Watts are not watts: heart rate, power and stress

There are many reasons given by people for not using heart rate to help guide their training. But, probably one of the biggest unspoken reasons is our egos. People want to see the watts on their head unit reflecting what they think they should be able to push.

Paying attention to heart rate often means doing less wattage – which no one seems to want to do! We know though, as Inigo San Milan said in his appearance on Peter Attia’s The Drive podcast “watts are not watts”. A great line.

A past experience by a Sans Chaines athlete was an example of this concept - that watts are not watts. This athlete had had a great winter of training, mainly indoors, and headed south for a week of training. Before leaving there had been a positive discussion about the need to take the opportunity in the warmer weather to do some long steady days – proper endurance - and a reminder that the transition outdoors can sometimes be a bit rough.

Come the first real ride the message came back from the athlete that they had bonked and had a terrible day. This was surprising given the discussion around doing some proper endurance and the athlete’s level of fitness. They had already been south earlier in the winter and done some amazing long days of riding. On a closer look at the training file though, it became clear that the expectation of riding at a certain wattage for endurance had over-ridden the rider’s attention to their heart rate.

Looking at the ride data, the power was indeed within the endurance zone, and even the recovery range. Totally manageable theoretically, and yet the athlete had bonked.

But when checking on the heart rate we get a very different picture. Plenty of time at tempo.

And more clearly within the workout itself we can see that there was a straight block of time spent at tempo heart rate, at the end of which the athlete bonked.

You can also see in that time the steady decline in power across the ride.

Why the disconnect between power and heart rate?

The trip south had been more stressful than expected, with an arrival that was later than planned. The first ride, an easy day, was then spent riding to various bike shops to try and sort out an issue with the bike. A stressful couple of days getting down south and getting settled. For good measure to mess up the body add in a change of environment, probably a couple of bad sleeps, a big change in the weather, and the first outdoor riding in a couple of months. These stresses are as real as any training stress.

At Sans Chaine we would say that this is why we use heart rate. The stress of getting down south was reflected in the rider’s heart rate - an internal measure of a rider’s workrate. The wattage - an external measure - didn’t show the true impact of the ride.

What is happening when someone is riding at their endurance wattage but their heart rate is elevated?

They are running hotter than the wattage shows. Their fat/carb mix – the fuel they are using to produce their wattage – has moved more to a carb heavy mix than would normally be the case. The metabolic area at which they are riding isn’t where they would typically be, and so the sudden bonk this rider experienced.

As athletes we have to be careful that we aren’t chasing the watts all the time. Our heart rate shows us nicely what is happening inside the body. Wattage is great but doesn’t reflect the internal environment. As the world of cycling talks more and more about polarized training we need to understand that we are in fact talking about working on a metabolic/lactate spectrum. The power we see on our head units doesn’t reflect this spectrum, heart rate does. Don’t discount it’s value and how it can help guide your rides.