Don't throw away your heart rate sensor!
With the advent of power training, heart rate monitors seem to have been relegated to the sidelines. But are they really useless?
Almighty power
There are many advantages to training based on power (watts). It provides objective, reliable, and consistent feedback. It is also instantaneous, allowing you to perform short bursts of effort (such as sprints or short intervals) with a high degree of accuracy in the data.
What do we mean by "objective"?
Simply put, watts do not take internal factors into account and measure the force exerted. So, if you place two cyclists of identical weight side by side on the same road, under the same conditions, the one who exerts the most power (i.e., watts per kilogram) will reach the top first. The respective heart rates of our opponents are irrelevant. Power is therefore the ultimate measure of external energy, or if you prefer: how hard you push on the pedals, regardless of how much energy you expend to do so.
Inspect the engine
But how much fuel did you leave on the road during this effort? How hard did you really push the engine? And what condition is it in afterward?
Measures such as TSS and IF give an idea, but heart rate is very useful for gauging the overall condition of the engine—your body. Measures based on calculations using power output do not take into account your level of fatigue, nutrition, hydration, and stress. These factors influence the body's ability to generate maximum power using minimum resources. They are therefore crucial to your preparation and recovery.
Avoid overtraining and monitor your progress
For short bursts of exercise, or exercise that varies fairly regularly, a heart rate monitor is not particularly useful.
However, it can show how quickly you recover, which is not without interest, quite the contrary. And not just during a workout, but over the course of several days.
Taking your resting heart rate every morning allows you to gauge your level of fatigue: a rapid heartbeat upon waking indicates that you have not recovered properly. Significant variations during exercise are also important. If you are unable to reach your usual maximum or, on the contrary, if you exceed your maximum at the slightest effort, it may be time to take a break and, above all, discuss this with your coach to adjust your training load.
In addition, measuring your heart rate can help you gauge whether your fitness level has improved or deteriorated. For example, if you repeat a workout at an average of 300 watts a few weeks apart, but your average heart rate drops by 20 BPM between the first and second tests, then you have probably improved your fitness level.
Two workouts where it's useful
During rides (3 to 6 hours) in zones 1 and 2, i.e., at very low intensity, it is best to rely on your heart rate rather than your power output to ensure that you don't overdo it and get the most out of your training—instead of exhausting yourself.
Because here, the aim is to activate the body without ever exceeding a certain level of intensity. And it may be that, if you are tired, for example, you exceed the desired limit, even if you are exercising at a very moderate level.
For time trial training, which requires constant effort, it is also useful to measure the effects of accumulated effort over time in order to avoid burning out.
Be more sensitive to sensations
To sum up: your heart rate tells you what is happening inside your body. The Power Meter what is coming out of it. If you sleep poorly, drink or eat badly, or are consumed by stress or anxiety before an event, your heartbeat can sound the alarm.
More generally, the heart rate monitor teaches you more about the machine that is your body. It can therefore teach you to listen to your body better, to be more sensitive to sensations that we sometimes try to ignore, when we should be listening to them carefully.