Results: Most studies were done on young adults aged 16 to 35. All studies except two showed a statistically significant increase on flexibility in the hamstrings. The best results were given by Ferber et al. (38) who studied different PNF techniques, with a 15.66 ± 0.95° increase in hamstring flexibility. The PNF technique that gave this result was PNF Agonist Contract-Relax.