For amateur purposes, both chest straps and armbands are very accurate and generally sufficient. They capture two main metrics: heart rate and HRV (Heart Rate Variability). By design a chest strap works like an ECG, while an armband measures blood flow (PPG). This means armbands can have a slight lag, for example, during sudden acceleration, but honestly, this can be ignored unless you are doing short 10-second sprints. You can definitely use either to check your morning 'readiness' or track your running performance - both will provide very similar data. Heart rate fluctuates constantly, but you are likely referring to heart rate zones. These zones are simply a convenient, measurable way to reflect what is happening in your muscles. When you experience cardiac drift while maintaining a steady pace, it is because your muscles are fatiguing, your core temperature is rising, and lactate is building up forcing your heart to work harder. Pros verify their zones by taking blood lactate samples during exercise. Think of your heart rate zones as a slightly blurry window into your muscle metabolism - it's a good estimate, but not perfect. Long story short: yes, your fitness levels and zones change throughout the season as you progress. While most watches are good at estimating these, they don't perfectly reflect day-to-day muscle performance. Due to fatigue, temperature, or hydration, you might be training in a different zone than your watch shows. However, these are 'geeky' edge cases. For amateurs, a standard running watch-calibrated every 3-4 weeks-is more than enough. Pros, on the other hand, rely on blood testing or advanced sensors like Q-LAC.

As I said I crunch my own numbers. It keeps me sort of in tune with my body and pushes me to explore further. I try not to overcomplicate things and instead look for the best angles though. For example, I was on the train yesterday, resting and doing some online learning, but my HR and HRV this morning are terrible. My body hasn't recovered, so it definitely doesn't make sense to train today - it wouldn't contribute to my HM140 goal. Was it a lack of hydration, junk food at the station, or just the stress of traveling? Actually as a result of all this, I’ll probably look into how dehydration specifically affects muscle recovery and performance…