I've confirmed that you don't even need to be doing something in the workout that requires you to stand on one leg.
Just load up any workout (I did this on the 10 Minute Solutions Cardio Kickboxing workout, which is under the "Rapid Results Fat Burner" group), and stood on one leg after the workout started. You can snap the achievements if you want, to see how much more you need... it will update after you replant your other foot.
And to confirm what Tom RT said, it is indeed cumulative. I started off at 23% and it took about 40 seconds for the achievement to pop.