The purpose of the present study was to examine the effects of high intensity static stretching for 20 seconds on flexibility and strength in the hamstrings. Seventeen healthy participants (13 men and 4 women) underwent static stretching for 20 seconds at three different intensities based on the point of discomfort (POD, 120% POD, and MaxPOD). To examine the change in flexibility and strength, range of motion (ROM), passive torque, muscle-tendon unit stiffness, peak torque of maximum voluntary isokinetic concentric contraction, and knee angle at peak torque were measured. To evaluate a time course of pain, a numerical rating scale (NRS) was described. ROM (percent change; POD = 113.5 ± 10.4%, 120%POD = 127.6 ± 18.8%, MaxPOD = 135.6 ± 18.5%) (p < 0.01) and passive torque (percent change; POD = 124.2 ± 38.9%, 120%POD = 143.4 ± 65.1%, MaxPOD = 171.8 ± 83.6%) (p < 0.01) were increased at all intensities. Muscle-tendon unit stiffness was decreased at 120%POD (percent change; 72.4 ± 36.2, p < 0.01) and MaxPOD (percent change; 56.6 ± 30.0, p < 0.01). Peak torque showed no change at all intensities (percent change; POD = 99.1 ± 14.0%, 120%POD = 95.4 ± 17.4%, MaxPOD = 98.4 ± 20.1%, p > 0.05). There were significant correlations between the intensities and relative change of the ROM (r = 0.57, p < 0.01), passive torque (r = 0.46, p < 0.01), muscle-tendon unit stiffness (r = -0.53, p < 0.01) and knee angle at peak torque (r = 0.50, p < 0.01). NRS increased with the intensity of static stretching (median; POD = 1, 120%POD = 3, Max POD = 8), though the pain disappeared immediately after the stretching (median = 0). In conclusion, static stretching for 20 seconds at high intensity was effective for a decrement in muscle-tendon unit stiffness.