Take on this intense ab challenge to strengthen and tone your core before the new year! Complete 200 mountain climbers, 100 ...

Training my abs is always tough, so I figured why not push it even further? I swapped out the usual sit-ups and Russian twists for a four-move dumbbell workout to really spice things up. I've been ...

Add Yahoo as a preferred source to see more of our stories on Google. When you buy through links on our articles, Future and its syndication partners may earn a commission. Credit: Getty Images While ...

This ridiculously hard L-sit exercise works your entire body without moving a muscle. These are the progressions you can use to get strong enough to do it. Everyone likes a good challenge. That’s why ...

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 ...