The question of the strongest bone in the human body is complex, as the answer depends entirely on how biological “strength” is defined. While most people assume strength means being difficult to break, scientists measure this property in several specific ways related to a bone’s function. The bone best at resisting a crushing force is different from the tissue best at resisting abrasion. To determine the strongest bone, we must look at the mechanical forces the skeletal system is built to endure.
How Biological Strength is Measured
Biomechanics, the study of mechanical laws relating to living organisms, defines strength based on the types of force a material can tolerate before failure. The primary metric for weight-bearing bones is compressive strength, which is the maximum stress a material can withstand under squeezing or crushing forces. This resistance is important for bones that support the body against gravity.
Another important measure is tensile strength, the ability of a material to resist being pulled apart or stretched. Bones are also subjected to shear stress and torsional loads, which involve forces pushing in opposite directions or twisting the bone. The overall strength of any bone is a combination of its ability to manage these forces simultaneously. The types of loads placed on various bones explain why their composition and shape vary throughout the body.
The Champion: The Femur
Based on the capacity to withstand immense crushing force, the strongest bone is the femur, or the thigh bone. This long bone extends from the hip to the knee and is built to support the entire body’s weight. Its shape is cylindrical with a slight anterior bow, which provides maximum resistance to bending and compression forces.
The load-bearing capacity of the femur is remarkable, particularly its ultimate compressive strength, measured at approximately 205 megapascals (MPa). This means the femur can withstand a compressive force of roughly 1,800 to 2,500 pounds, or up to 30 times the weight of an average adult, before fracturing. This resilience is necessary because the force exerted on the femur increases significantly during dynamic movements like running or jumping. The thick layer of compact bone, known as cortical bone, makes up the shaft of the femur and provides the structural density needed to handle this pressure.
The Materials That Give Bone Strength
The strength of the femur, and all bone tissue, comes from its unique composite material structure. Bone is a blend of organic and inorganic components, not merely a hard, uniform substance. Approximately 70% of a bone’s mass is an inorganic mineral called hydroxyapatite, a crystal form of calcium phosphate. This mineral phase provides rigidity and hardness, contributing significantly to the bone’s high compressive strength.
The remaining 30% is primarily an organic protein called Type I collagen, arranged in a highly organized fibrous network. Collagen acts like reinforcing steel, giving the bone elasticity and resistance to being pulled apart (tensile strength). This combination of a hard mineral and a flexible protein creates a lightweight but strong composite material. The relationship between the rigid hydroxyapatite and the resilient collagen allows bone to absorb shock and resist fracture better than either material could alone.
Hardness Versus Load Bearing: Other Dense Tissues
While the femur is the strongest bone by load-bearing and compressive metrics, it is not the hardest substance in the human body. That title belongs to tooth enamel, the outer coating of the teeth. Hardness, in this context, refers to a material’s resistance to scratching or abrasion, which is a function of mineral density.
Tooth enamel is composed of up to 96% mineral content, making it the most highly mineralized tissue in the body. This density allows it to withstand the constant forces of chewing without wearing away. Conversely, bones like the mandible (jawbone) are sometimes cited as the strongest due to their high density, but they do not surpass the femur in total compressive capacity. The femur maintains its status as the strongest bone because its primary function is to support the body, making its load-bearing ability the ultimate measure of its strength.