I was very excited to try these shoes since I walk my dogs 4 times a day. They felt only slghtly unstable at first, but I knew that was because they were working stabilizer muscles I don't usually use. My calves were the first place I felt a bit "work-out-sore"... and then my but. I felt I had done some lungs or squats. The outer lower hips/but, which I find difficult to work out, are definitely being worked. The feeling is similiar to some yoga or balance ball moves. Following the directions on how to walk on them is important (but easy). I'm going to get a pair for my husband.