Unlocking Athletic Performance: The Power of Combining Power and Heart Rate Data in Training

With the advent of power meters in cycling and, more recently, power calculations on running watches, many athletes: cyclists; triathletes and runners alike, have started treating heart rate as a less significant training metric. The advantage of measuring and using power in our training sessions, especially interval type training, is that the measurement is instantaneous. Using heart rate to guide our effort for shorter intervals requires a bit of self-knowledge and the moderation of the effort using perceived effort, because of a phenomenon called ‘heart rate lag’. Simply put, heart rate rises gradually in response to an increase in effort and the fact that your heart rate at the beginning of an interval will not match the actual effort that you are putting out. The true value of both power and heart rate as measurements of training output comes when we use them together. Taking note of how much power we can generate at specific heart rates and how that changes over time. 

For example: If we have a regular training route we can compare our average heart rate and power from different sessions to gauge things like, improvement, fatigue and recovery. Using round numbers, if we averaged 200W and 120bpm four weeks ago and now either average 200W at 110bpm or, 220W at 120bpm, these are both positive indicators that our training has been effective. Simply pushing harder would have produced a higher power average but that higher effort would have been flagged by an elevated heart rate. 

This is a very simplistic method of using the relationship between power and heart rate though but not necessarily one to completely dismiss. One of the best methods of determining our level of aerobic conditioning is by calculating ‘Aerobic Decoupling’ during an extended aerobic effort. As the name suggests, aerobic decoupling analyses the degree of separation between power and heart rate during one session. The lower the percentage of decoupling, the more aerobically conditioned we are. 

The gradual rise of heart rate during a constant aerobic effort is known as ‘Cardiac Drift’. There are environmental factors that can play a role in this and the biggest culprit is dehydration. As we dehydrate, our blood plasma level drops, forcing our body to increase stroke rate in order to supply the same amount of oxygen and nutrients to the working muscles. We are interested in the physiological reason for cardiac drift though. Again, simplistically put, when we start an aerobic effort, our body recruits as few muscle fibres as possible to achieve the power required. As these muscles fatigue, they cannot produce as much force, so your body calls on more and more muscle to achieve the same output, which we measure as power. As more fibres are utilised, the demands for oxygen increases and the body needs to increase heart rate in order to meet the demand. This is what causes aerobic decoupling. 

What aerobic decoupling measures is our muscles resistance to fatigue. The better conditioned our muscles are, the longer they can produce the power we require, thus reducing or delaying the need to recruit more muscle fibres and therefore, reducing the rise in heart rate over time at that constant power. The length of time that we measure aerobic decoupling should correspond to the event that we are training for. If our race will take three hours, calculating our aerobic decoupling percentage in an hour long aerobic effort will have less significance as a measure of our aerobic conditioning or fatigue resistance. Obviously, if we are preparing for a very long event, we don’t want to be doing very long intervals in training that match the race duration. In order to get an accurate measure of our fatigue resistance over multiple hours, we should perform back-to-back days of solid aerobic work, building up fatigue, before doing a shorter test to measure the decoupling in a weary state.

One of the key terms here is ‘aerobic’. Anaerobic efforts will completely ruin the calculation of aerobic decoupling. It needs to be measured in a completely aerobic effort. So, not exceeding the anaerobic threshold, the level at which work-rate exceeds the body’s ability to supply the demands for oxygen to the working muscles.  

So, how do we calculate aerobic decoupling? The easiest way is to push our workouts from our devices to one of the many training analysis software suites available. These will do the work for us and pop out decoupling numbers from the data recorded. Using very simple math though, the aerobic decoupling percentage equals – power divided by the corresponding heart rate at the beginning of the effort, subtract the power divided by the corresponding heart rate at the end of the effort, and then multiplying that number by 100.

Example:  [(200W / 120bpm) – (200W / 130bpm)] x 100

  1.66 – 1.54 = 0.12

  0.12 X 100 = 12%

A 5% aerobic decoupling for a workout lasting close to the duration of our target event is considered good.

Always remember that our bodies are susceptible to environmental factors and these will affect heart rate and therefore the results of a decoupling calculation. We have mentioned dehydration as one of the biggest factors to take into account but temperature, humidity, sleep and nutrition will all play a role in how our body performs under load for extended periods of time.