13. Plank is one of the biggest trends in the fitness world because it supposedly works on your core stability and therefore, your abs. This is also one of the easiest exercises because it doesnt require any movement, but that doesnt mean that it doesnt cause any pain. On the contrary, you will feel the pain, which can strain your muscles. Also, this exercise contains no motion, which means it doesnt burn calories! That being said, planks are pretty worthless too.