Amongst the sea of data in cycling, there are three fundamental training metrics. These are rate of perceived exertion, heart rate, and power. These data are all connected and essential to a deeper understanding of training.
The Cause, the Effect, and the Result
I like to think of the three fundamental metrics as linearly connected. Rate of Perceived Exertion, or RPE, is the first. RPE is how hard you and your body feels that you are going. This is the first step in measuring your training. Recall the basics of training; stress-rest-overload. In order to stress, you must have some idea of how hard or stressful your ride is. Rate of Perceived Exertion is a real time, highly accurate function of how hard you are stressing your body. RPE is the cause. You cause yourself to pedal, at some perceived exertion.
The next fundamental metric is Heart Rate, this is measured by a heart rate monitor. HR is your bodies reaction to the work you are doing. If RPE is the cause of the work, HR is the effect of the work on your body. As most of you know, the harder you exercise, the higher your heart rate will go. With a few specific efforts, you can determine your peak heart rate, and your heart rate threshold. Peak heart rate is just what it sounds like, the highest your heart rate can get. Heart rate threshold is the highest effort your heart rate can sit at for an extended period of time. From these stats you can determine training zones from heart rate. These zones can be used to determine how hard your are going, and predict what work in each zone will do to your body. Read my blog about exercise zones here. With your HR Threshold and heart rate data from a ride, you can calculate a training stress score. This score(HRSS) is a generalized score of how hard a ride is, and is an essential metric to be used for training. While RPE tells your how hard you rode, HRSS objectively quantifies it into a number.
The last fundamental metric is Power. Power is a measurement of work/time. Essentially it is a measure of how quickly you can accomplish an amount of work. This is measured by a power meter. A power meter is a device that is attached to your bike that measures both how hard you are pushing on the pedals, and how quickly you are spinning them. Power meters are the most expensive of these data recorders, but are becoming cheaper and more common by the day. Power is essentially what your body is able to output. Your body is a machine that intakes food and outputs kinetic energy through your muscles into watts. Your power meter gives you your real time output, despite feeling or HR. Power data is also used to determine thresholds and training zones. It is also used to create a training stress score, TSS. If your heart rate threshold and your power threshold are set correctly, HRSS and TSS should ideally be very close. Besides the point. Power meters are considered the gold standard for modern training and tracking of training.
What are their Functions?
Rate of Perceived Exertion is the primary metric used when training or racing. It just so happens that the human brain to body connection gives the most accurate predictor of ability. I say this because before using power or heart rate, one must pace themselves with their own intuition. The most aware riders plug their large number of data sets(previous workouts and races) into their own personal super computer(their brain), to be able to detect signs and symptoms their body exhibits to predict the best way to ride. In no case should a power meter or heart rate monitor override a decision made by a properly calibrated human brain. RPE is always the most fundamental and the first metric a rider should use. This is not to say heart rate and power are not useful, in fact they are essential to calibrating the human mind.
Power meters specifically help calibrate the human mind. This is true for many specific cases. The first being when someone first gets a power meter and dips their toes into structured training. In this case, the rider does not know what different intensities should feel like. For example, a rider might have never done a proper over under workout before, so a rough power model helps guide them into training properly. An experienced rider has enough previous data points to know what a proper workout should feel like, so they are able to use RPE to complete the workout to better fulfill its intention. A rider can also use a power meter to understand their fitness.
This is done by comparing RPE with the power you are doing. If today it feels easier to ride at 300 watts, you are likely to be able to do more power and ride stronger than yesterday. Each ride is a data point, where each data point helps you better decide how strong you will be able to ride. This is why older or experienced riders typically pace themselves well, they have experience knowing how they are feeling versus their output, and they can estimate how hard they can ride any given day very accurately.
Power meters are also important for another reason. Power meters should be used as tools to track fitness. While heart rate and RPE give valuable data, they do not give a measurement of strength. Power meters are the only device to objectively measure your fitness, so they became the gold standard for training. While RPE is vital for training in the sense that it guides workouts, power is essential for training as it tracks progression. One of the training fundamentals is progressive overload and objective measurements makes that possible.
