 2: 121.9 (5.2) N.m) or at 270 degrees per second (Week 1: 88.1 (3.4) N.m; Week 2: 88.6 (4.9) N.m). These findings suggest that it is possible to improve flexibility using three PNF stretch cycles performed three times a week without altering muscle isokinetic strength characteristics.