Objectives: To ascertain the dynamic stretch effects of flexibility of the hamstrings on lumbar spine and pelvic kinematics. Background: Tight hamstrings are positively correlated with low back pain. However, it is unclear how flexibility of the hamstrings affects spino-pelvic rhythm. Methods: Twelve healthy men participated in the study. The straight leg raising (SLR) angle, finger floor distance (FFD), and spino-pelvic rhythm was measured before and after the 6-week stretching protocol. The forward bending task was divided into 4 phases. The paired t-test was used to determine significant differences before and after the FFD, SLR angle, lumbar motion, and pelvic motion, and spino-pelvic rhythm in each phase (p<0.05). Results: After 6 weeks of stretching, significant improvements were seen in the FFD with maximum forward bending and in the SLR angle. Total pelvic rotation was also significantly increased in contrast to total lumbar flexion. A decreased spino-pelvic ratio was seen in the final phase. Conclusion: Dynamic stretching could change the spino-pelvic rhythm to a pelvis-dominant motion, indicating that flexible hamstrings are important for preventing low back pain. J. Med. Invest. 63: 85-90, February, 2016