Quite simply, I have never been confronted by a more constant and seemingly endless debate than the use of power data vs heart rate (HR) to classify the intensity of efforts on the bike. For me, as a physiologist, I can easily see it both ways, thus building arguments towards the implementation of both variables separately as well as simultaneously. However, when it comes to creating synergies between real-time metabolic events (like anaerobic threshold), using heart rate INITIALLY (and I’ll come back to this later) is the only means of accurately determining how and when these events take place. Wattage is a phenomenally consistent guide as a means of addressing energy expenditure, but when you think about the sources by which energy is produced, HR is, simply, much more reliable.
First of all, I know that might be a bit hard to swallow for anyone whose just spent $2000 on a power meter, but before you launch into an opinionated rant against my central thesis, please just maintain an open mind for a few minutes longer… and you’ll be happy you did. Afterall, we are both on the same side, we just differ on our logic regarding the most accurate device for registering what is actually happening inside our bodies. Think of it like this- if we all had internal tachometers with built in power displays (much like those of sportscars) this point would be moot and we could all go back to arguing about the ratios for the functionality and art of Shimano vs Campagnolo. However, we aren’t equipped with such devices, so we have to use what we can from the bodies’ internal and external feedback systems in order to provide a contextual framework of intensity and degree of effort.
Lets look at HR. First, just one simple question, what makes your heart rate increase? Have you ever really thought about this? I mean, it seems somewhat illogical not to have at least pondered what controls “the” organ responsible for delivering vital nutrients and, lest we forget, oxygen to every other tissue in the human body. On the outset, it seems as if it’s a question with no obvious answer, but thanks to an understanding of modern physiology, we know exactly what controls your HR. It’s called arteriovenous difference or “A-Vo2 diff” for short. Basically, A-Vo2 diff is a ratio of the amount of oxygen in your arteries to the amount of oxygen in your veins. As you know, arteries carry oxygenated blood away from the heart and to the working muscles. As the need for oxygen increases due to increasing energy demand from muscles, more and more oxygen is harvested from the arteries thus increasing the difference between the amount of oxygen in the arteries and the amount of oxygen in veins (A-Vo2 diff). This difference will become larger in proportion to the amount oxygen that’s needed as the catalyst for producing energy. Your heart and brain collectively recognize this increasing A-Vo2 diff and take steps to increase cardiovascular output by heart beats that are more forceful and more frequent. These two strategies both have the goal of offsetting your usage of oxygen with an accelerated delivery of oxygen. All this is very scientific, yes, but how can it be used effectively to ACCURATELY measure your effort levels while on the bike? It’s simple. Oxygen consumption is DIRECTLY linked to HR. Wattage is not. Not only can HR be used as a proxy for measuring your oxygen consumption, especially at intensities beneath your anaerobic threshold, but it can serve as an indication of your overall economy and sources of produced energy. This means that you, as an athlete, can accurately determine based on your HR, your usage of both carbs and fats while training or racing thus giving you an indication of how you need to fuel yourself for maximal performance at your current intensities.
The Problem with Wattage:
First of all, lets define exactly what constitutes a “watt”. A watt is a unit of energy consumed per unit of time. Think of a 60W light bulb. This lightbulb is using 60 units of energy (joules) every second. Now, if we think about watts in terms of what we see on the screens of our powermeters, we know that any increase in power can be instantaneous. However, the processes that provide the energy necessary to power such in immediate increase in wattage (especially those that are dependent upon the use of oxygen) are FAR from immediate. Because oxygen transfer takes time, our aerobic metabolism suffers from what physiologists call “metabolic inertia”. In simpler terms, aerobic metabolism is slow. Thankfully, muscles have ways of generating energy without the usage of oxygen, and this is something we all know. Whether it involves anaerobic forms of glycogen burning or the usage of other, more immediate methods (as is the case with the phosphocreatine pathway [P-Cr]) these immediate solutions to the energy void still can’t prevent massive effects on A-Vo2 diff at the muscular level. This begins a long signaling cascade that eventually terminates in a larger bulk of oxygen/min (VO2) being delivered to the muscle via an increased HR. However, because the need is so immediate, and because takes some time to mobilize the oxygen necessary to power these strong muscular contractions (a la sprinting…) you need anaerobic respiration to suffice until the aerobic systems are fully active. All the while, your powermeter is still reading the same number even though the systems required to put that number in place are in a state of complete turmoil.
Therefore, your powermeter isn’t accurately depicting what is happening inside the muscle in terms of energy production at all. It’s depicting energy that’s being consumed without depicting any information on how it was generated or by what means. This has massive repercussions on your training status and overall efficiency as an athlete.
Your muscles work under the assumption that there is an endless supply of energy and couldn’t be less concerned with how or what-with it’s produced. Think of it like a teenager that’s given free access to their father’s bank account. Their spending rate (wattage) is hardly reflective of what it takes to generate that capital and when it’s gone, it’s gone… at least until their father receives another payday (oxygen delivery). Muscles work precisely according to this mechanism. You’ll continue putting out watts until there isn’t any more energy available to the muscle or the conditions within the muscle become highly unfavorable… then you see the numbers on your powermeter start to tumble. See, muscles (and generated watts) have no understanding of what systems are currently operating in the background to provide them the energy they so desperately need to report that nice big digital readout on your powermeter. Because the body has several systems to produce both immediate and long term energy, oftentimes those systems are used simultaneously depending on the immediacy of each situation. Sprinting, for example, requires immediate energy that the aerobic system just isn’t capable of producing. But, given enough time, the aerobic system can and will “catch up” with the energy demand and fulfill it via increased oxygen delivery. However, the only way of knowing this is by monitoring heart rate as it is in direct contact with the systems that regulate oxygen consumption. When you think about it, it makes perfect sense – the organ responsible for oxygen delivery is in direct contact with the systems responsible for oxygen consumption. All of this underwrites your ability to produce long-term energy for your muscles.
From here, you can go varieties of different directions. With advanced metabolic testing, similar to the type performed by Sigma Human Performance, you can pinpoint the energy contributions of fats and carbohydrates independently, which is important for anyone looking to increase their efficiency as an athlete, prolong their fuel stores during a race, or focus training attention on the intensities that provide the greatest amount of aerobic stress. Because all this can be achieved by using HR as a proxy to differing rates of oxygen consumption, you can then train your body to work and be efficient within the boundaries of specific zones that are independent of power. This becomes incredibly useful especially in sports that demand long durations of steady-state exercise such as long-course triathlon. Moreover, using HR is a much more affordable method of monitoring the activity of your aerobic system especially for athletes that don’t depend on short explosive bouts of exercise. Using HR as a strain gauge to achieve maximal economy is a fantastic way of training your aerobic system to become more resilient to metabolic stressors. However, as all of this is wonderfully useful, does using HR alone come without limitations? I’m afraid not….
To be continued…