20 sec intervals? I did some 15 second stomps for a block a few months back and they were one of the least suffering intervals I've done, but actually quite rewarding. And I was in a huge gear, for me at least, going from 50rpms to 100-110rpms. Sure the muscles hurt a bit, but nothing like some of the other intervals that make the body feel like it's going to quit. Were you winding out the gear, or just increasing the effort and maintaining comfortable cadence? I could see it being painful if you were doing anaerobic workout and doing those 20s back to back without much of a rest.