I was asked a few weeks ago by a client whether she burned any calories while lifting weights. I assured her that she was providing a potent stimulus for her body to burn body fat (at a rate higher than at rest). But, the exact caloric expenditure is difficult to quantify because strength training's (and other types of high-intensity exercise) impact on the regulation of fat and carbohydrate metabolism over the next 24-48 hours. High-intensity strength training (moderate to heavy resistance) stimulates what is known as the metabolic sink .
When a muscle is stimulated to contract against a heavy resistance, additional, rarely-used muscle fibers are activated to provide more tension in the muscle. These rarely-used (in our daily activities) muscle fibers are very metabolically inefficient and selectively utilize glycogen (stored chains of glucose) stored within the muscle fibers. Not surprising, these fibers don't have a great capacity to resist fatigue and don't endure for very long.
After a high-intensity strength training workout, the enzymes that resynthesize glycogen are extremely active. Additionally, the glucose receptors on these muscle (fiber) cell walls are also very active, thus shuttling in glucose from the blood to be stored as muscle glycogen. The glycogen-depleted muscle fibers act as a sink for glucose to drain out of the blood.
The significance of the metabloic sink is two-fold. First, blood glucose is not available for storage as body fat (even if a meal of high carbohydrate content is consumed for many hours after a workout) as it is being shunted into the recently-worked muscle fibers (especially those rarely-used, inefficient fibers). Second, there is a shift to a greater use of fatty acids for energy in those muscle cells (it takes energy to resynthesis glycogen from glucose or amino acids) and other body tissues.
The high-intensity strength training not only burns extra calories while you are performing the demanding workout, but it also creates a fairly long-lasting shift in energy utilization. This is a big reason why strength training (with proper nutitrition) can cause a significant change in body composition even if it is only perfomed a few times per week and with no other formal exercise. In fact, I have had many female clients decrease their body fat levels by 5-8 percent (that is a 135 lb female can lose 12 pounds of fat and add 6 pounds of muscle in 12 months). Diabetics would also benefit from the metabolic sink created by high-intensity strength training. Unfortunately, it is rarely a recommended option for diabetics to help manage of their disease.