Having a strong core is really good for you.
So we asked a fitness expert to put together a 21-day core-strengthening challenge that takes only about 15 minutes a day.
You can do them first thing in the morning, add them to your current workout, or even squeeze them into your lunch break.
The important thing, Matheny says, is to complete each day's reps with proper form and control. You're not trying to blow through them as quickly as possible; you're trying to maintain stability and form with every single rep. That means you can take rest as you need it, particularly toward the end of the challenge (when you're doing a lot more at once).