Changes in the activity of hip adductor muscles with increased running speed were investigated in 4 male sprinters (personal best for 100 m: 10.58±0.26 s). The subjects were instructed to run at three different speeds (3-4 m/s, 6-8 m/s and 9- m/s). The surface electromyograms (EMGs) of 10 muscles around the hip joint were recorded, and whole-body motions were also filmed with a high-speed video camera (150 fps). Regardless of running velocity, the adductor longus (AL) showed activity concomitant with the rectus femoris when the hip joint was in extension. This suggested that the AL functioned as a hip flexor. On the other hand, the adductor magnus (AM) showed activity when the hip joint was flexed, suggesting that the AM assisted hip extensors such as the gluteus maximus. During high-speed sprinting, the AL was also activated when the hip joint was flexed. Similarly, the AM also showed activity when the hip joint was extended, corresponding to the latter half of the support phase. During the support phase, the AM may serve to stabilize the frontal plane by co-contracting with hip abductors such as the gluteus medius and tensor fasciae latae. Furthermore, the AL and AM showed increased activity while the hip was fully flexed and extended. This remarkable muscle activity around the flexion-extension reversal point during high-speed sprinting may stabilize the hip joint so that it resists dislocative force through the unique anatomical features of the hip adductor muscles, i.e. “shunt-” rather than “spurt-type” architectural characteristics.