It’s workouts like today that really spark my curiosity about exercise physiology.

I’ve been having a great string of workouts, and I’ve been feeling pretty sharp.

I will admit that I was feeling tired when I got up this morning, but I got a good night’s sleep.  But I was looking forward to a nice long, easy cruise.

The plan called for a 2×45’/4′ rest.  I decided to substitute a half marathon at the same power just to change it up a bit.

What happened in the workout was very strange.  My HR started low, and just kept going up.  I didn’t feel like I was working extra hard, but my HR just never plateaued.

Just look at the difference between this and the 2×45′ session I did on Sunday.

• Same power
• Same Stroke Rate
• Wildly different HR

So, what’s going on?  A few theories

• Accumulated fatigue?
• Warmer today than Sunday
• Fasted vs not fasted
• Morning vs Afternoon

I’d love to understand this stuff better.