Background: The hamstring is a muscle that crosses two joints, that is the hip and knee, and its flexibility is an important indicator of physical health in its role in many activities of daily living such as sitting, walking, and running. Limited range of motion (ROM) due to hamstring tightness is strongly related to back pain and malfunction of the hip joint. High-frequency diathermy (HFD) therapy is known to be effective in relaxing the muscle and increasing ROM.
Objects: To investigate the effects of HFD on active knee extension ROM and hamstring tone and stiffness in participants with hamstring tightness.
Methods: Twenty-four participants with hamstring tightness were recruited, and the operational definition of hamstring tightness in this study was active knee extension ROM of below 160° at 90° hip flexion in the supine position. HFD was applied to the hamstring for 15 minutes using the WINBACK device. All participants were examined before and after the intervention, and the results were analyzed using a paired t-test. The outcome measures included knee extension ROM, the viscoelastic property of the hamstring, and peak torque for passive knee extension.
Results: The active knee extension ROM significantly increased from 138.8° ± 9.9° (mean ± standard deviation) to 143.9° ± 10.4° after the intervention (p < 0.05), while viscoelastic property of the hamstring significantly decreased (p < 0.05). Also, the peak torque for knee extension significantly decreased (p < 0.05).
Conclusion: Application of HFD for 15 minutes to tight hamstrings immediately improves the active ROM and reduces the tone, stiffness, and elasticity of the muscle. However, further experiments are required to examine the long-term effects of HFD on hamstring tightness including pain reduction, postural improvement around the pelvis and lower extremities, and enhanced functional movement.

INTRODUCTION
MATERIALS AND METHODS
    1. Participants
    2. Intervention
    3. Outcome Measures
    4. Statistical Analysis
RESULTS
    1. Active Knee Extension ROM
    2. Peak Torque for Passive Knee Extension
    3. Viscoelastic Property Variables of the Hamstring
DISCUSSION
CONCLUSIONS
REFERENCES

• Ye Jin Kim(Department of Physical Therapy, The Graduate School, Yonsei University)
• Joo-Hee Park(Department of Physical Therapy, College of Health Science, Yonsei University)
• Ji-hyun Kim(Department of Physical Therapy, The Graduate School, Yonsei University)
• Gyeong Ah Moon(Department of Physical Therapy, The Graduate School, Yonsei University)
• Hye-Seon Jeon(Department of Physical Therapy, College of Health Science, Yonsei University) Corresponding Author