(Difficulty: Advanced)
Description:
Start in a normal plank position. Keeping your back flat. Carefully extend your left arm out in front of you bracing your core and holding yourself with your right hand firmly planted on the ground. Hold for a couple of seconds then switch arms.
Modification: You can also do it on your elbows and instead slide your arms out in front of you bracing your body with both arms still firmly planted on the ground.