After 26 Marathons, Heart Rate Trends Significantly Lower in Subsequent Races

What I found absolutely fascinating after my last marathon was how my heart rate hovered in the Zone 2 and 3 for most of the race. I had turned off the audio on my watch because frankly I did not want to hear the Garmin lady’s voice tell me my heart rate and pace every 3 minutes. I used to like it, but I have found it to be more distracting than anything else. So, the truth is, I had no real idea where my heart rate was during this race. I only knew that my performance status was a +2 a couple of miles into the race, and I was cruising along at a decent pace and not far off my personal best times within a marathon. I ended up finishing the marathon in 3:58:16 which was decent. Although the course only had two hills, I did not feel like the 26.2 miles was a walk in the park either. It took effort on my part, and I was fatigued in the later miles, especially as the full sun was shining down on me. Once I got home and started analyzing the data, I was so surprised to see this trend in my heart rate though. Although parts of the race were tiring and difficult, my heart rate never dipped into the dreaded Zone 5 (>181 beats per minute) and only fell into Zone 4 (161-180 bpm) for a mere 7% of the race. That was an incredible insight because it told me that I can run a marathon without overly stressing my heart. It also made me feel like maybe I did not work hard enough (LOL) but the soreness that had overcome my body in the days after the race was telling me otherwise. However, my overall recovery has been quick.

So, being a true data nerd, I wanted to go look at my heart rate data for the first marathon I used my Garmin which was almost 3 years and 13 marathons ago! I noticed that my heart rate was hovering in Zone 4 and Zone 5 for the majority of that marathon. At that time, in early 2019, my VO2 Max was 49 and max heart rate during this marathon was 192 bpm compared to my most recent marathon where my VO2 Max was 54 and max heart rate only being 168 bpm! Then I decided to compile all the heart rate data for all marathons over the past 3 years and that has led me to perform additional analysis and write this article.

The data shows that a dramatic change in my heart rate occurs during my 27th marathon. During my 26th marathon (Atlanta, warm and full of hills and my 4th marathon in a mere 4 month's time), my heart rate was higher for hours in Zone 4 and 5 and then for the 27th marathon (East Canyon, hot, downhill), my heart rate was lower and remained in Zone 3 and 4. What happened during this interim time that contributed to such a decrease in heart rate? Due to the pandemic, the 27th marathon was 188 days AFTER the previous marathon, so I thought the large lag time between the marathons was indicative of a rested body and heart. We all know that the more marathons you run in a compressed timeframe usually means more physical stress and a diminished performance. That could be one reason, but I knew there was more to it. Over the 6 months that had passed, it was as if a switch was flipped. And although this was the first marathon where the heart rate was lower in Zones 3 and 4, this is a trend that has continued with all subsequent marathons to the present day. I started digging into the data and found several interesting trends. For the six months prior to the 26th marathon and 6 months prior to the 27th marathon, I started looking at workout types, tempo and long runs, sprints, total miles, and number of rest days. I wanted to understand what led to such a dramatic change in my heart rate over the course of a marathon. Aside from going slightly more than six months without running a marathon at all, the only major data point that was significant was the number of mile repeats I ran over this 6-month period of time. For the four months prior to the 27th marathon, I ran 14 all-out sprints / short intervals and 64-mile repeats for a significant number of miles. The majority of these tempo-like runs (faster than marathon pace but slower than 5K pace) took place in the immediate few months prior to the 27th marathon. This training was completed at the right time in the training cycle. The workouts were never too close to the marathon and never too far away from it either. The truth is, due to the pandemic, I had quite few marathons cancelled so I was never sure when my next marathon would happen. The combination of sprints and mile repeats clearly led to increased capillary and mitochondrial density which led to greater oxygen delivered to muscles because you have more red blood cells circulating throughout your body. What also parallels this timeframe is that after my 26th marathon (higher HR in Zone 4 and 5), my VO2 Max was at 50 in March 2020. In May 2020, at the same time when I ramped up these mile repeats, my VO2 Max increased to 51 and then again to 52 in June as I further increased these workouts. There is no doubt that the intensity and speed of these mile repeats pushed the lactate threshold and led to an increased capacity to consume oxygen. Because the body is better able to utilize oxygen and deliver it to muscles, the heart can work more efficiently and perform comparably with less stress and maintain a lower heart rate during race performances. It is absolutely fascinating to understand the physiology behind your running performances. These mile repeats and tempo workouts make a significant difference in running economy and that is where you want to get to in training. Ultimately, we all want to become a more efficient runner so we will be able to push our bodies to run harder and faster on race day!