The ACSM flexibility training recommendations emphasize proper stretching of muscles supporting the major joints, but there is a little evidence to support this recommendation in terms of effectiveness, and which stretching parameters (technique and single stretch duration) are more adequate. A randomized controlled clinical trial design was use to investigate whether the ACSM flexibility training recommendation parameters improve hip flexion range of motion. A total of 173 subjects, 122 men (21.3+/-2.5 years; 176.33+/-8.35 cm; 74.42+/-10.80 kg) and 51 women (20.7+/-1.6 years; 163.43+/-6.57 cm; 60.12+/-7.88 kg), classified as recreationally active young adult university students were randomly assigned to 1 of 7 groups: 1 control group (no stretching) or 1 of 6 stretching groups. All stretching groups performed 12 weeks of flexibility training with a consistent stretch daily dose (180 s) and frequency (3 days per week) parameters and different stretch technique (passive or active) and single stretch duration (15, 30, or 45 s). Hip flexion passive range of motion (PROM) was determined through the bilateral straight-leg raise test before, during (at 4 and 8 weeks), and after the program (12 weeks). All stretching groups performed hip flexion PROM after flexibility training. A significant improvement was identified in mean PROM for each stretching group, but no significant differences were found between stretch technique and single stretch duration (p>0.05). The control group's mean PROM decreased (Delta PROM: -0.08 degrees, 95% confidence interval [CI]=-2.3 to 5.3), whereas all stretching groups increased PROM (Delta PROM: 15.14 degrees, 95% CI=10.19 to 23.56) in hip flexion after 12 weeks of stretching (p<0.05). The present study suggests that the current ACSM flexibility training recommendations are effective for improving hip flexion ROM in recreationally active young adults.