If you're trying to get (remain?) fit as you accumulate birthdays, you need to become acquainted not only with the usual corporeal suspects -- your heart, lungs, muscles and that so-trendy core. You also need to say hello to your tendons, ligaments, synovial fluid and articular cartilage.

So says Nicholas A. DiNubile, an orthopedic surgeon specializing in sports medicine and author of "FrameWork" (Rodale, 2005), a fitness guide focusing on the often-neglected skeleton and connective apparatus that, he argues, is the source of most pain and injury to aging bodies.

While it's not for casual readers, the book is a worthwhile guide for midlifers generally and particularly those with bad backs, bum knees, recurring sprains and that achy thing in your shoulder when you sleep wrong. (And you know who we are.)

DiNubile's program, laid out in seven jargon-laden but highly detailed steps, is founded on a simple premise: To find and address your weak links -- a necessary step to safely advancing your fitness program -- you need to know your body, more intimately than you ever imagined.

Structural weaknesses, such as old sprains that never fully healed and an uneven gait practiced for decades, open all of us to potential injury but are even more threatening to those old enough to remember Indian clubs in PE class.

His cataloguing of possible woes is a little deflating. ("Making matters worse, as we get older, our joints lose their lubrication, our bones lose the minerals that give them stiffness and strength and our soft tissues lose the suppleness that gives us flexibility." Yeesh.)

But DiNubile then explains how these physiological degradations occur and emphasizes through anecdotes and study citations how you (yes, you!) can combat this natural process.

Step One leads readers through a 22-page self-assessment of baseline frame condition, including flexibility, strength, anatomical oddities, balance, body mass index and posture. The goal: Learn your problem areas so you can adjust your workouts to maximize benefit and avoid injury.

Step Two is a treatise on aerobic fitness which, DiNubile asserts, "provides protective benefits for your frame." He cites a landmark study of firefighters that showed that those with better cardiovascular conditioning had lower incidence of back injuries than their peers. "[M]ovement lubricates your joints and enhances elasticity in your tendons and ligaments," he writes. But he adds, "It doesn't do much good to have a heart conditioned to run a marathon if your knees are ready for a joint replacement."

Then the book covers core fitness, with emphasis on working all aspects of your torso, not just the xylophone ridges of the abs. Then there is that expediter of degradation, stress. DiNubile says that it holes up not only in your neck and back but also in your core. The "compensatory pain it causes can radiate almost anywhere" in your body, potentially causing imbalances and "significant problems."

Among the book's most valuable sections is an appendix listing the 20 most common ailments that orthopedists see -- from rotator cuff injuries and tennis elbow to hip bursitis and torn knee cartilage -- and specific exercises to rehab and strengthen those weak links.

Provided you can muscle (or muddle) through the dense physiology, "FrameWork" is an excellent single source for safely starting and building a lifelong fitness regimen. It's certainly a fresh way to look at that increasingly familiar body of yours.

No chat today, but back online next Tuesday. Send e-mail to move@washpost.com.

-- John Briley