If you are looking for a real challenge, then try doing the RKC Plank. Invented by a former Soviet special forces trainer, ...
Planks are a simple, equipment-free exercise that strengthen your core, improve posture, and protect your back ...
YOU PROBABLY THINK you know how to do a plank. The ab training staple is relatively simple, after all—boiled down to the basics, all you need to do is get down on the ground and hold in place.