Though not every athlete is planning on competing in the 100 meter, short bursts of sprinting coupled with explosive starts make up a large portion of required skills in a multitude of sports. Whether coming off the line of scrimmage or attempting to steal second base, going from a complete standstill to moving like a freight train in a blink of an eye is vital to an athlete’s overall success. This article will discuss the basics of the mechanics of sprinting, especially in regards to the start.
To reduce it to its absolute basics, sprinting deals directly with taking a body that is at a standstill, and bringing it to a maximum speed in the shortest amount of time. Understanding that, let’s break down the steps.
Many factors make up a successful sprint – leg strength, stride length, and stride frequency, for example. However, an athlete must develop maximum horizontal velocity, and in order to do this, the body must be placed in a position which produces the greatest mechanical advantage, allowing for the maximum generation of force.
The first step, literally and figuratively, is overcoming inertia. Newton’s 1st Law of Motion states: An object at rest or in motion will stay at rest or in motion in the same direction and speed unless an outside force acts upon it. In other words, when you are standing still, your body wants to stay standing still. At the same time, when in a competition, you do not have all day to start moving in order to make a play. So, how does physics allow you to generate the maximum amount of force in the shortest amount of time in order to get your body on the move?
Let’s begin with the start. How important is it? During a sprint, 70% of the acceleration takes place in first 7 steps of the run. To begin with, situating your body at an approximate angle of 45 degrees and moving your Center of Mass (COM) lower and forward allows the most advantageous positioning of your body to generate maximum force in minimum time. Newton’s very famous 3rd law of Motion (For every action, there is an equal but opposite reaction) comes into play here in two very big ways. First, in what’s known as triple extension (open ankle, open knee, open hip),, your rear leg pushes against the ground to launch you forward. As you push, the ground pushes back with what is known as a ground reaction force, allowing you to move forward.
At the same time, as your rear arm swings back, it also helps to move you forward. Think of a rocket ship in space. It has nothing to push against, but yet still moves when it fires its thrusters. How? Back to Newton’s 3rd Law – as it “throws” its exhaust away from itself, the 3rd Law demands that there is an equal but opposite reaction, which moves the rocket forward. Don’t believe me? Try this: Sit in a chair with wheels and on a surface that allows easy movement. With your feet off the ground, take a medicine ball and throw it away from you. I guarantee you will move in the opposite direction.
Both of these actions cause what is known as forward rotation around your COM, which shifts your weight forward. This is countered by your free leg and forward arm. This is what helps keep you upright, and not running in circles, so to speak.
Even the flexing of your arms and forward leg is important. Your arms will act like pendulum, and can affect your running mechanics. Think of an old grandfather clock with a long pendulum as it swings back and forth; the longer the pendulum, the longer the time to swing it. Of course, the opposite is true for a short pendulum. By keeping your arms bent at the elbows, you decrease the pendular swing. This decreases the amount of rotary inertia. Think back to Newton’s 1st Law – as the arm is swings one direction, it wants to keep going in that direction, until an outside force acts on it. By bending your arm, it takes less energy to counter this inertia, and allows you to recover faster and to keep generating force forward.
Flexion of the forward leg does the same thing, and allows for a faster recovery and transfer of position in to the generation of force. This is especially important in making your supporting leg’s foot land below your COM. By doing so, you eliminate the deceleration that would occur if the foot was placed in front of your COM. In other words, if you swing your forward leg in front of your COM, you have to slow it down more (remember the pendulum) and generate more force to do so (Newton’s 1st Law), before you could place it on the ground to start generating force to propel you forward.
When all of this comes together, you will notice that the COM of a sprinter rises and falls very little when at full speed. This ensures proper and not wasted rotation and force production, which move you forward faster, which, of course, is what we always want.