Talk to an average gym rat about "core" training, and he'll name his favorite ab exercises. Talk to Bill Hartman, P.T., C.S.C.S., and the conversation shifts to the strength and endurance of all the muscles that act on the spine, pelvis, and hip joints. That's important, because no matter what kind of lift you're performing, your core has to support it. And if your "support system" is weak, you're limiting the amount of weight you can lift in every exercise. That means you're robbing yourself of at least 20%-30% more muscle growth and risking injury.
Test 1: Get into the back-extension apparatus (but don't use the 45-degree back extension—it's too easy). Cross your arms over your chest and raise your torso so it's parallel to the floor—hold as long as possible. If your torso drops below horizontal before two minutes, you fail.
Test 2: Grab a broomstick and get into the plank position (shown above), with your weight on your forearms and toes. Have a friend place the stick on the middle of your back so it touches your head, upper back, and tailbone. Hold until the stick loses contact with one of those three points. Again, if you can't last two minutes, you fail.
The Prescription: If you fail both tests, you need more core work. Simply perform the tests as exercises, three to four times a week—doing 1-2 sets for 50% of your max time for each—before you do any other lifts. Hartman also advises that you not use more than 20% of your body weight on any lift until you achieve a passing score. That may seem drastic, but your core will get up to speed quickly, and you'll immediately be able to lift heavier.