Why some of us have sculpted, toned and visible muscles and some of us carry a little more flab depends on a lot of factors.

Part of it is how and how much you work out, explains Todd Schroeder, Associate Professor of Clinical Physical Therapy at University of Southern California and Director of the USC Clinical Exercise Research Center. Resistance training, for instance, is what causes muscles to grow versus weaken and waste away. You need to do it to build muscle mass, which is what makes muscles look defined.

But it’s also things like what you eat, how you rest, and where you store your fat, Schroeder adds.

No matter how strong your muscles are, how much fat you store under the skin covering your muscles (the visible subcutaneous fat) affects what they look like from the outside, Schroeder explains. Things like what you eat and the calories you burn in any given day affect how much of this type of fat you have. “Everyone has six-pack abs, you just can’t see them if you have too much abdominal fat covering those muscles,” Schroeder says.

And a big part of the equation is your genetics — the body you’re born with. Some of us have body types that inherently cause us to store more fat, burn more calories or build muscle quicker, Schroeder says. “If two people perform the exact same workouts and eat the same diet, they will likely have varying results [in terms of what they look like],” he explains.

All of that’s to say, pounding out countless reps at the gym alone is likely not going to get you the results you’re looking for if your goal is more-defined muscles. What will get you there is being smart about your goals, doing the right types of workouts, eating right, and getting the rest your body needs.

Here’s what to do:

1. Set a goal that’s realistic for your body type

“Toning up” means different things to different people, says Chris Gagliardi, a personal trainer certified by the American Council on Exercise and based in El Cajon, California. Getting really specific about what your goals are and what changes you want to see can help determine the steps you need to take to get there. (Turn to personal trainers, fitness instructors, coaches, and your doctors to help figure out plans to meet those goals.)

And be realistic about goals. Consider where you’re starting from and how much time you can actually commit, Gagliardi adds. If you currently struggle to find time to work out, starting a training regimen with twice-daily workouts is probably unrealistic for you. Maybe aim to fit in a 30-minute workout five times a week to start. If you’re currently overweight and not exercising, start by setting a realistic weight-loss goal (losing no more than one to two pounds per week) — and when your overall fitness improves, set a more specific goal like becoming more toned or lifting a certain amount of weight. Setting intermediary process goals along the way can help, Gagliardi says.

2. Do both cardio and strength training

Resistance training helps grow muscles and define their shape. But you’re also going to have to do some cardiovascular exercises (the ones that get your heart rate up) to burn calories to get rid of those extra layers of fat between the muscles and the skin. The right mix depends on your current fitness level and body type, but the general idea here is that to see a change (like more definition in your muscles), you’re going to need to add to your workout routine. Be realistic, Gagliardi says. “Start from wherever you are and start building.”

If you’re not working out at all, work toward meeting the general exercise guidelines from the U.S. Department of Health and Human Services of 150 minutes of moderate aerobic exercise per week and strength training at least two days per week, he says. If resistance training is new for you, start with a program that works all of the major muscle groups two to three days per week, he says. And then start upping the intensity of your workouts so they continue to challenge you (more on that below). And if you’re already doing a lot of resistance training, consider adding some cardio to increase calories burned overall and boost cardiorespiratory fitness.

3. Make sure you’re doing exercises correctly and completely

It may seem pretty straightforward, but you have to do exercises correctly to get the most out of them, Gagliardi says.

Most strengthening exercises consist of three phases: lengthening the muscle during which elastic energy is stored for the third phase (such as in a bicep curl when you lower the weight), the amortization phase (a fancy term for the pause after you lengthen the muscle and before you do the third phase), and shortening the muscle when that stored energy is used (in a bicep curl, it’s the part where you lift the weight back up again). Focus on completing each phase, Gagliardi says.

Other types of resistance exercises, like holding the plank pose or holding a squat do not include these three phases because they are isometric exercises — ones during which a muscle is contracted the whole time. Focus on form during these exercises.

