“It kind of feels like pinching right here.”
“How come that one hurts my back?”
Reason #1: Pelvis tilted too far forward (arched back): I would say that 95% of the time this one is the cause of back pain while planking. When people get down into a plank position, most of them align everything properly (shoulders – hips – knees should be a straight line), but the common tendency is to let your stomach sag towards the ground. This creates a significant arch in your lower back and people complain of a “pinching” in their lower spine. In this position you’re relying primarily on your back vertebrae and spinal ligaments for support rather than the core muscles which should be holding you up.
The solution: Concentrate on keeping your back in a “flat” or “neutral” position. Engage your quads, but, and abs to “tilt” or “roll” your pelvis backwards thereby flattening your back out and eliminating the pinch-inflicting arch.