Heart Rate is lowest than ever even during summer but pace & effort is the same
I’m in a strange situation where even despite the summer weather, my heart rate is 3–5 bpm lower at the same pace (easy, intervals) than when I started the Norwegian Singles method two months ago. However, the effort still feels about the same, if not worse with my legs feeling more fatigued & tired.
For example, I recently did a 5K time trial in high heat and humidity and ran 30 seconds faster than I did in my initial 5K at the start of the plan. Both 5K trials were untapered, with the same seven-hour weekly load and a long run 2–3 days before. Even so, during the most recent 5k w/ the big PR, my heart rate was just at lactate threshold, and it still felt pretty rough.
For context, I’ve been running for about three years and decided to try Norwegian Singles after plateauing for the past year. I’ve been doing a vanilla Norwegian Singles ( 3x10, 5x6, and 10x3) with about seven hours of running per week.
I also don't think I am overtraining. My HRV is still in a good range and if anything has gone up significantly. My resting heart rate has also fallen by 1-2 BPM. My HR monitor also doesn't seem to be incorrect. Does anyone know what could be going on? Am I just getting fit? Could it be the summer heat making everything feel worse even despite my cardiovascular fitness getting stronger?