The human body on a bike is a system, just like a car, engine, and road are a system. One can measure power and training effectiveness at various points in that system.
The heart is the fuel pump so the amount it is pumping is a good sign of how hard the engine is working. Unfortunately the heart can lag behind real power output. So, when doing a really quick, short sprint the rider can get upto max speed and be back down to a slower speed or even stopped by the time the heart catches up and hit’s it’s max.
On the other end of the spectrum there is power output measured in watts. That’s a standardised measure that the cycling industry likes to believe is consistent across riders.
They’ve come to realize it is not consistent so when talking technical power it is often watts/kg or watts/lbs (watts per kilogram or per pound).But, there’ no marketing or device that can measure watts/kg so it’s not widely hyped. So, the first issue with power is that it does not take into account the weight being moved. This is why in vehicles there is the standard “foot pounds of torque”; how much power does it take to move 1lbs 1ft.
There is another, even less discussed inconsistency in power, related to weight, and that is the time component. This is why electric grids measure power by KwH (Kilowatt Hours); how many kilowatts are output for how many hours. For simple math, if I have a light that requires 1 kilowatt to operate, running it for 1 minute is a very different power consumption than 1 hour.
So, when you see the power output in watts from a cyclist ask what the time interval of the power measurement is. Is it being measured every pedal stroke, every second, every minute?
One final note on power is to consider inefficiencies and friction in the system. The simplest explanation is to compare a mountain bike to a road bike. I can get on the rollers on a mountain bike and it takes a massive amount of power to ride at 30km/h. I then get on the exact same rollers on a road bike and 30 is easy. The difference between these 2 bikes are the rider position and most importantly the tires. Now, applying that to the real world, you train on the rollers so know that you need to put out 200 watts to ride at 32km/h in a race but then you go and race and you’re putting out 200 watts but lose the race and average 25km/h. Turns out the rollers had less friction than the road and didn’t simulate wind resistance.
Conveniently, that brings me onto the next measure of training; speed. Speed still have the same flaws that power has but speed is ultimately what we are training for so makes the most sense to measure. Conveniently speed is also one of the most cost effective and accessible KPI’s for training. Whether a speed sensor on the rollers or GPS within a phone, it is extremely cheap to measure speed and very easy to judge if current speed is adequate/meets training goals.
While the heart is the fuel bump of the body, the muscles are the engine and the veins are the fuel/oxygen line. In sports medicine centres that often measure blood oxygen saturation; the amount of fuel supplied/used by the engine. Another measure could easily be blood pressure. Finally, one could look at the nerve firings or tension/movement of the muscles (for example in an MRI).
These are often costly, weighty, constrict movement, or simply not viable.
The bottom line, in my opinion, is determine goals and determine the measures that will allow you to train to achieve those goals. Don’t buy a power metre simply because the marketing hype says you need one. I’ve never owned a power metre; the closest I’ve come is the PowerTap HR PowerCal and I have used my wifes power metre to test the accuracy. Even that, simply satisfies my need for data rather than a KPI used for training.
If you are training for racing, setup up rollers or go for a ride as similar as possible to races. Then ride at race speed and distance. Then, when it comes time to race, taking advantage of draft, wind, and racing smart should be easier than training making for a winning combination.