Can anyone explain to me this "Power" rating. Why is it important? How is it measured? What function does it have to do with training? I'm not a geek. Dumb it down. Danke.

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ZimZam wrote:
Can anyone explain to me this "Power" rating.
Objects that are moving possess a certain amount of energy due to that motion, called "kinetic energy." Higher mass and higher speed both result in more kinetic energy: doubling an object's mass doubles the kinetic energy, doubling an object's speed quadruples the kinetic energy. Energy is measured in joules: a 1kg object moving at 1 meter-per-second has .5 joules of kinetic energy, while a 2kg object moving at 2 meters-per-second has 4 joules of kinetic energy.

"Power" is the rate of change of energy. It's measured in watts, where 1 watt is 1 joule-per-second. If the aforementioned 2kg object is accelerated using 1 watt of power, it will take four seconds to reach the 4 joules where it's moving at 2 meters-per-second; if it's accelerated using 2 watts of power, it will only take two seconds to reach a speed of 2 meters-per-second.

But in the real world, there are also resistances to overcome, such as aerodynamic drag. Resistances apply power against the kinetic energy of an object. If a power source is trying to accelerate a moving object with 1 watt, but the object is also facing 1 watt from resistances, the object will not accelerate or decelerate, but rather continue moving along at its current speed.
Resistances generally become larger as speeds increase. If a power source is trying to accelerate a moving object with 2 watts, and it's only facing 1 watt of resistance, the object will accelerate until it's going fast enough that it's facing 2 watts of resistance. Similarly, if a power source is trying to accelerate a moving object with 1 watt, but it's facing 2 watts of resistance, the object will decelerate until it's only facing 1 watt of resistance.

On a bicycle, the rider's pedaling is a power source used to accelerate and maintain speed. Opposing the cyclist's motion are resistances like aerodynamic drag, rolling resistance, and gravity (if the cyclist is climbing).

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Why is it important?
In a given circumstance, pedaling at more watts means you will maintain a higher speed.

And: reducing your resistances, such as by using fast tires that have less rolling resistance, allows you to maintain a given speed at lower power (and therefore lower effort).

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How is it measured?
Most power meters take the place of drivetrain components, such as pedals or crank arms or rear hub. They measure the forces that your pedaling is putting on the drivetrain, and use that to infer the amount of power being delivered.

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What function does it have to do with training?
However much you want, really. Power tells you how much your pedaling is really doing, in a way that metrics like heart rate or perceived effort do not.

When used carefully, a power meter can also be used to characterize how efficient your bike setup is. So if you do something like change your posture on your bike, you can check whether the change increased or decreased your aerodynamic drag.
It's a consistent measure of how hard you are training/racing. In running and swimming, pace is a pretty decent proxy for effort, especially if you are swimming in a pool, and running in fairly consistent conditions. In cycling, weather conditions, road surface, gradient, group riding, etc mean that speed is largely meaningless as a measure of effort. E.g. Yesterday I did a flattish group ride that averaged 25mph, and my average power was lower than it would be for a hilly solo ride the week before averaging 18mph.

Why does it matter that you can measure how hard you are training? Because it allows you to increase your training load incrementally so that you are improving your fitness steadily without overdoing it and getting injured. It also helps on race day to pace yourself at a steady effort regardless of course conditions (e.g. not going too hard into a headwind trying to chase a target average speed).

If you don't have power then a combination of HR and RPE are the next best thing, and even if you do have power then you should still pay attention to HR and RPE as they may well be telling you something that power isn't (e.g. even though you've comfortably held x watts a dozen times in training, today is a bad day for whatever reason - illness, fatigue, nutrition, etc - and you need to back off a bit)