Reebok master trainer Sara Haley took this series up a notch to increase its fat-burning and muscle-toning potential. Pair these moves with either your lower body or cardio upgrades (or both!), depending on your Week 4 daily schedule. And if you feel sore the next day, take it easy, not off. "Consistency is key," Haley says.

For this first move, begin in elevated plank position, with legs on a chair or low table. Lower yourself (next image), then push up, kicking right leg toward left shoulder (third image). Repeat, switching legs. Do 8 to 12 reps.

Holding a set of 3- to 5-pound hand weights (or full water bottles), start in a squat with a flat back, arms at sides with thumbs to ceiling. Lift arms straight out to sides, forming a T, squeezing shoulder blades. Lift arms, pressing shoulders away from ears to form a Y. Do 6 to 10 reps.

Elevate upper body on chair or bench with arms straight, elbows soft. Stretch legs out long, balancing on heels. As you lower down, lift one leg up while maintaining form (next image). When you reach the top, lower your leg and lift the opposite leg on way back down. Alternate legs, and do 5 to 7 reps for each side (10 to 14 reps total).

Using a set of 3- or 5-lb weights (or full water bottles), press palms to the back of the room, squeezing shoulder blades together. This week, hold here and pulse for a count of 8. Then, rotating thumbs to ceiling, raise straight arms up on an angle, no higher than your head (next image). Hold here for 8. Do 20 reps.

Elevate upper body on chair or bench with arms straight, elbows soft. Stretch legs out long, balancing on heels. As you lower down, lift one leg up while maintaining form (next image). When you reach the top, lower your leg and lift the opposite leg on way back down. Alternate legs, and do 5 to 7 reps for each side (10 to 14 reps total).

Using a set of 3- or 5-lb weights (or full water bottles), press palms to the back of the room, squeezing shoulder blades together. This week, hold here and pulse for a count of 8. Then, rotating thumbs to ceiling, raise straight arms up on an angle, no higher than your head (next image). Hold here for 8. Do 20 reps.