Heart rate ......... there's never ending knowledge and things to learn about our body.

Keeping your heart rate at about 90% max is tough, how long did you hang in there? Lactic acid buildup is near definite when you push yourself so much. Have you been exercising regularly before commuting to work?

I think it's just a temporary overtraining syndrome? What I do know is that when you overtrain, you have a tough time reaching a target heart rate. I think it's just your body not allowing you to take yourself too far, thus you are unable to reach above 75% of max HR. It could also be that you're not fit enough since after 12 hours, the lactic acid is still there. It should have sort of cleared after so long. You should include some stretching anyway, it'll definitely clear up most of the lactic acid. Take it easy anyway Matt, there's no point reaching such a high heart rate. It's only a commute right? I know guys do have the competitive spirit in them, but try to control it, do only such tough commute once in a while.