Back-to-the-fundamentals exercises that cleverly integrate optional intensity modifications. It’s easy to stick with the base move or follow the corner-of-the-screen inset to see the more advanced variations (e.g. instead of the base plié, you’ll do a double-time plié or a plyometric plié jump). It’s an effective way to keep challenging yourself as you get stronger. For balance and variety, each lower-body exercise is followed by a complementary upper-body routine (e.g. squats, then bicep curls; reverse lunges, then tricep rows). Erin O’Brien is a down-to-earth, straight-forward instructor — she always lets you know what you’re doing and why. Simple set, one-on-one cuing. Requires 2 to 5 lb. dumbbells.