So what is it now, you medical experts? We need to learn how to sit? Oh, puhleeze. We’ve been doing it all our lives. For many, sitting for eight hours straight is pivotal to the job, not to mention that post-work leisure time plopped on the La-Z-Boy watching TV or playing video games. Oh, occasionally we’ll get up to go sit in our cars in order to sit in restaurants and eat.

You’d think, therefore, we’d have this sitting thing down by now, that we’d be no slouches when it comes to taking a load off.

Right?

Not so.

Turns out, we literally are slouches. Doctors, chiropractors and ergonomics experts, who make a nice living off our backs, say poor posture while sitting is something of an epidemic.

Eighty percent of Americans will cringe with back pain at some point in their lives, and back injuries prove the top reason for missed work, according to the National Institutes of Health.

This is something San Francisco chiropractor Gregg Carb, for one, just won’t stand for.

Carb has written a self- published book, “The Science of Sitting Made Easy” (Posture Press, $14.95), to address the problem. Boiled to its essence, Carb’s message is the same as Mom hectored you with for years: Sit up straight, will ya?

Our spines are strong and resilient, Carb says, but not impervious to the deleterious effects of slouching, craned necks, twisted trunks.

“When you hold any body position for long periods of time, your spine is gradually reshaped into that very position through an adaptation of the connective soft tissues,” Carb says.

“Everyone has their own style of sitting, so to speak. But no one’s immune to gravity and, therefore, you will experience (back pain) as a result.”

This is not some breakthrough discovery, back experts concede. But, just as a dentist reminds people to floss, a spinal specialist will preach posture and body alignment, especially when we’re on our duffs.

“It’s a huge issue,” says Dr. George D. Picetti III, spine surgeon at the Sutter Neuroscience Medical Group in Sacramento, Calif. “Sitting is very hard on the spine, mostly in the lumbar (lower back) region. The longer you sit, the more you compress the discs. Throughout the day, the water content of each disc declines. You’re very vulnerable to lifting or something like that.”

Years of improper sitting can lead to disc degeneration, which is permanent. But, Carb says, mobility and comfort can be restored through rigid adherence to, well, sitting rigidly.

“You will get some actual form changes over time if you improve your posture,” Carb says. “It’s almost like having braces (on your teeth). But we’ve found we can loosen people up in a matter of weeks.”

What not to do while sitting: Roll your shoulders inward, jut your head forward, round out your lower back, sink your chest.

What to do to avoid those bad habits: Set your seat back to a nearly upright position and sit as far back into the seatback as you can, keeping your rib cage and trunk upright and your head aligned directly above your shoulders.