Physics for Coaches: Measuring Force vs Guessing It
Summary
Every coach wants to know how hard an athlete is really working. Not how fast they moved, not how high they jumped, but how much force they actually put into the ground. The trouble is that most of the tools on the...
Physics for Coaches: Measuring Force vs Guessing It
Every coach wants to know how hard an athlete is really working. Not how fast they moved, not how high they jumped, but how much force they actually put into the ground. The trouble is that most of the tools on the market do not measure force at all. They measure something else and then guess the force from it. Here is the difference, in plain terms, and why it matters for your athletes.
Two ways to get a force number
Imagine you want to know how heavy a truck is.
The first way is to watch it pull away from the lights and time how quickly it speeds up. With enough maths, and a few assumptions about the engine and the road, you can work out roughly what it weighs. That is a clever estimate. It can be close. But it leans on every assumption being right.
The second way is to drive the truck onto a weighbridge and read the number off the scale. No maths, no assumptions. Just the weight.
That is the difference between inferring force and measuring it. Most performance tools take the first route. They track how fast the athlete or the sled moved, then calculate the force from how that speed changed. A jump mat does the same kind of thing in reverse: it times how long the athlete hangs in the air and works backwards to a force. These methods are useful, and plenty of good coaches rely on them. But they are estimates built on top of assumptions.
A loadcell is the weighbridge. It sits in the line of the pull and feels the actual force the moment it happens. No guessing.
Sport is horizontal
There is a second thing worth getting straight. Most of what your athletes do happens horizontally. They sprint, they accelerate, they cut, they drive into contact. All of that is force pushed backwards into the ground to move the body forwards.
A lot of testing, though, is vertical. Jump tests and barbell lifts measure force going up and down. They are not wrong, and they tell you useful things, but they measure a different direction to the one the athlete competes in. If you want to know about acceleration and sprinting, it makes sense to measure force in the direction the athlete is actually trying to produce it. That is what a force-measuring sled does.
Why a coach should care
Two reasons stand out.
First, trust. When you build a picture of an athlete across different loads, light to heavy, an estimate carries a little error at every step, and those small errors add up. A measured number does not stack up error the same way. You can trust the picture more.
Second, left versus right. One of the most useful things you can learn is whether an athlete pushes harder with one leg than the other, which matters a great deal coming back from injury. You cannot pull a left-right difference out of a single speed reading, because speed only tells you about the whole system at once. You need force measured for each side. Only direct measurement gives you that.
The simple test
You do not need to be a physicist to use any of this. You just need to ask one question of any tool that gives you a force number: is it measuring the force, or working it out from something else?
Both have their place. But when the goal is an honest, sport-specific picture of how hard your athlete is really pushing, measuring beats guessing. That is the whole idea behind a sled that reads force directly: it tells you how hard they pushed, not how fast the sled went.
Mark Fisher
More from Sports Science
