Body weight is the most significant factor in determining how many calories you burn for any activity. Heavier people burn more calories because they have more weight to move around. Your body composition tells the relative proportion of lean tissue and fat in your body. When comparing two women of the same weight, the one with the higher percentage of muscle burns more calories during physical activity.
Body Composition and Cardiovascular Exercise
Cardiovascular exercises such as running, swimming and biking burn a lot of calories. When walking at 3.5 miles per hour, a 160-pound woman can expect to burn approximately 5 calories per minute, while a woman weighing 200 pounds burns closer to 6.5 calories per minute. Heavier people burn more calories because they have more weight to move. When comparing two women who weigh the same doing the same activity, the woman with the most lean muscle has the greater caloric burn. In basic terms, muscle works and fat doesn't, at least metabolically. Muscle needs energy to do its job and creating that energy takes calories. Fat is a good source of energy in the body, but it doesn't work like muscles so additional calories aren't needed to sustain it. When doing cardiovascular exercise, your body engages its largest muscles and uses them repeatedly. Having and using more muscle will burn more calories in your workout.
Muscle Mass and Resistance Training
Just as in cardiovascular training, a woman with more muscle burns more calories during strength training than her peer with less muscle. Resistance training has an added benefit as it causes your working muscles to become more metabolically active over time. With regular strength training, a pound of muscle will burn approximately 1.5 additional calories per day. That may not sound like much but, when multiplied by every pound of muscle in the body, it adds up. In fact, regular strength training may increase your resting metabolic rate by seven percent. To illustrate, an untrained 140-pound woman burns approximately 1400 calories per day at rest, while the same woman burns nearly 1500 after regular strength training.
Other Factors to Consider
Changes in body composition are sometimes reflected on the scale. Adding muscle or losing fat causes your weight to fluctuate. Weight is the predominate factor determining the number of calories burned, but body composition plays an important tole. As you add muscle, your resting metabolic rate and your body weight both increase and you burn more calories for any given activity. Over time, your body fat begins to decrease and you lose weight, which also means that your caloric needs also decline.
Body composition heavily impacts the amount of calories you burn during exercise. While numbers on the scale are easy to measure, knowing your body composition helps understand whether you're improving your overall fat to muscle ratio. Measuring your body composition isn't an exact science, but skinfold measurements and water submersion testing provide a general sense. If you want a better sense of how your body is changing as you exercise, take a body composition test when you start working out and have another one completed three to six months later. You may find that the numbers on the scale haven't moved as much, but your body composition may be significantly more healthy.