When the foot first hits the ground, the entire limb must be supple in order to absorb shock and accommodate discrepancies in terrain, while shortly thereafter, these same structures become rigid so they can tolerate the accelerative forces associated with propelling the body forward.

Cortical bone, which is surrounded by the pain-sensitive periosteum, is extremely powerful and resists bending forces like a steel pipe. In contrast, spongy bone is softer and is loaded with small chambers that allow for the production of red blood cells.

maintain economy, the best hybrid runners must “fine-tune” the positions of their limbs prior to contact in order to provide just enough stiffness to absorb and return energy at each successive running speed: Too much stiffness, and the center of mass moves up and down excessively and impact forces increase; too little stiffness, and the pathway of

we were to walk with our knees locked and our pelvis stiff (e.g., with a Frankenstein-like gait), the body’s center of mass would move up and down through a series of abruptly intersecting arcs, which would significantly increase the metabolic cost of locomotion because specific muscles would tense to accommodate the exaggerated up-and-down

contact, midstance, and propulsive periods. Although running is also divided into the same three periods, the increased speed and the need for a more forceful propulsive period changes the timing of the events: The contact and midstance periods are slightly shorter, and the propulsive period is longer

While tendons connect muscles to bones, ligaments connect neighboring bones to one another. They are structured similarly to tendons, except they contain more blood vessels to enhance healing, and more elastin fibers to make them stretchable (arrow). Ligaments are not as strong as tendons, and the higher elastin content makes them prone to tearing.

By the age of six, children continue to burn more calories while walking and running. Fortunately, by age 10, mechanical efficiency is equal to that of an adult, and after almost a decade of practice, children are finally efficient at getting around on two legs.

The neurological mechanisms necessary to complete the gait cycle are unusual in that swing phase motions are reflexive and present at birth

The gait cycle consists of two distinct phases: stance phase, in which the foot is contacting the ground; and swing phase, in which the lower limb is swinging through the air preparing for the next impact