Last Thursday I was intending to so several hard 15 minute intervals on my cyclo cross bike on grass. We had significant rain so I decided to do the workout on the trainer in the basement. It was my first day on the trainer since April. I decided to do the intervals with 15 seconds of out of the saddle every 3 or 4 minutes of the 15 minute intervals. On the 2nd interval I began to feel calf cramps and finished up the interval. On the 3rd interval I had to quit after about 1/2 way done due to cramps in both my calves. I've been doing the intervals every other week since August and the only thing that changed is the rigidly fixed bike on the trainer. I don't think the cramps were the result of conditioning. The cramps resulted in putting my body in a fixed isloated position that it was not accustomed to be in for sustained periods.