We've said it before and we'll say it again — when it comes to building a rock-solid core, endless sit-ups and planks alone won't do it. Not only does switching things up help you beat boredom, but it ...

Add Yahoo as a preferred source to see more of our stories on Google. You can achieve a lot in just 12 minutes, especially if you spend that time doing this core workout from fitness trainer Kat Boley ...