The plank looks like the easiest exercise in the gym, which is exactly why it goes wrong. Most lower back ache from planking comes from holding on long after the abs have quit. A good plank feels like hard work in your midsection and shoulders, and nothing in your lumbar spine.
Line up before you lift
Set your elbows directly under your shoulders, forearms flat on the floor, feet about hip width apart. Before you move, check the line: ears over shoulders, shoulders over hips, hips over ankles. If the floor version is confusing, practice the same stack standing first.
Then tuck your tailbone slightly and squeeze your glutes. That small tuck is what keeps your lower back neutral for the entire hold.
Brace, then lift into position
Take a breath, breathe it out, and tighten your abs as if bracing for a light poke. Now push your forearms into the floor and lift into the full position, keeping the ribs pulled down toward your hips.
Hold that tension without letting go. The moment you feel the work slide from your abs into your lower back, the set is over — that early cut-off is the whole point of the exercise. If you cannot feel your abs at all by the end, the hold was too relaxed: go shorter and harder.
Hold less, hold better
Twenty to thirty seconds of a genuinely hard brace does more than two minutes of sagging. If you cannot yet hold twenty seconds cleanly, do it from your knees — the bracing rules are identical and the line stays honest. Start with holds you can finish in the same straight line you started in, and stop each one a breath before your hips want to drop.
Rest about a minute, shake out and reset completely between sets. Three clean 25-second holds beat one long collapsing minute every time.
Fix the two classic breakdowns
Hips sinking toward the floor mean the abs have switched off and your lumbar spine is taking the load. Drop to your knees, reset the tuck and shorten the next hold. Sagging is the main cause of lower back pain from planks.
Butt up in a pike is the opposite problem: it turns the plank into a rest position, so lower the hips back to the line. Also check your head — look at the floor between your hands — and push the floor away instead of sinking into your shoulder blades.
Progress by making it harder, not longer
Once you can hold three clean 40-second sets, stop adding time. Move to a harder version instead: reach your arms further forward for a long-lever plank, lift one foot, add slow shoulder taps or turn to a side plank.
Log each hold in Bulky with its length and how steady it felt. Watching holds lengthen week by week, or seeing a harder variation become comfortable, is progress you can actually trust.