I just finished a week of BBC. Arms and Abs and Butt and Legs cardios are the hardest. I'm not a fan of the Chest and Abs cardio (I am not a fan of getting the heart rate up high and then laying down for ab work right away) so I ran instead. I tried to do the Shoulders and Back cardio because I do like it but my calves weren't going to be able to withstand all of the jump rope this time so I opted for a sub for that one too. I love that series though. It really is tough!