It's getting hot! Even if you've logged some serious mileage on the track or treadmill this spring, you may not feel ready to let loose in summer shorts and bikinis. Maybe you're one of those naturally skinny girls, an ectomorph who just can't add pounds no matter how many milkshakes you pound. Or maybe you're a cardio junkie, and you've lost the curves you crave. Whatever the reason, if you put the "rail" in "frail," this article is for you.

If you feel less than sexy and you know it, then step off the treadmill and pick up some weights! Cardio burns calories, but it doesn't build the definition in your arms and legs that screams, "I'm strong and hot!" It's time to add some assets to your slender frame and carve a cat-suit-worthy physique. (Halle Berry, anyone?)

What's the great news? Just as any woman can successfully lose body fat, so can she build more muscle—and with it, those enviable curves. Here's how it's done.

1. Eat Up

I know what you're thinking: "Wait? What? You're telling me to eat more?" I know this might be a total mind-blower for you ladies who are constantly told to watch what you eat, but you need to eat more to build muscle. The foundation for all your hard work in the gym is the food you eat!

Many women are misinformed about their eating habits; they think they have to eat much differently than men do. Really, ladies looking to build muscle can approach their eating habits similarly. The only real difference is the amount of calories. Because men tend to have more muscle and less fat, they can eat more.

Building muscle requires energy, and energy requires fuel, so fuel up! Without a caloric surplus, the body will simply maintain its current size. Women who want to add lean muscle and shape to their figure need to eat more calories than they burn during the course of the day. This equates to around 200-250 calories over maintenance. For most women, about 15 calories per pound of body weight should be enough to maintain.

Now, "eat more" doesn't give you free reign to raid the cookie jar, so don't overdo it. Instead, add more quality calories to your actual meals. Focus on a balanced diet comprising wholesome foods from a variety of food groups: complex carbohydrates, lean proteins, and healthy fats. Set a caloric intake range and try to stay within that range to maximize calories available to ensure muscle gains while limiting fat gain.

Nutrition Tip

Getting enough protein is especially important for muscle gain. Try to eat one gram of protein per pound of bodyweight. It might seem like a lot, but it will help you build those shapely curves.

2. Heavy As She Goes

Many women are afraid that merely touching a dumbbell will turn them into bulky, masculine, mass monsters. Have no fear! Lifting heavy weight will only enhance your results, not turn you into the She-Hulk.

If you want to build muscle, you need to set down those five-pounders you've been using for biceps curls and go heavier. Your muscles will only grow when they're stressed. Heavy resistance training breaks down muscle tissue; through a process of repairing and rebuilding, the muscle comes back stronger and larger than before. You will develop strong curves when you're brave and challenge yourself in the gym.

If you're still worried about getting huge, remember this: Women produce much less testosterone than men. Normal testosterone levels for men are 200-1,200 nanograms per deciliter (ng/dl), compared to women's 15-70 ng/dl. Huge difference! Testosterone is a crucial muscle-building hormone, so lower levels mean you won't see quick mass gains. Your muscles will grow slowly, and you'll have much more control over how much muscle you'd like to see.

Muscle Building Tip

If you're not reaching fatigue by the tenth rep of an exercise, it's time to increase the weight and lift heavier. Heavier weights lead to growth.

3. More Is Better

Many articles in popular women's fitness magazines show exercises that work just a single group of muscles. Isolation work can be necessary, especially for bodybuilders, but the exercises aren't always the best use of time. If you want results, focus on compound movements, which involve more than one muscle group. Think squats, deadlifts, presses, pull-ups, and rows. These exercises allow you to lift heavier weight and train more muscles per movement. They're easy to progress and a lot of fun, which will encourage you to work harder and more frequently.

Performing these exercises will challenge your body, build muscle, and add curves in a hurry. You could spend your time isolating particular muscles with curls and leg extensions, but you could wind up with some serious imbalances and slow results. Who wants to rock a Beyonce booty with Paris Hilton biceps? Nobody! Break up with your favorite isolation machine, start a lifelong relationship with a barbell, and save some time.

4. Crank up the Intensity

Stop walking, stop jogging, and start sprinting! Cardio is important for health, but excessive aerobic training may actually hinder the growth of your lovely lady lumps. Prolonged aerobic exercise is catabolic, depleting energy that is crucial to building muscle.

High-intensity exercise burns calories during and after exercise, and it increases the secretion of hormones that stimulate fat breakdown. Lengthy sessions of low-intensity cardio, on the other hand, only burn calories during exercise and can eventually use carbohydrates or even protein as a fuel source. While you might be comfortable watching Friends reruns on the elliptical for hours, your body can actually deplete your carbohydrate supply and turn to the protein stored in your muscle for energy.

Substitute those monotonous workouts on the treadmill with high-intensity interval training (HIIT) sessions. HIIT alternates between high- and low-intensity exercises. The combination maintains the intensity of a workout over a longer period of time than high-intensity exercise alone. Twenty-minute bouts of interval sprints will strengthen your entire lower body, reveal your incredible physique, and help you develop awesome glutes.