1.2 Maximum Velocity
Let's start exploring maximum velocity sprinting with subsection 1.2 in this first chapter
The previous post ended with these words: “let’s leave the acceleration phase and fast-forward a few tens of metres when you reach maximum velocity.” But not so fast…
1.2 Maximum Velocity
I said I’ll fast-forward the acceleration phase since I’ll discuss this in detail later in this series. But here’s a preview that’s necessary to understand maximum velocity.
In your first step, when you start accelerating, you’re producing a lot of horizontal force since you’re leaning forward. However, not all the force you produce is horizontal. Some of the force is in the vertical direction.
Here’s a reason why you cannot produce 100% of the force in the horizontal direction:
Do you remember the stick figures I introduced in the previous section? You can only produce force in the direction of the line connecting your feet to your centre of mass. You’d need to be lying flat on the ground to be close to achieving this. And you don’t need me to tell you why this can’t work!
But I will. You need to keep your body up in the air for the duration of your step. Indeed, in the first step, you need to raise your body since you’re leaning forward in a low position, especially if you’re using starting blocks.
You need a vertical force. Without vertical force, you’d fall flat on your face as you leave the blocks. That’s not nice. And it won’t help you win the race, either!
In these first few steps, your horizontal velocity is increasing with each step. Your body covers more ground in each of these early steps. Therefore, you need more vertical force to keep your body up for a longer distance.
But the total amount of force you can produce is limited by your strength and the ability to create force effectively, which comes from training. You can only produce more vertical force at the expense of horizontal force.
So, the faster you go, the more vertical force you need to keep your body up in the air for the duration of each step. This requirement for more vertical force reduces your horizontal force, which reduces your horizontal acceleration.
With every step, as you accelerate, your acceleration decreases. Recall that this doesn’t mean that your velocity decreases. Acceleration refers to an increase in velocity. So, as your acceleration decreases, your velocity increases by smaller amounts.
You’ll reach a speed at some point where the only way to keep your body up in the air for the whole step distance is to use all your force for this – all the force you produce is vertical. (We’ll refine this a bit later.)
Maximum velocity is when all the force you can produce is needed to keep you in the air. (Almost) All of it is vertical force. There’s no more horizontal force to accelerate you forward.
Our friend Newton has some insights on this, too. A body will maintain its speed if no force acts on it. No horizontal force means no acceleration.
But there’s air resistance. And there are braking forces when the foot hits the ground. We’ll talk about all of this in more detail in a later chapter. These create horizontal forces in the opposite direction to the one you’re sprinting in. These forces cause deceleration. Therefore, you still need a small horizontal force component to overcome these decelerating forces and still maintain constant velocity.
But the main point stands. You start sprinting by applying significant horizontal forces to accelerate and just enough vertical force to keep your body up. With each step, you apply more vertical force and less horizontal force. Eventually, almost all the force is vertical. This is the force that keeps your body in the air as you travel at high horizontal speed.
Fascinating! So, let’s assume you have a certain maximum speed. Now, if you get stronger and learn to apply force more effectively over time, you’ll have some force left to apply horizontally when you reach your previous maximum speed. Therefore, you can accelerate a bit more, reaching a higher top speed. But eventually, you’ll reach the point when you need (almost) all your force to be vertical to keep you up in the air for a longer distance. Everyone has a limit on the maximum speed they can achieve!
We’ll return to this key point when we talk about the biomechanics of maximum velocity sprinting in Part 2 of this series: Maximum Velocity Biomechanics. You’ll explore how the various phases of the stride affect maximum velocity, including the contact of the foot with the ground, but just as importantly, the phases leading up to the all-important ground contact. Every phase of the maximum velocity stride is linked!
Chapter 1 also includes another subsection, which I’ll publish soon in a separate post here on How to Sprint: Understanding Biomechanics, which is part of Back on the Track.
–> Next: 1.3 You Push The Ground. You Don’t Pull It! (Coming Soon)