We aimed to investigate the change rate in the cross-sectional area of each hamstring component to evaluate muscle extensibility and to contribute to the studies on hamstring strain prevention. Fifteen healthy young males volunteered to participate in this study. They performed a knee extension test. For the measurements, we used multi-posture magnetic resonance imaging (gravity magnetic resonance imaging), the open shape of which allows performing body scanning in various positions. We measured the maximum cross-sectional area of the hamstring during the initial and stretch positions from the obtained images. Then, for each muscle, we calculated the maximum cross-sectional area change rate relative to the initial position. For all hamstring muscles, the maximum cross-sectional area during stretching was significantly smaller than that in the initial position. The maximum cross-sectional area change rate of the semimembranosus was significantly smaller than that of the other 3 muscles (there were no significant differences among these 3 muscles). The results suggest that the semimembranosus has higher resistance to morphological change than the other muscles, which could be an important limiting factor for the extensibility of the hamstring muscle group.