Biomechanics and the Olympics:Part X

Since the Olympics are ending on Sunday, this will be the last post of the series.  Again, with track and field being the focus of the Olympics this past week, some of these athletes are putting up really fast times.  Newton's third law is a critical factor in a person's capability to run fast.  Newton's third law is the law of action/reaction: "for every action there is an equal and opposite reaction."

In order for a person to walk or run, they exert a force from their foot into the ground.  The ground will push back with the same amount of force, but in the opposite direction.  This force from the ground is known as the "ground reaction force" and can be measured in a laboratory setting using an instrument called a force platform.  This force is measured in three directions: 1) vertical (straight up and down), 2) anterior-posterior (forward and back), and 3) medial-lateral (side to side). 

The way a person moves is influenced by the magnitude of the force applied as well as the direction the force is applied in.  If you want to jump as high as possible, you push straight down into the ground, and the ground pushes you straight up.  Almost all of the ground reaction force is in the vertical direction.  If you want to jump for maximum horizontal distance, you will apply a force both down and back into the ground, and the reaction force will push you up and out (consider a long jumper, they are trying to jump as far as possible, which has both a horizontal and vertical component).  When a person is running, they have a greater posterior ground reaction force when the foot hits the ground, and a greater anterior reaction force when they push off, which will propel them forward.  If a sprinter wants to increase his or her running velocity, he or she will need a greater anterior ground reaction force than posterior ground reaction force.  These sprinters are able to apply a large amount of force into the ground, at the optimal angle, in order to maximize the ground reaction force and run at very fast velocities.