The Number on Our Screen Isn't What We Think It Is
We've seen it a hundred times. A cyclist posts their ride and leads with the number - 287 watts. Their training partner replies: "Nice power." But here's what neither of them said out loud: 287 watts of what, exactly?
Most athletes assume a power meter measures effort. It doesn't. It measures torque and angular velocity - the force we apply to the crank, multiplied by how fast the crank is spinning. The device then does the math and displays a number in watts. That number is real. What it represents is more complicated.
Power meters measure mechanical output at a specific point in the drivetrain - a crank arm, a pedal, a hub, a spider. They're strain gauges. They detect tiny deformations in metal caused by force, then convert that signal into watts using a calibration formula. They're measuring what the bike receives, not what your body produces.
This matters more than most athletes realize. A significant portion of the energy our body generates never reaches the pedals. Heat production, internal friction, the muscular cost of stabilizing our hips and core - none of that shows up in the number. A well-trained athlete delivers a higher percentage of metabolic output to the drivetrain than a less-trained one, even at the same watt figure. Same number, different physiological story.
Here's where athletes - and sometimes us coaches - get into trouble. Power is "context blind". 250 watts on a Tuesday, moderate ride and 250 watts in the final mile of a race look identical on a graph. One is sustainable. The other might be the last thing our legs ever do at that intensity at that moment. The meter doesn't know the difference.
Temperature, fatigue, altitude, hydration status, and time of day all affect what a given wattage costs our bodies. Our power output can stay flat while our internal load climbs. This is why athletes who train exclusively by power numbers sometimes dig holes they didn't see coming - the number held, but they didn't.
None of this makes power meters bad tools. They're excellent at removing perception bias from pacing - a problem that heart rate alone can't solve, since cardiac response lags behind effort. Power also gives us coaches a reproducible, weather-independent load metric as well as helping us target specific energy systems via training zones. A 45-minute threshold interval at 265 watts is the same training stimulus in July heat and October chill in ways that Rate of Perceive Exertion (RPE) simply isn't.
The utility of power data scales with our understanding of its limits. Used well, it's a reference point - one input among several. Used poorly, it becomes the whole conversation, and we start chasing numbers instead of developing fitness.
Our power meter measures what the bike sees. It does that job accurately. What it cannot do is measure us - our fatigue, our adaptation, our readiness, our ceiling on a given day. That part still requires the oldest instrument in training: paying attention to how we actually feel (a.k.a. Perceived Exertion).
The watt is a unit of physics. Training and racing happen in biology. It's good for us to keep that gap in mind.
Gratefully,
Mark
Team MPI
