Today was my first day back to Insanity. I started with the Plyo Cardio Circuit, and did fairly well until the plank knee in's/side-to-sides! I need to build my arm/shoulder strength before I will be able to hold plank that long with all of the jumping! Push-ups are always a challenge for me too. Does anyone have supplemental workouts or advice to help with these areas of opportunity?? Thanks in advance!