I bought an ergonomic chair. Then a standing desk. Then a lumbar support cushion. My back still hurt by 4 p.m. every day. The problem was not the furniture — it was how I sat in it. I had been chasing expensive solutions instead of fixing the basics.

The Three Rules of Sitting

Feet flat on the floor. Not crossed. Not tucked under you. Not dangling. If your chair is too high and your feet do not reach, get a footrest. A stack of books works.

Knees at or slightly below hip level. When your knees are higher than your hips, your pelvis tilts backward and your lower back rounds. That rounded position is what causes the dull ache that builds through the day.

Screen at eye level. The top of your monitor should be at or slightly below eye level. If you are looking down at a laptop, you are tilting your head forward. Your neck muscles have to hold up a ten-pound head in that position — they get tired, and the pain radiates down into your shoulders and upper back.

The One Thing Nobody Tells You

The best sitting position is your next position. No posture is comfortable forever because your body is not designed to be still. Stand up every thirty minutes. Walk to the kitchen for water. Do a quick stretch. The best ergonomic setup in the world will still hurt if you sit in it motionless for eight hours.

One Stretch That Helps Immediately

Stand in a doorway, put your forearms on the doorframe at shoulder height, and lean forward gently. Hold for 30 seconds. This opens your chest and shoulders — the muscles that tighten up from hunching toward a screen. Do this twice a day and your upper back will thank you.