Women’s bodies are more flexible than men's to allow women to carry and eventually hold a baby, according to Harvard University. This is caused in part by the hormone estrogen, which promotes the lengthening of muscles, according to Krav Maga for Fitness.
Harvard researcher Katherine Whitcome said a woman's body has evolved in order to help a woman survive pregnancy. This is due to the fact that pregnancy often presents an big challenge in female body, notes Whitcome. During pregnancy, the body undergoes major changes to accommodate the baby, and these changes influence a woman’s posture.
A woman gets her additional flexibility from structural differences in her lower spine. The curvature of a woman's spine is spread across three vertebrae, while the curvature of a man's spine spans just two vertebrae. The increased flexibility also allows the expectant mother to maintain her normal activities through her pregnancy, and also allows her to lean back and adjust the fetus's position, according to Harvard University. By adjusting the fetus's position, she shifts her own center of gravity and this positively impacts her stability.
Muscle mass also has an impact on flexibility. Because men typically have more muscle mass than women, they are less flexible, according to Fit Day.