Losing muscle mass isn't nearly as common of a goal for most men as building muscle, but if you're turned off by bulging biceps and tremendous triceps, you might have decided now is the time to lose some mass. Endurance athletes such as long-distance runners or cyclists may also benefit from having less arm mass, as this increased muscle weight can slow you down and isn't functional to your sport.
Reduce your calorie intake. Your diet is a main player in whether you lose or gain muscle mass. The greater your calorie deficit, the more muscle you'll lose, writes nutritionist Lyle McDonald in "The Rapid Fat Loss Handbook."
Lower your carbohydrate intake. Carbs are key in retaining muscle mass, according to nutritionist and bodybuilder Dr. Joe Klemczewski, so limit your intake of starchy foods such as bread, pasta and cereals, along with high-sugar fruits.
Stop training your arms. Your muscles require stimulation to grow and this stimulation comes from weight training. Progressively overloading your arms with more weight, sets and reps causes muscle breakdown. This tissue then repairs and regrows bigger than before. If you stop training your arms, the muscles won't get any stimulation and will cease growing. Avoid all arm training, either direct arm isolation exercises such as curls and press-downs, or compound upper-body moves like bench presses and pullups.
Increase the intensity and duration of your cardio sessions. High-frequency, high-intensity cardio programs don't allow for adequate recovery and can put your body into a catabolic state, where it breaks down muscle tissue, according to the American Council on Exercise. Look to add more running, sports, cardio machine work, fitness classes or general physical activity to your current routine. This can lead to muscle breakdown all over if you don't include strength training for other muscles though, so make sure you keep working your lower body and core in your resistance-training sessions.