I mountain bike about 4 times a week - 90 minutes on weekdays and 3 to 4 hours on weekends. The rides are pretty intense - steep uphills followed by steep downhills. My heart rate obviously goes up and down a lot. How does this fit in with the primal view on exercise? On the longer rides on some uphills my heart rate will stay elevated (80 to 90 % or even 100% at times) for 20 minutes or more at a time.