Higgs, F and Winter, SL. The effect of a four-week proprioceptive neuromuscular facilitation stretching program on isokinetic torque production. J Strength Cond Res 23(5): 1442-1447, 2009-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 proprioceptive neuromuscular facilitation (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 were measured at 120 and 270°·s−1. Subjects then participated in a 4-week quadriceps flexibility training program consisting of 3 cycles of PNF stretching performed 3 times a week. The range of motion was recorded before and after the first stretching session of each week. At the end of the 4-week period, the peak isokinetic quadriceps torque and flexibility were again measured. The mean (SE) improvement in the knee flexion range of motion over the whole program was 9.2° (1.45°), and typical gains after a single stretching session were around 3°. Post hoc analysis showed that the pretraining session range of motion was significantly improved in week 4 compared with the pretraining 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°·s−1 (week 1: 121.9 (4.6) N·m; week 2: 121.9 (5.2) N·m) or at 270°·s−1 (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 3 PNF stretch cycles performed 3 times a week without altering muscle isokinetic strength characteristics.