The problems: Many people raise their hips to high, or allow them to sag, increasing strain on the lower back.
How to fix it: To feel what it’s like to nail proper form on the plank — stand with your back to a wall and your heels against the baseboard. Your butt, upper back, and the back of your head should touch the wall.
This is the position your legs, trunk, and head should assume when you perform plank variations: a straight line from your head to your heels. Additionally, make sure your weight is on your forearms, your elbows are directly below your shoulders, your core is braced, and your glutes are clenched. Your feet should be hip-width apart.
Having trouble finding that position when you’re horizontal? Grab a long dowel and have a training partner place it on your back. The dowel should touch your glutes, upper back, head, and heels — just like the wall does when you stand.