Short-sleeve shirts and tank tops are ideal for warm weather and workouts. If you have sagging underarms, however, you may be tempted to hide in hot, long-sleeved shirts. Although you can't spot reduce one area of your body, you can target the triceps -- the major muscle at the back of your arm -- with a series of resistance exercises. Talk to your physician before you attempt a weight-loss and exercise program, particularly if you have a history of poor health.
Lose weight. Exercise may tone the muscles on the back of your arm, but you won't see definition until you reduce your overall body fat. The basic formula for healthy weight loss is to eat less and exercise more. Shave several hundred calories off your daily diet by reducing serving sizes at meals, replacing soda with water and skipping second helpings. Aim for a minimum of 150 minutes of aerobic exercise and two full-body strength-training sessions each week.
Hit the mat. Assume a pushup position with your shoulders in line with your wrists, your body straight and your toes curled under. Slowly lower your body until your elbows are at a 90-degree angle. Straighten your arms to complete the pushup. To place more emphasis on the back of the arms, point your fingers forward and keep your elbows tucked into your body. If you are a beginner, drop down to your knees to modify the exercise.
Pick up a dumbbell. Overhead triceps extensions are an excellent way to tone the back of your arms. Stand with your feet together and your core engaged. Grip a dumbbell with both hands and bring it over your head. Your arms should be fully extended. Slowly lower the weight behind your head by bending your elbows. Push the dumbbell toward the ceiling to return to the start position.
Pick up two weights. Hold one dumbbell in each hand with your thumbs against your hip bones so that your forearms are perpendicular to the ground. Lean forward slightly and engage your core. Press the weights back by extending your forearms. Keep your upper arms against your torso. Bend your elbows to bring the weights back to the start position.
Perform two to three sets of eight to 12 repetitions of each upper-arm exercise. The last few repetitions should be difficult to complete with correct form. Incorporate the sets into a total-body strength-training routine.
Individuals who have lost a lot of weight may need a surgical procedure called brachioplasty to remove excess skin under the arms.