Howdy all, I've been doing stationary bike twice a week in the gym and I've noticed something odd. I use a consistent cadence and resistance level every workout, but sometimes even pushing hard, my HR never goes out of zone2, where as normally I spend most of my time in zone3 and zone4. My pre-workout habits, sleep, restfulness, are all the same each workout as I keep to a regular schedule.

My question is does this happen to you and have you found any correlation that might explain why? I'm wondering if dehydration might play a part. I've also had my heart checked out by a cardiologist in the last few years and I'm not worried about any health issues. It's just a strange thing I'm trying to figure out.

The images below are 2 sessions. My METS, avg watts, cadence, were all nearly identical.