The sit-up puts 3,300 newtons on your lower back every rep

A 60-second floor swap called the dead bug builds core stability without the compression.

In 2020, the US Army removed sit-ups from its required fitness test after linking them to more than half of soldiers’ fitness test injuries. The replacement was a plank. If your back aches and you still do crunches, the dead bug is an alternative you can start today.

McGill’s biomechanics lab at the University of Waterloo measured approximately 3,300 newtons of compressive force on the lumbar spine during a standard sit-up. That matches the NIOSH action limit for repetitive low back compression, the threshold linked with higher injury rates in workers. Each rep meets it. Callaghan and McGill (2001) applied repeated flexion to 26 pig spine specimens under modest loads. Disc herniations appeared consistently within 22,000 to 28,000 cycles, and with heavier loads the number dropped to 5,000 to 9,500. The study used an animal model, not living humans, but sit-ups involve the same repeated flexion under load.

The dead bug keeps the spine neutral while the limbs do the work. It trains anti-extension: the ability to keep the lower back from arching while arms and legs move. Lab studies estimate lumbar compression during dead bugs at levels well below a sit-up. Hodges and Richardson (Spine, 1996) found that in a healthy back, the transversus abdominis fires first no matter which direction the body moves. In people with low back pain, that contraction was significantly delayed. The dead bug may help retrain that firing pattern.

That 44% comes from a 2019 CDC survey of nearly 32,000 adults. The swap costs nothing, needs only a floor, and fits between commercial breaks.

Where do your knees spend the most time bent?

Walking loads the kneecap briefly with each step, then releases it. Sitting holds the load in one position for hours, which is why the stiffness shows up in the chair and not on the sidewalk. Start with the periodic extension breaks in the Fix above. If stiffness persists past a few weeks or gets worse, an orthopedic or sports-medicine clinician can evaluate the cause.

Got an ache question? I read every one.

Keep moving,

The Ache Report

P.S. Set a 30-minute timer at your desk and do 3 dead bug reps per side each time it goes off. Over a 4-hour stretch that adds up to 8 sets and 24 reps per side, no separate workout needed.

Was today useful? One click tells me everything.