Hi, reba! I think the reason for this is the Firm's unique aerobic weight-training concept. On Tough Tape, you are doing quicker reps, with little rest, so your heart rate stays elevated throughout the workout. Cathe's MIS is serious heavy weight lifting. She knows this, and puts lots of rests, and stretches throughout the video. She even comments that you don't want your heart rate high during this workout, she wants you to concentrate on building strength. MIS is a classic, one I have been neglecting lately, because of my Power Hour addiction!:-rollen