Strengthen your core without the mat ...

Doing the classic sit-up involves having a partner hold your feet or anchoring your feet under a brace as you lift your torso toward your thighs. This variation may be common, but it isn't necessarily ...

Beth Skwarecki is Lifehacker’s Senior Health Editor, and holds certifications as a personal trainer and weightlifting coach. She has been writing about health for over 10 years. Sprint interval ...