Running doesn't just happen in some upright fashion with your foot striking down underneath you. At some point you have to accelerate to get there. How does acceleration differ from the common, upright form of running? I have done all of the thinking so you don’t have to. Let’s go through the differences of the mechanics between acceleration and max velocity and what drills make sense for each.

Acceleration

Acceleration is the change of velocity of an object. Also, it is an important part of your sprint ability. Every time you stop, you have to start again. And this starting again is when you accelerate. When it comes to sports, you can imagine this happens pretty often. Up to 75% of maximum running velocity comes from as little as the first 10 yards of your sprint. I have even more numbers for you: the highest rates of acceleration occur in your first 8-10 strides.

These first 8-10 strides are accompanied by a forward lean. When you sprint at full speed, you lose this forward lean and have an upright posture. Why the forward lean?

I want you to imagine you’re in a hot rod about to tear down a drag strip. You rev your engine, watch the lights go from red, to orange, to green: Go! You pop off the clutch, jam down on the gas, and begin barreling down the strip. The front of the car lifts as the tires squeal and you pick up speed at an alarming rate. What happens to you as you sit in this car? You get pressed against the seatback as you desperately grasp the wheel.

Why this analogy? I want you to think about two things: Why do you get pressed into your seat, and what direction do the wheels generate force in relation to the ground?

You get pressed into the seat because of your inertia, or because an object, in this case your body, resists any change in velocity. The wheels rotate forwards. However, in relation to the ground and your car, they produce a horizontal force in an opposite direction to your car's change in velocity. You accelerate forwards, so your wheels produce force backwards (remember Newton’s Third Law?).

Just like the tires in your hot rod, when you run, your feet need to produce force in an opposite direction to the direction in which you are trying to go. In running, you must push back to go forwards. This force is called the ground reaction force, and it has been confirmed that the direction of this force is more important than the magnitude when it comes to sprinting (Nagahara et. al.). In order to do this, you need to plant your feet behind your body's center of mass (line of gravity) to cause an equal and opposite forward progression. To make this possible, you must lean forward so that there is available space behind your body’s center of mass in order to plant.

Your body’s inertia is also partly compensated by this forward lean. A weak core and hips can produce a backward lean. This will also cause you to lose position when you plant your foot into the ground. In this scenario, you’ll either plant below or in front of your center of mass. If you remember from the foot strike conversation, this is less than ideal.

You can see from this video that within his first few steps, my client has an aggressive forward lean, is attacking the ground behind his center of mass, and when he drives his leading leg forward, he keeps his heel close to the ground. The purpose of this piston-like motion, you ask? On the last running mechanics blog, I noted that athletes need to increase their stride length and frequency without overstriding. Overstriding is the act of reaching in front of your center of mass to increase stride length, but it usually leads to decreased stride frequency and a higher chance of injury.

To prevent this, keeping your heel close to the ground is key.

Max Velocity

Before you run out of the blocks with your aggressive forward lean, let’s talk about how to get you upright. If you keep your lean for your entire run, you’ll do 1 of 2 things: fall on your face or run slow. Obviously, when we are trying to sprint and sprint well, this isn’t ideal.

Before I get into the meat and potatoes of this part of our conversation, let me give you some more numbers. Max running speed is reached within 4 to 5 seconds of your sprint. This usually will happen around 40-60 yards if you’re a speedy individual. There are a few things that separate an elite sprinter from a novice and I’ve mentioned them numerous times now. One of which is the amount of vertical force a sprinter puts into the ground and another is their stride length. The average stride length for an elite sprinter is 2.70 meters, while a novice is only hurdling forward 2.56 meters each stride.

The amount of vertical force is the key factor in the difference in stride lengths between elite sprinters and novice ones. For acceleration, the force vectors direction was more important. For max velocity, upright sprinting, the force vector’s magnitude is now more important. However, it should be noted that this force should be vertical and not horizontal as it is during acceleration. And, guess what, I’m going to tell you why.

What’s the big difference between max velocity mechanics and acceleration mechanics? Inertia and momentum. When you’re running at max speed, you no longer have to overcome your stubborn inertia. The nice part is, you also have momentum (mass x velocity). So, your goal at this point, is to maintain speed, not increase it.

This changes your mechanics for the same Third Law of Motion I mentioned earlier. Instead of planting behind your center of mass to have an equal and opposite reaction to start to propel you forwards, you need to plant underneath your center of mass to keep propelling you forward. The vertical force downwards is going to help you fight gravity as well as keep you upright and maintain speed. However, gravity is not the only force you have to fight. You are also up against air resistance. So, some horizontal force is needed.

This horizontal force comes from you cycling your legs. Unlike the piston motion of acceleration, your legs are going to now make cyclical, circular motions. This also means that you no longer have that low heel recovery that we mentioned earlier. Watch this video of me running the 40 yard dash.

You can see that my leg plants straight underneath my body vertically, pulls the ground slightly, and then practically kicks my butt to do the whole thing over again. Also, it should be noted, that if you were to draw an outline of the motion of my leg, it would make a circle, not an ellipse. If it were an ellipse, it would cause me to pull the ground too much and not utilize enough of that sweet, sweet vertical force.

Max Velocity and Acceleration Drills

Now you know the difference between what these two phases of sprinting are. How does the training for them differ, though?

When it comes down to it, it is all about the drill selection. Acceleration drills are going to be much more focused on utilizing that piston-like motion, while max velocity drills are going to by more focused on teaching you how to cycle your leg and produce vertical force.

Here are a few drills that I use often to teach Acceleration:

Here are a few drills that I use often to teach max velocity:

We have talked a lot about speed training these past few weeks. If you don’t feel like you have the tools you need to be fast now, I’m sorry, but you’re a lost cause.

References

Brown, L. E., & Ferrigno, V. (2005). Training for speed, agility, and quickness. Champaign, IL, IL: Human Kinetics.

Haff, G., & Triplett, N. T. (2016). Essentials of strength training and conditioning. Champaign, IL, IL: Human Kinetics.

Nagahara, R., Amini, E., Marcon, K., Chen, P., Chua, J., Eiberger, J., . . . Gujar, T. (2019, May 31). Influence of the Intention to Lean the Body Forward on Kinematics and Kinetics of Sprinting for Active Adults. Retrieved June 27, 2020, from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6628312/

‍