Carbohydrates are efficient sources of energy. They are broken down into sugar units by the digestive system and quickly absorbed into the blood. From there, they are transported to the cells and used to create a form of molecular energy known as ATP. However, the body is constantly burning a combination of carbohydrates and fats, and sometimes even proteins. The exact proportion depends upon the precise nature of the physical activity that you embark upon.
Sources of Energy
Carbohydrates are efficient because of the way in which they are used by the body. The muscles actually liberate about five calories of energy from carbohydrates per liter of oxygen, versus only 4.86 calories from a combination of carbohydrates and fats. The sugar glucose can also be utilized under anaerobic conditions. The muscles, along with the liver, are the only organs in the body that have the capability to store carbohydrates, which is done in the form of a molecule called glycogen. It is devoted exclusively to fueling the body's activities.
Muscle Fiber Types
Glycogen is generally activated during short bursts of high-intensity activities such as sprinting and weight lifting, both of which utilize a higher proportion of muscle strands known as fast-twitch fibers. Type II B fibers, as they are also called, contain relatively few mitochondria, the powerhouse of the cell, and rely more on quickly burning glycogen. The mitochondria can process carbohydrates, proteins and fats for energy, but only carbohydrates constitute glycogen.
Weight lifting may burn a higher proportion of carbohydrates, but it still burns fewer calories overall. For a 160-lb. individual, lifting with free weights will only burn about 219 calories in an hour. Even vigorous weight lifting, which requires heavy weights and few reps, will only burn about 420 calories for that same individual. Running at 8 mph, on the other hand, burns 986 calories in a 160-lb. individual. Tae kwon do and rope jumping both burn 730 calories.
Weight lifting can be an intense activity at the peak of exertion, but there are far too many breaks in between exercises to keep your heart rate consistently elevated. Therefore, weight lifting is quite good at burning carbohydrates in the moment, but aerobic activity can burn more over longer durations. If you want to exhaust glycogen stores, for example, then endurance exercises usually work better. You can even deplete full stores of glycogen after only about 90 minutes of aerobic exercise.