Looking up at a mirror or clock during your plank may help you check your form or stay on track, but it can also wreak havoc on your form, Whitney says. The key to a plank lies in keeping your entire body in a straight line from head to hips to heels.
Glancing at a clock or mirror on the wall will throw your body out of alignment. And holding your head in this position for even a few planks a week can cause soreness in your neck and tightness in the upper back muscles, Whitney says.
Keep your neck long and eyes on your hands (in forearm plank) or straight down in front of you (in high plank). This will help prevent the neck from craning and trap muscles from bunching up.