You’re out for a long run and you notice your heart rate starts to drift up – or your pace starts to dip down beyond your control. What’s going on? This could be a sign of aerobic decoupling – a helpful metric to pay attention to and assess your durability for long endurance workouts.
What is aerobic decoupling?
Aerobic decoupling occurs when you are running at a constant speed or riding at a constant power, but your heart rate is drifting up (sometimes called “cardiac drift”). It can also occur when you are maintaining the same heart rate range, but your speed or power start to drift down.
In other words, it’s comparing your internal measures of intensity to your external workload.
Let’s think about this practically. Once you settle into a comfortable speed for a run or ride, you generally want your heart rate working in parallel to that effort.
For example, if you look at your Strava or Garmin data after a ten-mile steady state run, you’d hope to see the same general timing across each mile, as well as a somewhat similar heart rate throughout the run (after the first mile or so – it takes some time for your body to get into the rhythm).
Now, there naturally is some cardiac drift that occurs for most athletes over that ten miles, so just because your heart rate happens to go up a few beats per minute doesn’t mean you’re in dire trouble.
Generally, you’re looking more at the severity of decoupling, and how early in the workout it’s starting. Those can give insights on durability and endurance.
For example, this would be a graph that would indicate minimal decoupling and high aerobic durability:
You can see that the pace stays steady, and for the most part, the heart rate does too. There is some slight cardiac drift, but very small and normal increases there.
On the flip side, the graph below illustrates excessive decoupling.
In this case, despite a steady pace, there are sharp increases in heart rate throughout the workout. There’s about a 10% rate of decoupling between the first half and second half of the workout.
Research on aerobic decoupling
An interesting 2022 study in Sports Medicine looked at decoupling among more than 82,000 marathon runners. They compared the percentage of max heart rate to speed relative to the runner’s estimated critical speed.
They looked at these metrics during the 5-10 kilometer portion of the marathon and compared it to the 35-40 kilometer portion of the race. The ratio was classified into three categories: low, moderate, or high decoupling.
The results? The low decoupling group:
- Completed the marathon at a faster relative speed
- Had better marathon performance
- Typically experienced the first instance of decoupling later in the race
Interestingly, females tended to have lower decoupling and experience the onset later in the race compared to males.
Another 2019 study looked at cardiac drift among 280 marathon runners. They divided them into two groups – those who had a significant decrease in speed starting around 26 kilometers (the “fallers”), and those who didn’t.
The “fallers” had poorer performance and higher cardiac drift (aka decoupling) compared to the other group. (This could have been the result of poor pacing at the beginning of the race – runners went out too hard then faltered.)
Of note, though, is that there’s a bit of a chicken and egg scenario with this research. It’s unclear if runners with lower decoupling have engaged in certain aspects of training that enhanced their durability, or if these runners have some natural durability adaptations (for example, genetic differences in muscle fibers or commonalities between the way they adjust gait over a long run) that increase their endurance.
Why does decoupling occur?
There’s no standard consensus on this, but here are a few theories:
- Underdeveloped fitness level for that particular workout (this doesn’t mean a low fitness level total – but rather that for that particular workload, your body may still need time to train and adapt for efficiency)
- Additional strain to cool the body by sending blood to the skin (hot days)
- Increased core temperature
- Excess stress (physical or mental), fatigue, or overtraining
- Poor pacing early on in a training session or race
- Genetic or gender-based differences in athletes
Some of these can’t be controlled, while others you can work to improve.
Why does this mean for training?
Theoretically, athletes that have less decoupling may indicate more durability for longer distance events. They may be able to maintain better running economy – using less energy at a particular pace – for a longer portion of the run or ride. This is important for endurance training and competing.
How to calculate aerobic decoupling
As a coach, first let me say – if you are just getting started with running or cycling or triathlon, don’t get caught up in all this. Work on exercise consistency for a few months first. Your body needs time to adapt to that initial stimulus before you can really start any structured training or dig into any of this data.
Also, keep in mind that data is just that – data. It can’t ever reflect the subjective measure of how you feel, which I think gets lost these days. I like to look at everything holistically.
