If you feed yourself properly, including taking a multivitamin/mineral daily, you would only add muscle by performing high intensity cardio just like many sprint-type athletes. Additionally, when it comes to accomplishing a fat loss goal, the “fat burning heart rate range” is of no value. It is only of value to endurance athletes so they can conserve glycogen. Who cares what percentage of the energy you expend during an activity comes from fat? It’s the total energy you expend for the entire day compared to how much energy (calories) you consume that produces the body fat change. In fact, the higher intensity cardio you perform, the more absolute fat you will burn during and after the workout. So bust out of your “target heart rate” anytime you want because the harder you work the more fat you will lose—just don’t put it back on by eating too much.