E3 Environmental Mechanical Interactions

Topic

The interaction of the foot or hoof with the ground generates the ground reaction force, which, along with gravity, is the only external force capable of altering a mammal's momentum during terrestrial locomotion. This ground reaction force comprises three components: a vertical component, which supports body weight and generates upward propulsion; an anteroposterior component, which decelerates the body during the impact phase of the stance and propels it forward during the push-off phase; and a mediolateral component, which stabilizes the body laterally.

Throughout the stance phase, the vertical component of the ground reaction force follows a temporal pattern that varies according to the gait type: during walking, reflecting the inverted-pendulum behavior of the center of mass, this vertical component exhibits two peaks separated by an intermediate trough; conversely, during running, reflecting the spring-like behavior of the center of mass, the vertical component displays a single peak throughout the stance.

Within this temporal pattern, the speed at which the vertical component of the ground reaction force rises from zero to its first peak—the loading rate—serves as a key indicator of injury risk: the more abruptly the force rises at the onset of the stance, the greater the risk of overuse injury to the structures absorbing the impact.

The maximum magnitude attainable by the combined anteroposterior and mediolateral components of the ground reaction force—those acting in the horizontal plane—is limited by the coefficient of friction between the foot and the ground: the higher the coefficient of friction, the greater the maximum horizontal force the foot can exert against the ground without slipping, whereas a lower coefficient results in a lower maximum available horizontal force. The precise point on the sole of the foot where the ground reaction force is applied at any given moment—the center of pressure—does not generally coincide with the projection of the center of mass onto the ground; the distance between these two points defines the external moment that the ground reaction force exerts on the animal's center of mass.

The exact way in which the ground reaction force alters the animal's momentum—as mentioned earlier—is determined by the integral of that force over the duration of the ground contact; this integral, known as impulse, is equal in magnitude to the change in the animal's momentum produced during that same contact phase.

When comparing ground reaction forces across mammals of vastly different body sizes, the peak value of the vertical component—expressed as a multiple of the animal's body weight—turns out to be approximately the same for a given mode of locomotion, regardless of whether the animal is small or much larger. This relative consistency allows the locomotion of mammals of widely varying sizes to be treated as equivalent within the context of this interaction with the ground.

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