By: Adam Clark, B.S. Physics, B.S. Mechanical Engineering, push athlete at USA Bobsled, training for the 2014 Olympics this February
I’ve heard this argument waged by elite strength coaches and athletes alike. Is it important for sprinters to perform heavy resistance training? There are famous coaches on both sides of the fence here and they’ve argued the finer points of the sport to death but made little progress toward actually answering this question. When Ryan asked me to write something about sprinting, I thought I’d take the opportunity to come at this argument from a different direction. Rather than taking the point of view of an elite speed coach (which I am certainly not) I decided to take the point of view of a mechanical engineer (which I in fact am) and see if I can shed some new light on the argument.
Years ago, before I knew a lot of things that I know now, I used to want to be an engineer. I wanted it so much that I spent 6 years in college studying physics and mechanical engineering just to do it. After two and a half years of staring at the inside of a cubicle I realized the error of my ways and changed my focus to strength and conditioning. Some might say that’s a waste of an education but I disagree. I will, of course, grant that I don’t get much use out of Electrical and Magnetic field theory but the Dynamics and Mechanics I learned apply just as much to the human body as they did to all those imaginary rockets we solved problems about.
We learn in Dynamics that Newton’s second law states: the net acceleration of an object (the human body in this case) is equal to the net force applied to that object divided by that object’s mass as shown below.
Ahhh math! Stay with me.
Let’s define the terms. The mass is easy and for most athletes it is constant (at least we hope). Assuming all races are run on Earth the mass of an athlete is basically his weight (sorry engineers). For example the mass of a 220pound athlete is 100kilograms (we won’t go into English units). The net acceleration of an athlete is the acceleration we observe while watching the race. If it is positive then we see the athlete speeding up, if it is negative then we see the athlete slowing down, if it is zero then the athlete is maintaining the same speed. The net force is the sum of all the forces acting on the athlete at any given time as shown in the diagram below.
As you can see in the picture some forces are helping the athlete (Those forces are shown as F1), and some forces are working against the athlete (Those forces shown as F2). F1 is generally equal to the force produced by the athlete; the forward force the athlete applies to the ground with each foot strike. F2 is the sum of resistive forces such as the backward force the athlete applies to the ground with each foot strike, the friction between the athlete’s foot and the ground, and the aerodynamic drag produced by the wind. For the sake of this discussion we’ll ignore the frictional and aerodynamic forces because they are relatively small when compared to the backward force of each foot strike. We will also assume that for a given athlete F2 will be constant in each phase of sprinting because his running mechanics are approximately constant in each phase of sprinting. Considering these things the net force is equal to the positive forces (F1) minus the negative forces (F2).
Most sprinters and sprinting coaches view a race like the 100m sprint in two parts: the acceleration, or drive phase, and the top speed phase. Let’s look at both of those phases and what Newton’s second law means to an athlete in each.
The Drive Phase
The drive phase extends from the moment the gun goes off and the race begins until the moment when the athlete reaches top speed (generally between 40 and 60 meters depending on the athlete). During the drive phase the athlete is increasing in speed, aka he is accelerating, so the net acceleration is positive (greater than zero). What separates great starters is their ability to produce a large net acceleration (accelerate very rapidly) and Newton’s Second Law shows us how this is possible.
There are only two factors in the equation given above, net force and mass. Since we’re assuming the mass of an athlete is constant that means that an athlete’s ability to accelerate rapidly is directly related to their ability to apply net force to the ground in the forward direction. Based on Newton’s Second Law, if the athlete can increase the net force in the forward direction then they will accelerate faster.
The Top Speed Phase
You’ve probably already guessed at this point that the top speed phase extends from the moment the athlete hits top speed through the finish line. In reality sprinters are actually slowing down slightly during this phase do to the depletion of energy stores within muscle tissue and other movement errors but for the sake of this discussion let’s assume that an athlete is able to maintain top speed through the end of the race. As we said earlier, when an athlete is maintaining the same speed their acceleration is equal to zero. What separates the fastest athletes is the speed at which this occurs (net acceleration =zero) and again Newton’s Second Law shows us how this is possible.
Based on the equation given, if the net acceleration of an athlete equals zero, then the net force must also equal zero because mass cannot. This means that the positive forces (F1 in the diagram) and the negative forces (F2 in the diagram) are equal. This is the same for every sprinter. When they reach top speed the net force is zero and the value of F1 is equal to that of F2. Since we are assuming that in each phase of the race the negative forces are given for an athlete based on their technique that means that an athlete’s ability to achieve a higher top speed is again directly related to their ability to apply net force to the ground in the forward direction. For example, if an athlete is hitting top speed at 40m (F1 = F2 at 40m). F2 is some given value at that point. If the athlete can increase F1 beyond that given value for F2, then the net force, and thus net acceleration, will again be positive. The athlete will continue to accelerate until F2 increases to match the new F1 and the net force again becomes zero at the athlete’s new top speed.
It has been shown above that in both phases of a 100m race an athlete’s performance can be improved by increasing the net force applied in the positive direction, but how does an athlete do that? The force an athlete can apply in any sport is the product of two things: Strength and efficiency. In other words the maximum amount of force an athlete can apply and the ability the athlete possesses to correctly apply that force. When we’re talking about net force production the maximum force an athlete is capable of producing is always multiplied by the efficiency with which we applies it as shown below.
Increasing either of the above values will increase net force production. Some athlete’s may be capable of producing 1000pounds of force but only capable of correctly applying 50% of that. Conversely, other athletes may only be able to produce 600pounds of force but are 95% efficient at applying it. In both cases it is the athlete with the highest net force production who wins.
Efficiency can be increased and maximized through proper coaching and attention to detail over an athlete’s career. In reality an athlete can never reach 100% efficiency although they may come close. There are always losses produced by small flaws in technique or anatomic abnormalities. As an athlete’s career progresses, increases in efficiency will become smaller and smaller until they are undetectable or entirely non-existent. This is known as the law of diminishing returns and is the reason that athletes must always continue to work on the other side of the puzzle, maximum force production.
Once increases in efficiency have taken you as far as they can the only way left to make improvements in net force production, the only way to increase acceleration, the only way to increase top speed is to increase maximum force production and that’s done through, you guessed it, heavy resistance training, aka lifting big weights. Of course you cannot just do any heavy training for this to work. A sprinters program must be made to increase maximum force production in the way in which they need it. Ground contact times for sprinters are very short and even more so the faster an athlete runs. It does no good to drive an athlete’s maximum force up to 1000pounds if it takes them half a second to produce it. Sprinters must be trained in such a way that they can produce maximum force in on the order of 90 milliseconds, or 0.090 seconds. This still requires the use of heavy resistance training but in a way catered to the sport of sprinting.
In closing, I hope I have shown here that for a sprinter to reach their genetic potential in the sport they MUST maximize their ability to produce force AND maximize their efficiency in applying that force. If the athlete is doing anything less, then they are leaving money on the table. Maximizing only one of the two will never get an athlete to their genetic potential. They must both be maximized in order for the athlete to achieve their true best.