The femur is the longest, strongest, and heaviest bone in the human body. It’s protected by various muscles and helps you maintain your posture and balance. The femur connects important muscles, tendons, and ligaments in your hips and knees, helping you carry your body weight and maintain stability to prevent falls. It also contains bone marrow, a soft, fatty tissue made of stem cells that can differentiate into other cell types, forming red and yellow bone marrow. Red bone marrow cells produce blood components, while yellow bone marrow cells store fat. As you age, red bone marrow is gradually replaced by yellow bone marrow. Understanding the importance of the femur can help you avoid injury and protect your bone health.