Heart rate is a good calibrator for RPE as well. Often times in a workout it can become harder and harder to ride at a specific pace. At a constant pace RPE can slowly creep up, and if gone unnoticed, the focus of the workout could change. Say you plan to do 3 tempo intervals. You may complete two of the intervals at the correct power, RPE, and HR, but on the third maybe the temperature went up, or your fueling faltered, or you simply fatigued, and your third interval kept the same pace, but it was harder and actually stressed a different system, one unrelated to the intention of the workout. Keeping an eye on your heart rate keeps you honest, and perhaps makes you lower some wattages on workouts, but keeps the integrity of your workout.
Heart rate also serves as a good pacing strategy. For certain races it can be advantageous to pace yourself instead of letting others dictate your pace. This is most often seen on climbs in the world tour. There are certain climbers who attempt to maximize their results by riding their pace, keeping themselves from blowing up trying to follow someone else. Heart rate can be a very important tool for doing this. For both training and racing heart rate often serves as a good reality check to keep us from riding too hard.
How to Combine all Three
All three of these metrics are essential training tools, both for what they provide by themselves, but also for what they can add to each other when paired together. For most of these data pairs you get two tools to use. One being the main tool, typically combining the best of both worlds. When we combining these two data sets, we compare them against each other. When we compare RPE with power outputs we can get a sense for how we are feeling on a day, giving ourselves a pacing strategy. When we combine heart rate and power, we can get a sense for long term fitness changes. If your bodies reaction(heart rate) changes for a specific output, we know a rider’s fitness has likely changed. When we combine RPE and HR, we again get a better pacing strategy. In longer races it can be easy to go to hard early in a race when you feel good, keeping an eye on your heart rate and keeping it from elevating is a good pacing strategy. All these are the face value benefits of combining training data.
The second way to compare these data sets is an informative tool to help determine what might be going wrong. This comparison method has to do with decoupling. Decoupling is the separation of previously linked systems so that they may operate independently. When two data sets are compared they often are linked or related, e.g. when one goes up the other does as well. What we are watching for in a decoupling is seeing one data change and the other does not. The most famous of these decoupling is simply called aerobic decoupling. This is comparing heart rate and power. When we see heart rate rise and power stay the same this is known as aerobic decoupling. This mainly demonstrates a lack in endurance fitness. Read my thoughts about aerobic decoupling in my blog about durability here.
When we see a decoupling between RPE and power, this simply shows fatigue. It can be normal to get tired after a long hard ride, in fact this is important to racing and training. A decoupling between these data sets simply gives you information of how hard the ride was for you. Similar to a Power/HR decoupling, but being a bit more “real time” as you are riding.
The final decoupling is between RPE and heart rate. This one should happen less, as it isn’t a intended outcome of riding(i.e. getting a little tired). A decoupling between RPE and HR can indicate a few things. Typically a rise in RPE without a rise in HR(or power for that matter) indicates that you are bonking. This is because your body doesn’t have enough fuel to push watts, so your body reacts by increasing RPE, but heart rate does not elevate. Sometimes if you start your ride and no matter how hard you go you cannot get your HR up. This is typically extreme fatigue. The same can happen at the end of a very hard long ride. For the most part bonking and extreme fatigue should be avoided, so noticing this decoupling is typically a bad thing.
Conclusion
Cycling is becoming a more and more data driven sport. As technology improves people will search for ways to optimize sports with science and data. This has undoubtably made both peak performance go up, as well as giving new riders access to gains through trickle down technology. I think now more than ever it is important to remember the human side of the athlete(RPE). A hyper fixation on science can lead to tunnel vision and blind spots of fundamentals. A strong relationship with your self-intuition as an athlete is more fundamental than any fixation on science could ever be. RPE helps an athlete build this within themselves without getting clogged in the weeds of new technologies and methods. This is why I believe RPE is most fundamental, while HR and power data are essential add-ons to enhance performance.