How Hard is Hard

Over the past few years we've seen the introduction of a number of on-the-bike power measuring devices. The use of these devices has lead many riders, coaches, and physiologists to rethink and reexamine their basic training philosophies and principles. Above all, zone-based training for endurance sports seeks to find a way to measure or estimate a particular metabolic response, energy system, or fuel source being utilized by the body at a particular effort made by the athlete. The debate has always been about how to measure that effort and accurately determine the energy system being used, both in the lab and in the field. I consider four ways of measuring effort on the bike: perceived exertion, speed/distance, heart rate, and power. I've arranged them not necessarily in order of training importance, but technological advancement.

In the descriptions below, I've been deliberately vague with the term "threshold." We can talk about ventilatory threshold, anaerobic threshold, threshold at the Conconi test deflection point, or lactate threshold. Which term you use, from my perspective, is based mostly on the test or approach you're taking to measure it. However, almost all methods are an attempt to measure or estimate lactate threshold as the ultimate barometer of sustainable effort. I may broach those different approaches in another article, but for the purpose of this discussion, I will use the general term "threshold" to encompass all those approaches without a value judgement.

Perceived Exertion

Naturally, the rating of perceived exertion (RPE) is the oldest, and least technologically dependent way to measure your effort, but perhaps unfairly has become the least valued approach in this time of heart rate monitors and power devices because it seems to involve a lot of guesswork. RPE is essentially your own estimate of how hard it feels like you're going, and is relative only to you. There is a set rating scale for this, with a 6-20 point range:

Very, very light: 6 - 8
Very light: 9 - 10
Fairly light: 11 - 12
Somewhat hard: 13 - 14
Hard: 15 - 16
Very Hard: 17 - 18
Very, very hard: 19 - 20

Somewhat Hard would be what many programs refer to as Zone 3 -- something you could sustain for at least 30 minutes at a time, and do 1-3 total hours worth of work in. Hard would be Zone 4, just below the threshold point, and something you would be able to sustain for intervals of 5-30 minutes, and an hour's worth of work. Very Hard would be the point where you just at threshold, whereas Very, Very Hard would be clearly above it, at VO2max. This is a zone you could only maintain for less than +/- 5 minutes at a time.


When cyclocomputers first became widespread, I believe they were misused in the way that powermeters are often misused now. Riders thought that since they had to go 30 mph in their races, they best way to prepare was to ride at 30 mph in training, and simply try to gradually increase the amount of time they could ride at that speed. Obviously, factors like wind and terrain make speed an incredibly unreliable way to gauge effort, to the point of almost being irrelevant for road training. If you're measuring an effort in terms of how long it lasts, how hard it is, and what cadence you're pedaling at, speed doesn't factor in at all, since it doesn't tell us what "hard" is in any consistent way.

Where the converse might apply, though, is on the track, where you have shorter, set distances with specific time performance levels to aim for. You know if you want to do a certain time in the kilometer or pursuit, you have to go a certain miles an hour average. From there, you can calculate how many watts you need to generate to acquire and maintain that speed, and train accordingly. Terrain and weather are less of a factor, and speed can be more easily reproduced.

Heart Rate

The introduction of the portable, wireless heart rate monitor was probably the single biggest factor in determining the way we train for endurance sports today. For many of us, training by heart rate is a little bit of magic, similar to putting a key in your car, turning it, and having it start up and run. How many of us actually know what's happening in our bodies when we train in a particular heart rate zone? And is heart rate the best way to measure or estimate the metabolic effect we're looking for?

Aerobic training is 'aerobic' because oxygen must be delivered to the muscles for fuel. You don't need a heart rate monitor to know your heart beats faster relative to the increased energy demands of training as your body tries to deliver more oxygen to the muscles through the blood. There is some correlation between energy demand, lactate levels in the blood, and heart rate, so basing your training zones on heart rate has been the preferred method for gauging effort.

The drawbacks with relying on heart rate is that there are number of important variables that can skew the relationship between heart rate and lactate levels in the blood. 'Cardiac drift' -- the rise of heart rate at a steady effort over time — is one factor that may makes heart rate less reliable. Extreme heat, even without dehydration, is also a cause, as the heart works overtime simply to help cool the body, as much as it does to fuel working muscles. Heart rate is also a delayed reaction. It may take a minute or more to your heart rate to reflect an effort being made, so for short efforts it's a less useful or accurate indicator.

One positive aspect of training with heart rate is that in a trained individual, heart rate at threshold is fairly static over the course of a season, within a few beats. So, it can be useful to look at changes in power at threshold measured by heart rate to track improvements.


As I mentioned with speed, many people think of power as an absolute. If you want to go this fast, if you're this aerodynamic, and you weigh this much, you need to generate this many watts to do it. And so, they build their training programs around the watts they think they need to sustain to be competitive. I think this is a mistake, as it doesn't necessarily take into account the lactate produced to generate those watts at different points of the season. 400 watts for 5 minutes will be much more damaging and difficult in January than it will in May, and can be the difference between being aerobic and anaerobic. If you follow a program based on periodization principles that have you building an aerobic base first, this is clearly disruptive to that.

What's invaluable about power is that it gives tangible meaning to the efforts you make on the bike. If you know that you generate a certain wattage at threshold, you can rely more accurately on watts where cardiac drift or high temperatures make heart rate less reliable. You can also be very specific with the intensity and consistency of your intervals because of the instantaneous feedback you get from the powermeter, before you see a metabolic response from your body that would be reflected in your heart rate. Power can also tell you a lot about the nature of the efforts you make in races. An average heart rate over a set interval or race might not reflect the incredible variations in wattage your body creates to stay with or move around in a pack over changing terrain.

In many ways, these techniques of measuring effort are simply external methods of estimating lactate levels in the blood, and in particular lactate threshold. The lactate threshold or stasis point (the point at which your body produces as much lactate as it can clear) is the best indicator we have of exercise intensity and sustainability. Regardless of how you feel, how fast you're going, how hard your heart is beating, or how many watts you're producing, if your blood is filling up with lactate, your effort will soon come to an end. So, with every method of gauging effort, we're seeking a perceived effort, a speed, a heart rate, or a wattage at that lactate threshold point. But what if we could see lactate levels on the fly?

It's not unreasonable to think that in our lifetime we might see a device that would strap to your wrist and measure lactate production in the muscles, and lactate levels in the blood. We have portable lactate analyzers now, and nanotechnology promises things that only science fiction dreamt about. We couldn't have imagined tiny, portable, wireless heart rate monitors 100 year ago, so why not tiny, portable, wireless lactate analyzers 100 year from now? Training changes as the tools we have to measure the body's response become more accurate and detailed. It's important not to throw out the old with the arrival of each new, but to incorporate them as one more lens to with which to get a closer or different view.