Optimal intensity of PNF stretching: maintaining the efficacy of stretching while ensuring its safety

[Purpose] To investigate changes in hamstring flexibility in relation to intensity of proprioceptive neuromuscular facilitation stretching and changes in pain over time, and examine the correlations between pain level and target intensity or flexibility gain. [Participants and Methods] Sixty-one healthy adults were randomly divided into 4 groups (100% [P100], 70% [P70], 40% [P40], and 10% [P10] of maximum voluntary isometric contraction) according to intensity of hold-relax stretching. Hamstring flexibility was measured with the active knee extension test, and pain was measured using the visual analogue scale. [Results] Concerning hamstring flexibility, P100 showed significant differences from P40 and P10, and P70 was significantly different from P10. At post-stretch, P100 significantly differed from P70, P40, and P10 in visual analogue scale. At 1 day, P100 significantly differed from P40 and P10. Although there was a significant correlation between post-stretch pain level and stretching intensity, there was no significant correlation between pain level and flexibility improvement. [Conclusion] Repetitive high-intensity stretching may cause heavy burden on muscle tissues, and pain caused by high-intensity stretching can hinder muscle performance. Moderate stretching intensity is recommended and considered conducive to maintaining the effects of stretching while ensuring its safety.