Power, Heart Rate, and Rate of Perceived Exertion are the three most common and effective tools to measure fitness for cyclists. While each serves a purpose when it comes to training, these tools are not necessarily created equal. In this article we’ll be diving into all three and discussing their strengths, weaknesses and some of the common misconceptions associated with each. Let’s get into it!


Power is calculated by measuring force and speed (leg speed in most cases). This means power is dependent on both how hard you push on the pedals, as well as how quickly you turn them over. In short, putting out big power isn’t just pressing hard on the pedals; it’s a combination of pushing hard on the pedals and spinning them quickly.

When calculating power, force and speed are inversely related. A low cadence requires high force to output a given wattage. Conversely, a high cadence requires less force to achieve the same wattage.

When calculating power, force and speed are inversely related. If a low cadence requires high force to output a given wattage, conversely, a high cadence will require less force to achieve the same wattage.

Power directly relates to both speed (with other factors thrown into that equation such as weight, aerodynamics, etc.) and fitness. As a rider’s power increases, this is a sign that fitness is increasing. Assuming other variables are staying the same or improving (weight, aerodynamics, etc.), this rider is also becoming a faster cyclist, which is the ultimate goal of training.

Best Use of Power

Power is an objective measure of output that is reliable, precise, and comparable over time. To train with it, riders use their Functional Threshold Power, or FTP, as a fitness benchmark. This is essentially the amount of power you can hold for an hour. This figure is a fantastic representation of fitness and can be used to scale workouts to your fitness level as well as monitor output/performance during a race.

Heart Rate

Heart rate is measured in beats per minute. Each beat is a contraction of the heart muscles, which pump blood throughout the body. This process is what delivers oxygen and nutrients to muscles, while removing things like lactate, and other waste products of both aerobic and anaerobic work.

As workload increases, heart rate also increases in order to keep up with the higher demand of filtering and replenishing skeletal muscles. However, both the rate and the amount of this increase is different from athlete to athlete and really has little relation to fitness.

Heart Rate Difference

The reason for this is that we are all wired a little differently from a physiological standpoint and we all have different cardiovascular systems. One athlete may have a max HR of 200 that he reaches within 2 minutes of an all-out effort, while another athlete may have a max HR of 155 that he reaches after 5 minutes of the same effort.

Which rider is “more fit?” It’s impossible to tell with this information alone.

Misconceptions About Heart Rate

HR by itself is an insufficient indicator of fitness for several reasons. To best explain this, it’s important to start from the beginning – To improve the cardiovascular system is to improve fitness. However, HR is simply one variable in the very complex structure that is the cardiovascular system. Drawing conclusions about your fitness based on fluctuations in heart rate over time simply cannot be done without further information.

Say a rider’s max HR goes up 5 BPM after 6 months of training. This information alone means nothing in relation to fitness. However, if the rider’s FTP also went up 20 watts, it’s safe to say they got more fit. Conversely, if a rider’s max HR goes down after 6 months of training but their FTP still goes up, it’s still safe to say they got more fit and improved cardiovascular efficiency.

Without power data and left exclusively to the devices of HR data, you really wouldn’t know whether or not you are getting stronger.

Training with Heart Rate

While not impossible, training with HR is very challenging. This is because HR is a subjective measurement that can be effected by many variables, thus painting an unreliable portrait of effort and performance. Things like rest, stress, coffee/nutrition, hydration, etc. can all effect your HR values during a ride.

To illustrate this, let’s assume a rider has 2×20 min intervals at LTHR (lactate threshold heart rate) planned for back-to-back training days:

Day One: The rider has a pot of coffee in the morning before riding, and maintaining LTHR during the intervals seems incredibly easy because they’re not actually riding at their FTP.
Day Two: No coffee and they also didn’t get a good night sleep. Maintaining LTHR feels nearly impossible to the point where they are unable to finish the 2nd interval. While trying to hard to increase HR, they were actually riding over FTP.

