I'm not really sure, but I'm guessing the low impact and the consistent work to the lower body throughout all the workouts. That, and my modifying things I know would strain my knees. I'd venture to say it was also b/c I was doing less Jillian and less step workouts in general but even in the past when I had cut down on that, my quads never felt this strong and my jogs were never this pain free.
I also have noticed more definition in my shoulders and upper arms and I havent don much other non-LIS workouts for upper body in the past 6 weeks. I have done STS Upper Body premix once and then 2 weeks of STS Upper Body workouts, doing an STS upper body workout twice each of those weeks and that was about 4 weeks ago since those. My upper body continues to improve. I'm attributing that to the compound moves, the pushups and the sweat-fest these workouts still continue to give me.
This morning I did the AT Extreme premix followed by 4 sets of 35 situps and I'm feeling great!! Tired and sweaty, but accomplished