The foot and ground exert an equal and opposite force on each other. This is known as the ground reaction force.12 The position and acceleration of the runner's center of mass regulates the direction and magnitude of the ground reaction force. For instance, a runner sprinting from a standstill. Throughout the initial phase of acceleration, the body is inclined forward and therefore the center of gravity falls in front of the contact surface. After many gait cycles, the runner reaches top speed and their center of mass then moves backward. A participant who tried to accelerate with their body upright would tumble backwards as a result of the direction of the ground reaction force.12 The further forward the trunk of the body leans and the further
