Quick! Let’s free asso­ciate. Com­plete this sentence:

_ SETS OF _ REPS.

Did you answer 3 and 10? Of course you did. It’s the Pavlov­ian response. After all, any­one who’s ever picked up a dumb­bell knows that doing 3 sets of 10 reps of each exer­cise is the quick­est way to build muscle.

Except it’s not. In fact, it’s the quick­est way to get nowhere with your work­out rou­tine, says Michael Mejia, C.S.C.S., a long-time Men’s Health fit­ness advisor.

Truth is, today’s most sacred exer­cise guide­lines orig­i­nated in the ’40s and ’50s, a time when cas­tra­tion was a cutting-edge treat­ment for prostate can­cer, and endurance exer­cise was thought to be harm­ful to women. Worse, so-called fit­ness experts across the coun­try are still spew­ing these same old con­ven­tional wis­doms, despite plenty of research indi­cat­ing that they (the experts and the wis­doms) aren’t wise at all.

Chances are, these are the rules you exer­cise by right now. And that means your work­out is long past due for a 21st-century over­haul. We asked Mejia to do just that. Here are the five mus­cles myths he most com­monly hears. Hope­fully, we’re about to bust them for good.

BONUS TIP: Get back in shape—and stay lean for life! Check out our list of the 100 Best Fit­ness Tips Ever!

MYTH #1: DO 8 TO 12 REPETITIONS
The claim: It’s the opti­mal rep­e­ti­tion range for build­ing muscle.

The ori­gin: In 1954, Ian Mac­Queen, M.D., an Eng­lish sur­geon and com­pet­i­tive body­builder, pub­lished a sci­en­tific paper in which he rec­om­mended a mod­er­ately high num­ber of rep­e­ti­tions for mus­cle growth.

The truth: This approach places mus­cles under a medium amount of ten­sion for a medium amount of time—it’s basi­cally The Nei­ther Here Nor There Workout.

Here’s the deal: Higher tension—a.k.a. heav­ier weights—induces the type of mus­cle growth in which the mus­cle fibers grow larger, lead­ing to the best gains in strength; longer ten­sion time, on the other hand, boosts mus­cle size by increas­ing the energy-producing struc­tures around the fibers, improv­ing mus­cu­lar endurance. The clas­sic pre­scrip­tion of 8 to 12 rep­e­ti­tions strikes a bal­ance between the two. But by using that scheme all the time, you miss out on the greater ten­sion lev­els that come with heav­ier weights and fewer rep­e­ti­tions, and the longer ten­sion time achieved with lighter weights and higher repetitions.

The new stan­dard: Vary your rep­e­ti­tion range—adjusting the weights accordingly—so that you stim­u­late every type of mus­cle growth. Try this method for a month, per­form­ing three full-body ses­sions a week: Do five rep­e­ti­tions per set in your first work­out, 10 reps per set in your sec­ond work­out, and 15 per set in your third workout.

MYTH #2: DO 3 SETS OF EACH EXERCISE
The claim: This pro­vides the ideal work­load for achiev­ing the fastest mus­cle gains.

The ori­gin: In 1948, a physi­cian named Thomas Delorme reported in the Archives of Phys­i­cal Med­i­cine that per­form­ing three sets of 10 rep­e­ti­tions was as effec­tive at improv­ing leg strength as 10 sets of 10 repetitions.

The truth: There’s noth­ing wrong with—or mag­i­cal about—doing three sets. But the num­ber of sets you per­form shouldn’t be deter­mined by a 50-year-old default rec­om­men­da­tion. Here’s a rule of thumb: The more rep­e­ti­tions of an exer­cise you do, the fewer sets you should per­form, and vice versa. This keeps the total num­ber of reps you do of an exer­cise nearly equal, no mat­ter how many rep­e­ti­tions make up each set.

The new stan­dard: If you’re doing eight or more reps, keep it to three sets or less. If you’re pound­ing out less than three reps, you should be doing at least six sets.

BONUS TIP: When it comes to mak­ing lifestyle changes that will improve your health, your first step is the most impor­tant one. Start here: 20 Lit­tle Changes for a Health­ier Life.

MYTH #3: DO 3 OR 4 EXERCISES PER MUSCLE GROUP
The claim: This ensures that you work all the fibers of the tar­get muscle.

