Objective: Improving of muscle flexibility is an important issue in physiotherapy and sport sciences. There are many methods for increasing muscle length and decreasing muscle stiffness. In research findings, PNF method has been found to be better than static and ballistic methods. There is another method named Harmonic technique (introduced by E. Lederman 1997) that has been claimed to be more effective, but there is not enough documentation about this claim. Aim of this study was to compare effects of stretching by PNF and harmonic techniques on hamstring flexibility.

Materials & Methods: This research is a RCT study in that 45 colledge students aged 18-35 years were arranged in three groups (Harmonic, P.N.F, and control). Subjects haven’t had any painful pathology in low-back and lower extremities for last six months. Subjects had limited hamstring length (20 degrees deficiency in Active-Knee-Extension test) and hadn’t professional sport activities. Dependent variablies were muscle stiffness and hamstring length which popliteal angle in AKE test was its indirect index. In pilot study, reliability of measurement of these variables were approved. Then hamstring muscle of subjects in harmonic and PNF groups were stretched by harmonic and PNF methods for six weeks, 5 minute per day and 3d/wks, whereas control group hadn’t any exercise.

Results: Findings of this study showed that in both used techniques, changes of hamstring length were significant (P=0.000), but in control group there wasn’t significant change. There wasn’t significant differences between changes of hamstring length in PNF and Harmonic groups. Only in harmonic group, muscle stiffness had significant changes (P<0.03).

Conclusion: According to findings of this research, both harmonic and PNF methods equally increased length of hamstring, and harmonic technique can be used as an alternative stretching method for other techniques. Maybe harmonic technique is better than PNF, because the significant changes of stiffness observed only in harmonic group ie. not only length of muscle increased in this group, but also stiffness decreased significantly which can be an indicator of considerable biomechanical changes in muscle and possibly increase of muscle length in PNF group is not real improvement in flexibility, and indeed, tolerance of subjects to stretching in this group has increased.