So all that said – if you are an athlete that has trained for at least a few months and loves looking at data, it may be worth it to look at your pace/power and heart rate lines on a graph, and see if decoupling starts to occur at a specific point, and the degree to which it occurs. It’s probably obvious, but this means you’ll need a heart rate monitor and either a GPS watch (running) or power meter (cycling).
Once you’ve got the data: if you’re using a training software (like TrainingPeaks, for example), that should automatically calculate the rate of decoupling for you. If you’re not using anything like that, though, you can use this quick calculation:
- Average pace (or power) / average heart rate for first half of the workout = a
- Average pace (or power) / average heart rate for second half of the workout = b
- (a-b)/a = rate of decoupling
For example, on a long steady state run:
- 5 mph / 160 bpm = 0.031
- 5 mph / 170 bpm = 0.029
- (0.031-0.029)/0.031 = 0.0645 = 6.45% decoupling
There’s no hard and fast rule for what your percentage should be. Some coaches feel decoupling of less than 5-6% is ideal. I generally like to focus on overall trends over time.
Remember, some cardiac drift is to be expected. Where decoupling starts to become an issue is a corresponding increase in fatigue or effort, or trends that illustrate a decline in training adaptations.
How to assess aerobic decoupling
Look at what your “normal” amount of decoupling looks like on some of your long workouts or tempo workouts for a few weeks. Take a peek at when decoupling starts on a graph of your heart rate over these runs. Then, monitor future runs using those initial numbers as benchmarks.
If you start to see higher overall rates of decoupling or it’s occurring earlier in your workout, try to assess what’s going on. Here’s a few questions to ask yourself:
- Are you dehydrated? Try weighing yourself naked, before and after your workout (naked = no heavy sweaty clothes after). If you’ve lost more than 3% of your body weight, you may want to work on hydration a bit more.
- Did you come off a week of intense training? Your body may simply need more time to recover. If this is the case, odds are your next long run or ride will be back in line with the decoupling averages you took at the beginning.
- Did you have a stressful week at home and feel fatigued? Sometimes things going on in life can affect training – late days at the office, a baby up in the middle of the night, a family emergency…all of this can affect the body (as well as the mind). One week of this is probably not a big deal in the scope of training, but ongoing chronic stress can be detrimental. If this is occurring, it’s worth seeking out support from a therapist to help with ways to cope with stress or anxiety.
- Was it excessively hot outside? Perhaps it’d be better to train in the early morning or early evening to avoid the heat stress. Or, if your race is going to be in warm weather, you may need to do some training in the heat – but will need to ensure a proper training plan structure that allows you to acclimate.
- Are you going too fast? Long steady state workouts are generally done at a conversational pace, and tempo paced workouts require a very specific speed or power that doesn’t cross into too-intense territory. If you’re pushing yourself too hard, your body may not be able to maintain that throughout the entire workout, leading to decoupling and fatigue. If you feel like that workout was too tough, try slowing down a little and see if it helps with both subjective and objective measures.
- Did you skip a few workouts recently without adjusting your training plan? If so, you may be trying to jump into a workout you’re not ready for yet. Try going back a week or two in your training plan and repeating that week with all workouts included.
- Are you overtraining? Sometimes when people train too much, they may see decoupling as a result of overtraining. Other signs can include dead legs on workouts, fatigue, irritability, poor sleep, and decreased motivation.
- Have you built a good base? Any athlete should spend quite a bit of time building a running base prior to jumping into structured race training. If you’re a new runner in particular jumping into a race training plan, you may see decoupling occurring on long runs because you simply haven’t given your body time to develop a base level of aerobic endurance.
The Bottom Line
Decoupling occurs when your heart rate starts to rise despite a steady pace/power output, or your pace/power starts to drop while attempting to maintain a steady heart rate. This is a relatively new area of exercise research, and there’s no official consensus yet on exactly why this occurs, ideal rates, or definitive training you can do to reduce the level of decoupling. However, by keeping an eye on decoupling trends in your workouts, assessing common workout missteps, and comparing those to subjective measures (how you felt during the workout) – you can make adjustments that give you the best chance at a great block of training.
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