Background: Flexibility changes according to stretching intensity have been rarely investigated. I aimed to assess the effect of different stretching intensities on hamstring flexibility by measuring them in a setting similar to real clinical settings.
Methods: Stretching intensities were quantified using an easy method, and participants were grouped according to intensity: 100% (P100), 70% (P70), 40% (P40), and 10% (P10) of maximum voluntary isometric contraction. Proprioceptive neuromuscular facilitation stretching intensities were measured using a sling system and tension dynamometer. Hamstring flexibility was measured (before; immediately after; and 3, 6, 9, 12, and 15 min after stretching) using the active knee extension test. Flexibility was compared between subgroups, and longitudinal changes in flexibility were additionally observed in each group.
Results: At identical time points, no significant difference in hamstring flexibility was found between the high-intensity (P100) and moderate-intensity (P70, P40) groups. A significant difference was found between P100 and P10 immediately after and 12 and 15 min after stretching. Increased flexibility was maintained until the end in P100 and P70 but not P40 and P10.
Conclusions: High-intensity and moderate-intensity stretching increases flexibility compared with low-intensity stretching. Furthermore, high-intensity stretching was superior to moderate-intensity stretching in terms of maintaining flexibility over time.
Keywords: Dynamometer; PNF; flexibility; intensity; stretching.