To answer this question, some data were collected during a 20 km night run. It was mainly a pretty flat route in Freiburg and surroundings, except of a hilly part in Heuweiler, which is a very scenic and beautiful place nearby.

A brief look at the plots shows quite a strong correlation between the heart rate and the cadence, which is sort of expected.

hr_and_cadence_preview

After some adjustment period (around 2.5 km in this particular case) the heart rate looks very aligned with the cadence. This correlation has been observed multiple times in personal experience, and the topic is widely covered in popular running-related media and research works. Some sources even mention a physiological phenomenon called cardiolocomotor synchronization (CLS), when the frequency ratio of an activity (specifically, cadence in case of running) is coupled to heart rate in a 1:1 fashion, or at other simple integer ratios (e.g. 1:2, 2:3, 3:2).

Likely, CLS is not the case for this particular run (the data show that these parameters are very far from being fully coupled), but the cadence/heart rate ratio actually looks pretty consistent in the plot:

cadence_hr_ratio_preview

The ratio is slightly higher in the second part of the workout, possibly, fueling might be a reason for that - after pretty much inconsistent daytime intake, the first gel at 5km and the second one at 10km finally brought some energy to the body.

This leads us tho the main conclusion - it's not that simple. Especially, if it comes to regular hobby runners or running enthusiasts who have to balance their sport activities with other life responsibilities and deal with sub-optimal conditions. Plenty of factors can affect heart rate - current level of fitness, cumulative fatigue built up during a busy week without sufficient rest periods, lack of proper fueling, lack of sleep, daily stressful situations, injuries or physical discomfort, any kinds of emotional pressure. Even music in the headphones can be a stimulating or calming factor.

Still, awareness is a good thing. if the heart rate is unusually high, it might be a sign that something needs to be changed. Conditions around, training process, even expectations - finding out what can be adjusted isn't always a trivial thing, still, it can be rewarding. We're definitely going to cover this topic further in the next posts. Stay tuned!