Health&Fitness Aken: Tighten your core in 21 days

Week 1

Fix your form: “With planks, your form either makes or breaks the exercise,” says New York City celebrity trainer David Kirsch. Your body should form a straight line from head to heels. 

Begin by standing: Stand up straight, feet hip-width apart. Now have someone try to gently knock you off balance. Pay attention to the muscles you need to engage to stay centered:“That’s exactly how you should feel when you’re in plank,” says trainer Jonathan Ross, a senior adviser to the American Council on Exercise. 

Perfect your position: Lying on your belly, plant your forearms (or hands, if you prefer) directly under your shoulders. Come onto your toes and squeeze your glutes. Hold for 5 to 10 seconds. 

Start the timer: Hold your plank for 20 to 30 seconds or longer. If you need to rest, loweryour knees to the floor for a few seconds. 

Week 2

Boost endurance: If you feel any shoulder or lower back pain as you start to hold your static plank for longer, or if your butt creeps toward the ceiling, stop and reset. 

Master 30: Hold your plank for half a minute without resting. 

Add 15: Hold your 30-second plank, then rest in a downward dog position for five seconds, suggests Kirsch. Return to plank and hold for another 15 seconds or more. 

Hit the minute mark: Hold a 45-second plank, followed by a Downward Dog and another 15- to 30-second plank. 

Go for 90: Hold your plank for at least one minute. Rest in Downward Dog if you need to, then hold another 30-second plank. 

Week 3

Switch it up: Get the hang of the following variations individually (do each for a minute). Then tack them onto each other, aiming to eventually finish all three back to back. 

Move the center of mass: From a forearm plank, drop your right hip so your right thigh grazes the floor. Return to the starting position and drop your left hip. Repeat. (If you’re on your hands, your thighs may not reach the floor.) 

Test your balance: Extend your right arm straight out in front of you, parallel to the floor,without disturbing your form. Return to center, then extend your left arm. Repeat with your right and left legs. 

Change levels: Start in a plank on your forearms. Press up onto your right hand, then your left, so you come into a high plank. Return to your right forearm, then your left. Repeat the pattern, alternating the starting arm.