Using "Decoupling" to Monitor Your Fitness
By Ryan Drummond of Threshold Athletic Coaching
When a runner (or their coach) considers an increase mileage or intensity of training, it's good to have something measurable as guide. There are rules of thumb, such as "3-4 weeks to adapt to a new stimulus", or "no more than 10% increase per week", etc. These approaches, while time tested, lack direct feedback of the runner's actual performance.
"Coupling" (and de-coupling) refer to the relationship between the measure of effort (pace or speed) and the measure of aerobic stress (heart rate). When Coupled, these data remain more or less parallel to each other, as viewed on a chart like the one above. That is, when pace is held constant, HR rises to a level then also remains constant.
If enough time passes at that effort level, HR will gradually start to rise, or "drift". The amount of this drift over a given time period is "de-coupling": the HR stress is starting to diverge from the performed effort. (This is due to lowered heart stroke volume as exercise progresses.) Our aim in training is to observe low decoupling, which indicates the runner has adapted to the effort and duration. The duration of this measurement is proportional to the athletes goal event, and is especially important for longer events like the half marathon and above, as shorter events can sustain more HR drift.
As shown in the example, decoupling is calculated as a percentage change in two ratios of pace to HR. The first ratio is defined over the first half of the test duration, and the second ratio is defined over the second half. The difference of these ratios, expressed as a percentage of the first half ratio, is the decoupling. The benchmark figure is 5% or less to be considered adapted to the effort and duration. If decoupling is higher than that, one should consider reducing pace effort, taking further time to let the body adapt, or consider if external factors such as heat stress or dehydration are contributing to the drift.
To get a good estimate of decoupling, you need accurate averages of pace and HR, and it is best if pace can be held constant, such as the depicted treadmill workout. Your training platform or running watch may include this calculation (sometimes denoted PaHR% or PwHR).
References:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6162481/
https://www.trainingpeaks.com/blog/aerobic-endurance-and-decoupling/
https://www.polar.com/blog/cardiac-drift-effect-on-training/
https://www.sportsperformancebulletin.com/endurance-training/training-structure-and-planning/discontinuous-training-master-stroke-endurance-athletes/