Many people think that cardio workouts will only help you burn fat. However, cardio exercise can help you build muscle as well. Muscles adapt to any type of resistance and will grow to become strong enough and large enough to perform the demands you place on them. While you're not demanding to lift heavy weight with cardio, you are still causing hypertrophy to your muscles. Your body will react by developing the muscles used to be able to handle the workload.
Limit your cardio sessions to three times a week for only 20 minutes a day. If you are looking to add lean muscle, limiting yourself to this amount of cardio per week will ensure you are not burning muscle during your cardio sessions.
Do high-intensity interval training where you perform intense intervals of activity alternating between slow and fast speed to burn fat and build muscle in a short period of time.
Warm up by doing any activity that increases your heart rate slightly such as a brisk walk. Then, begin doing your activity of choice at full speed for as long as you can keep it up, followed by slowing back down to your warm-up pace only long enough to regain your breath and once again attempt the activity at full speed. Do this until you complete 20 minutes of exercise.
Do different exercises to keep your body guessing while using high-intensity training. Do interval training with sprints, jumping jacks, jump rope, or hitting a punching bag.