I started commuting to work on nice days last week. (40 mile round trip) On the ride into work (about 1 hour), I pushed myself fairly hard, keeping my heart rate at about 90% max. 12 hours later, time to head home. I could feel the lactic acid sitting in my legs, but figured I'd tough it out and head home staying around 80% max. I was heading into the wind on the way home and felt like I was working harder, but my HR would not go above about 75%, no matter what I would do.

Can someone explain what was going on with my physiology?

Matt