A large, triangular bone at the base of the spine, formed by the fusion of five vertebrae. It connects the spine to the pelvis via the sacroiliac joints and forms the posterior wall of the pelvic cavity, transmitting body weight to the legs.
From Latin 'sacrum' meaning sacred, short for 'os sacrum' (sacred bone). Ancient cultures considered this bone sacred, possibly because of its role in reproduction or because it was the last bone to decay, symbolizing resurrection.
The sacrum is incredibly strong because it has to transfer the entire weight of your upper body to your legs through the sacroiliac joints - it's literally the keystone of your pelvis! Many people experience sacroiliac joint dysfunction, which causes lower back pain that's often mistaken for a herniated disc but feels different because it's more to one side.
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