This week, you'll use the following upgrades to take Week 3's cardio moves up a notch. You should be pushing yourself enough so that you feel slightly breathless, says fitness expert and Reebok master trainer Sara Haley; try not to rest for too long between exercises so your heart rate stays elevated. Pair these cardio moves with either your upper body or lower body strengtheners (or both!), depending on your Week 4 daily schedule.

For this first move, perform the jump-rope motions as you did last week, but continue for two full minutes. Try to go faster than last week while still maintaining good form; by the end, you should feel slightly breathless and it should be difficult to carry on a conversation.

Instead of regular jumping jacks, this week do cross jacks: Start with knees bent and legs crossed in an X, and arms wrapped around your chest. Jump legs open and arms bent, parallel with the ground (next image). Jump back to starting position with legs in an X and arms wrapped. Continue for 60 seconds

Run in place, lifting knees toward chest, squeezing abs, and pumping arms (alternate angle shown in next image). Keep chest lifted. This week, pick up speed and while maintaining good form, lifting knees as high as possible. Continue for 45 seconds.

Step right foot out to the side in a low squat and quickly bring left foot in to meet it. Repeat two times in one direction, then rotate 180 degrees and continue in the same direction (next seven images). Keep switching every two shuffles. Continue for 30 seconds.

Step right foot out to the side in a low squat and quickly bring left foot in to meet it. Repeat two times in one direction, then rotate 180 degrees and continue in the same direction (next seven images). Keep switching every two shuffles. Continue for 30 seconds.