4. Make sure your workouts challenge you — and continue to challenge you

You don’t want to be so sore that you can’t walk after every workout you do, but your workouts should feel like work. And a little soreness the day after a workout means the muscle is growing, Schroeder notes. You want to work out hard enough during resistance training that muscles are fatigued by the time you finish. How to fatigue your muscles depends on the type of resistance training you’re doing.

If your goal is to strength train to achieve maximum strength, such as in body building, you’ll want to focus on more weight and fewer repetitions (“reps”), according to the American Council on Exercise — adding more weight to up the intensity of the workout. If you’re strength training for endurance — the kind of resistance training long-distance runners do, for example, to protect their knees, ankles, and other joints — you want to focus on lighter weight and more reps.

If you’re training for hypertrophy (to grow the size of the muscles), up the intensity by adding reps then weight. Start with six reps and add reps until you get to 12, Gagliardi says. Once that set becomes less challenging, add weight and drop the number of reps back down to 6. The idea is that you stay in that target repetition range, but you’re adjusting the number and weight accordingly so that after each set you feel like you couldn’t do one more.

5. Be consistent and patient

Don’t get discouraged, Schroeder says. It takes our bodies time to change. On a cellular level, you’d be able to see (with a microscope) changes in protein synthesis in the muscles in as little as six hours after a workout, Schroeder says. But it can take weeks or months to see changes that are visible in the mirror in your bedroom or locker room, he adds. “So don’t be discouraged if you don’t feel like anything is happening the first few weeks you start a new program.”

6. Don’t skip rest days

Rest days are just as important to training as the days you’re working out; they’re when your muscles actually grow. During your workout you’re causing tiny muscle tears that get repaired by the body after the fact — on your days off — and it’s during that muscle repair process that the muscles actually get stronger. But a rest day doesn’t mean you need to lie on your couch all day.

When resistance training for the major muscle groups (like your leg muscles, your core muscles, and your arm and upper body muscles) rest 48 to 72 hours before training the same muscle group again, Gagliardi says (and as recommended by the American College of Sports Medicine). That means you could strength train on as many as four or six days a week, as long as you’re alternating muscle groups, he says. You could also do cardio on opposite days that you strength train. Or you could fit in your cardio before or after a strength training session.

It’s okay to be active most days or even every day, Gagliardi says. “But the intensity and type of activity being done should vary.” (On days “off” from a set workout, try foam rolling, stretching, or light aerobic activity, like taking an easy walk or a hike, to help with muscle recovery.)

And don’t overtrain, he adds. Some common signs include: increased resting heart rate, impaired physical performance, reduced enthusiasm for training, increased injuries and illness, altered appetite, disturbed sleep and irritability.

7. Healthy up your diet

Remember the saying “abs are made in the kitchen”? There’s truth behind it because what you eat is one of the biggest determinants of how much fat you carry (along with how many calories you burn and your body type) all over your body. What specific combination of fat, carbohydrates and protein is ideal for your body, depends on your genetics, the workout program you’re following, and some other factors, Schroeder says. “But the bottom line is that you need to reduce the subcutaneous fat covering the muscle to have muscle definition.”

Clayton tells his clients to focus on improving nutrition overall, starting with making sure you’re getting plenty of protein (essential to building muscles) and fiber — and cutting out added sugars (soda, candy and granola bars). To help grow muscles and cut excess weight, a lot of people have success with higher protein, higher fiber diets, he adds. But his advice is to make small changes gradually to get to a point where you’re eating healthier, rather than making drastic changes at once. “One problem people make too often is over-exercising and trying to cut calories, leaving people starving,” he says.

8. Think lifestyle change

One of the biggest mistakes people make when it comes to trying to build muscle is focusing too narrowly on one part of the body (like doing a whole lot of arm exercises to try to lose fat there). “It doesn’t work,” Gagliardi says. You need to make changes throughout the entire body.

And remember the patience part, Schroeder says. It can take months to see the changes you might want to gain from a program. (So pick a diet and training plan you can maintain, he says.)

TRY THESE FITNESS ROUTINES

Want more tips like these? NBC News BETTER is obsessed with finding easier, healthier and smarter ways to live. Sign up for our newsletter and follow us on Facebook, Twitter and Instagram.