It’s safe to say that on the first day, the caffeine in the coffee was helping elevate the riders HR, allowing them to reach their LTHR without actually putting out power equivalent to their normal FTP.

Then, the second day they probably had some lingering fatigue from the previous day’s workout, combined with a lack of caffeine and a lack of sleep. Assuming these are acute problems that are not wearing the rider down day after day, the rider probably wouldn’t have any issue hitting his power target.

However, even small challenges in nutrition, rest and stress levels can greatly affect HR and prevent you from reaching your LTHR. With these assumptions it is safe to assume that the rider was probably working significantly harder than necessary to reach their LTHR on the second day.

Further, HR is a lagging indicator of effort. Say a rider wants to do 4×5 minute threshold intervals. Depending on how quickly that rider is able to get their HR up, the first couple minutes of each interval are unknown as to whether or not that rider is at, above, or below threshold. With only a 5 minute interval, often times more than half of it could be spent in the wrong zone while the rider waits for HR to reflect the effort.

Best Use of Heart Rate

The most effective use of HR in respect to endurance training isn’t actually on the bike at all. Rather, HR is best used as a window into a rider’s health and freshness level.

By monitoring daily resting HR upon waking up, athletes are able to begin to see trends and get an extra insight into their current state of both health and freshness. If a rider has a resting HR of 50 every day and then wakes up to 65, it’d be smart to do a quick self-assessment. How are you feeling? Could you be getting sick? Could it be time for a rest week?

Before skipping a workout, though, it’s important to understand that there are also plenty of acceptable reasons for an elevated resting HR that do not necessarily warrant an off day. Things like a tough workout the day before, nerves, acute lack of sleep, etc. could be the cause of the high HR value, in which case you could still benefit from a workout.

Resting Heart Rate

While resting HR can vary in small increments from day to day, the trending increase of 10+ BPM around Week 2 may be a sign of illness or over-training.

This is why resting HR data needs to be interpreted as trends, rather than single points of data. Further, it needs to be taken into consideration with the other things going on in your life and how you feel.

Rate of Perceived Exertion

First off, RPE is a liar. It is a subjective measurement that when used alone can be harmful to performance. However, this doesn’t mean that RPE is not informative. Due to the fact that RPE is based off of perception, it is necessarily subjective but informative nonetheless.

RPE is exactly what it sounds like – how hard it feels like you’re working. All of the tools that we have discussed thus far stem back to RPE and are always used in conjunction with it, even if you don’t want them to be (no hiding from the pain). It’s often represented on a 1-10 scale: 1 being barely even pedaling and 10 being everything you’ve got.

RPE can allow you to ride way harder than you should, or way easier. For example, during the first 2 minutes of a big TT that a rider has been waiting all year for, adrenaline may have them pumping out 600 watts during that first 2km. They get so fired up that their RPE insists that’s a good pace. A few minutes later they are blown up and wondering what went wrong. With just a quick glance at their power data, this mistake can be avoided by knowing 600 watts is not a sustainable power output for that athlete.

Best Use of RPE

RPE is something that should be taken into consideration in some amount during every part of an athletes training. Trends in workouts “feeling harder” or “feeling easier” can provide a lot of insight into how effective that athlete’s training is.

If a rider has been doing similar workouts for several weeks and they continue to feel easier and easier, it may be time to re-assess their FTP. Or, if the same workouts are feeling harder and harder over the course of several weeks, perhaps a rest week is necessary.


Training and building fitness are a balancing act. The more insight an athlete has into their performance and health, the smoother the road to their goals will be. While each of these tools discussed has a unique strength over the others, using each of them together, in the way that they are most effective, provides the most insight into fitness.

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Trevor DeRuise

Trevor DeRuise is a professional mountain bike racer, Level III USA certified coach, lead contributor to cycling forum conversations, and the main person behind all TrainerRoad’s tweets. Follow the latest TrainerRoad news and updates he’s sharing online.

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