A Small Running Experiment.

2026 Day 113. #RunnyDays.

With all these months of running I have been slightly unsatisfied in certain aspects. I do not fully understand it at the moment and that frustrates me a little bit. My bpm and breathing are not always directly linked with my pace. I could push harder or go slower and still end up with similar heart rate. This was just an idea I had built based on my previous runs so I decided to experimentally verify this.

I ran a 5K on two days, Day 1 and Day 2 (very interesting names, I know) at a 5.30 min/km pace and at a 6 min/km pace to compare. I let my body ease into it instead of pushing it or slowing it down. Once I set the pace in the first kilometre it was easy for my body to follow through over the rest of the 5K. These were back to back days. I have recounted some important considerations regarding both of these runs.

The data has sparked an interest in me. I want to learn, do some research, experiment with my runs and try and figure out the best forward. It might not be easy or straightforward but at least it is fun and brings me joy.

Below is the data from the Fitness app on my phone.

Day 1, 5K with a 5.30 min/km pace. I expected these to be drastically higher than this. It seems that all the months of training and running are actually doing me good.

Day 2, 5K with a 6 min/km pace. I felt that the result was the same as the previous day. What could have possibly caused this to happen. Is my training worthless and I just stay satisfied because the results on the days I am pushing are above expectations.

Some important considerations here are

1. Maybe the difference is subtle over a 5K and would be more apparent when I compare longer runs.
2. I ran along different locations in the same city. One was my usual route while the other I was only familiar with.
3. Both times I let my body set the pace. I tried focussing on running at a consistent speed instead of pushing or easing my body in a certain pace.
4. The days had similar temperature and climate overall with Day 2 being ever so slightly colder than Day 1.
5. I consciously activated my calves more on Day 1 whenever I felt my pace was slowing down. I would shift from moving my legs faster to increasing my stride length.
6. Day 2 was more consistent in this regard. I ran without feeling the need to strike a balance between the utilised muscles. I probably underutilised my calves on this run.
7. The first kilometre on both days starts of with baseline heartbeat. It takes time to ramp it up and that is why the bpm for the first kilometre is much lower than the other splits.
8. I practiced the same breathing pattern for both runs. Long, deep breaths and relatively quicker exhales. I am now used to nasal breathing only.
9. Heart rate was measured through an Apple Watch so there are obvious error rates that we should consider. Although I would assume both run could have had a similar error rate, and we can compare the results as is.
10. I had slightly less sleep the night before Day 2 compared to Day 1. I woke up a bit groggier but settled into my normal mood quickly.
11. I would say I put in similar effort in both the runs. One felt more laid back, the other did not feel like too much effort either but I would not say I was the same level of relaxed during both the runs.

Overall, I am still trying to understand why did I achieve basically the same heart rates at different paces. Maybe I need to run at 5 min/km and see how much different that makes. I know for a fact that 6.30 min/km is easily my Zone 2 pace, with my max heartbeat on those runs recorded at around 152. In my initial days the Zone 2 pace was around 7 min/km.

The improvement does make me happy but this little experiment makes me feel that a 30 sec/km difference in pace amounts to nothing. At the same time the difference between 6 min/km and 6.30 min/km would amount to a lot because the average bpm would change significantly.

I theorise that your body can sustain the paces in the range of x sec/km at the same heartbeat with minor changes in effort. It would make sense to practice more at the faster pace so you can push yourself harder. Practising at the slower pace will also lead to better results but your overall progress will be slower.

I am going to look further into this and maybe do a little more of experimental running. What I am going to try to do is do more of a bpm focussed running. I would plan a bpm and see how fast (and slow) can I run within a +/- 5bpm of this planned rate. If my theory does prove correct, I would be changing my training routine.

See you tomorrow.