It's common knowledge in fitness circles, but a lot of people don't know that the classic sit-up exercise — that staple of gym classes and boot-camp style training — is actually terrible for your back ...

Add Yahoo as a preferred source to see more of our stories on Google. It's common knowledge in fitness circles, but a lot of people don't know that the classic sit-up exercise — that staple of gym ...

The plank is one of the most effective exercises for building a stable core. But stick with it for too long and it can start to lose its edge, offering less of a challenge as your strength improves.