You’re Arching Your Back
One of the most common plank mistakes is arching the lower back, says April Whitney, CSCS. As your core starts to fatigue, it’s common to let your midsection sag toward the ground, causing your back to fall out of alignment.
Although this error doesn’t always cause immediate injury, this mistake causes stress on the lower back, which, over time, leads to lower back pain or sensitivity, Whitney says.
Letting the lower back arch also takes the core-strengthening benefits out of the exercise. That’s because your abdominals are no longer working to hold your body up, ultimately stalling your progress.
Tuck your pelvis under to keep your back straight, Whitney says. Think about drawing the belly button into the spine and tilting the hips up toward your chest to prevent the lower back from drooping.