The ori­gin: Arnold Schwarzeneg­ger, circa 1966.

The truth: You’ll waste a lot of time. Here’s why: Schwarzenegger’s four-decade-old rec­om­men­da­tion is almost always com­bined with “Do three sets of 8 to 12 rep­e­ti­tions.” That means you’ll com­plete up to 144 rep­e­ti­tions for each mus­cle group. Trou­ble is, if you can per­form even close to 100 rep­e­ti­tions for any mus­cle group, you’re not work­ing hard enough.

Think of it this way: The harder you train, the less time you’ll be able to sus­tain that level of effort. For exam­ple, many men can run for an hour if they jog slowly, but you’d be hard-pressed to find any­one who could do high-intensity sprints—without a major decrease in performance—for that period of time. And once per­for­mance starts to decline, you’ve achieved all the muscle-building ben­e­fits you can for that mus­cle group.

The new stan­dard: Instead of focus­ing on the num­ber of dif­fer­ent exer­cises you do, shoot for a total num­ber of rep­e­ti­tions between 25 and 50. That could mean five sets of five rep­e­ti­tions of one exer­cise (25 rep­e­ti­tions) or one set of 15 rep­e­ti­tions of two or three exer­cises (30 to 45 repetitions).

MYTH #4: NEVER LET YOUR KNEES GO PAST YOUR TOES
The claim: Allow­ing your knees to move too far for­ward dur­ing exer­cises such as the squat and lunge places dan­ger­ous shear­ing forces on your knee ligaments.

The ori­gin: A 1978 study at Duke Uni­ver­sity found that keep­ing the lower leg as ver­ti­cal as pos­si­ble dur­ing the squat reduced shear­ing forces on the knee.

The truth: Lean­ing your torso too far for­ward, so that your knees stay back, is more likely to cause injury. In 2003, Uni­ver­sity of Mem­phis researchers con­firmed that knee stress was 28 per­cent higher when the knees were allowed to move past the toes dur­ing the squat. But the researchers also found a coun­ter­ef­fect: Hip stress increased nearly 1,000 per­cent when for­ward move­ment of the knee was restricted. The rea­son: The squat­ters had to lean their tor­sos far­ther for­ward. And that’s a prob­lem, because forces that act on the hip are trans­ferred to the lower back, a more fre­quent site of injury than the knees.

The new stan­dard: Watch a tod­dler squat. Push your hips back as far as you can, while keep­ing your torso as upright as pos­si­ble. This will reduce the stress on your back and knees.

MYTH #5: WHEN YOU LIFT WEIGHTS, DRAW IN YOUR ABS
The claim: You’ll increase the sup­port to your spine, reduc­ing the risk of back injuries.

The ori­gin: In 1999, researchers in Aus­tralia found that some men with back pain had a slight delay in acti­vat­ing their trans­verse abdo­minis, a deep abdom­i­nal mus­cle that’s part of the mus­cu­la­ture that main­tains spine sta­bil­ity. As a result, many fit­ness pro­fes­sion­als began instruct­ing their clients to try to pull their belly but­tons to their spines—which engages the trans­verse abdominis—as they per­formed exercises.

The truth: “The research was accu­rate, but the inter­pre­ta­tion by many researchers and ther­a­pists wasn’t,” says Stu­art McGill, Ph.D., author of Ulti­mate Back Fit­ness and Per­for­mance and widely rec­og­nized as the world’s top researcher on the spine. That’s because mus­cles work in teams to sta­bi­lize your spine, and the most valu­able play­ers change depend­ing on the exer­cise, says McGill. Read: The trans­verse abdo­minis isn’t always the quarterback.

In fact, for any given exer­cise, your body auto­mat­i­cally acti­vates the mus­cles that are most needed for spine sup­port. So focus­ing only on your trans­verse abdo­minis can over­recruit the wrong mus­cles and under­recruit the right ones. This not only increases injury risk, but reduces the amount of weight you can lift.

The new stan­dard: If you want to give your back a sup­port­ing hand, sim­ply “brace” your abs as if you were about to be punched in the gut, but don’t draw them in. “This acti­vates all three lay­ers of the abdom­i­nal wall,” says McGill, “improv­ing both sta­bil­ity and performance.”

YOU, HEALTHY FOR LIFE! Don’t miss Dr. Oz’s 25 Great­est Health Tips Ever!