The 21-day challenge: Pick 5 days per week to do an ab workout. Take two days off each week, for a total of 15 ab workouts in 21 days. You can do these routines after a workout or as a stand-alone workout.

Day 1

Day 2

Day 3

Day 4

Day 5

Day 6, 9, & 13

Day 7

Day 8

Day 10

Day 11

Day 12

Day 14

Day 15