Directing your body to gain fat in a particular area is impossible, because your body has a set pattern for weight gain that's genetically determined. If your arms and legs seem disproportionately thin, fill them out with healthy muscle tissue, rather than fat. Although increasing muscle mass takes work, you can use exercise to encourage growth in certain areas. Adding healthy calories to your meals and snacks, as well as spending focused time at the gym lifting weights -- with a special emphasis on your biceps, triceps, thighs and calves -- will help you balance your frame.
Create a Calorie Surplus
To gain weight, you must create a calorie surplus, which means you need to eat more calories than you burn. Consuming 250 to 500 calories more than what you need daily to maintain your weight will encourage muscle mass growth. If you tend to gain fat easily in areas other than in your arms and legs, keep your additional calories toward the lower end of that range.
To determine your daily caloric needs for maintenance, consult a dietitian or plug your age, weight, gender and activity level into an online calculator. With added calories and focused work in the weight room, you can expect to add between 1/2 and 1 pound of weight -- mostly in the form of muscle -- per week. Because adding muscle mass is a complex metabolic process, you can't expect muscle to develop at a faster rate than this.
Hit the Weight Room
Simply adding calories will encourage you to fatten up everywhere. To build size in your arms and legs, you must do resistance training. A total body program that addresses all the major muscle groups -- including the arms, legs, chest, back, abs and shoulders -- builds balanced strength and a proportional frame. Include at least one exercise for each of these bodily sections, for at least two workouts per week on nonconsecutive days.
Focus on your arms and legs by performing two to four exercises for each muscle group during each session of the two- to three-weekly workouts. Arm exercises include hammer curls, preacher curls, concentration curls, triceps dips, overhead extensions, pullups, pushups, flyes and triceps kickbacks. For your legs, appropriate moves include squats, lunges, step ups, leg presses, calf raises, dead lifts and leg curls. If you're just starting a program, use your body weight or the minimal weight for just one set of eight to 12 repetitions. As you become stronger, lift heavier weights that fatigue you, in four to eight repetitions for each of the two to three sets.
Choosing Foods for Muscle Gain
Increasing your calorie intake by adding sugar, refined grains and saturated fats is contrary to your goal. These foods, when eaten in excess, usually cause weight to accumulate in your midsection. Instead, opt to increase calories from healthy sources -- mostly from lean proteins. Consume about 0.55 grams of protein for every pound of your body weight daily to encourage muscle gain. Spread out the protein over several meals; focus on eating a serving before and after your weight-training workout.
At meals, you can increase calories and protein intake by eating an extra ounce or two of chicken breast, flank steak, pork tenderloin or tofu. Snack time also provides an opportunity to boost calories with protein. A hard-boiled egg offers 6 grams of protein and 78 calories; a cup of low-fat cottage cheese has 24 grams of protein and 183 calories; and 2 ounces of deli turkey offers 66 calories and 12 grams of protein. Whey protein, Greek yogurt and tuna are other portable sources of protein calories.
Realistic Expectations for Weight Gain in Arms and Legs
Not every person gains muscle at the same rate. Some people are naturally lithe and may find that their limbs remain lanky, despite focused dietary and exercise efforts. You'll have to experiment to find the ideal number of calories that supports your workouts and that doesn't promote fat gain in your torso. Be patient with the process. Results will take several months to become evident.
Get adequate sleep of seven to nine hours per night and optimal hydration to support exercise and a healthy diet for muscle gain.