I did my first Master-Cleanse in the beginning of October- 10 days, no exercise- and I felt fantastic! Lots of energy, no detox side effects, everything was great.
I decided to cleanse again after Xmas. This time, I decided to add exercise to my cleanse. I stuck with my normal routine (jogging a few miles, several times a week) as to not add additional stress to my body. This time around, I felt HORRIBLE. Every time I jogged, I felt like I could barely get enough air in my lungs, and I was feeling tightness in my chest (never had the symptoms before). It made me really nervous. I was also exhausted all the time. I actually stopped the cleanse on Day 8 altogether b/c I was pretty scared.
I have no idea if exercise actually made this difference, but it was the only behavioral difference b/w the 2 cleanses. I am cleansing again now w/o exercise.... in my opinion, it's not worth the risk/strain to your heart.