Let's start by defining what metabolism means. According to the Dictionary of Sport and Exercise Science (Anshel 1991), metabolism is defined as "chemical changes that utilize energy and result in tissue and compound building (anabolism) or breakdowns of substrates and relase of energy (catabolism)."
There are only three different ways that your workout program can alter your metabolism - the actual workout session itself and the number of calories burned during the workout, post-workout oxygen consumption, and the addition of new muscle mass.
The addition of new muscle mass is the only way to permanently increase your metabolism because muscle is "metabolically active". In there words, muscle burns calories while doing nothing so the more muscle you have, the more calories you burn in a day.
The Workout Itself
Muscle contraction requires energy, i.e. calories. The number of calories depends on the muscle being worked and the level of resistance being used. This means that you will burn more calories performing squats, leg presses, or deadlifts than you will by performing concentration curls.
Intensity and the load itself play a large role in the number of calories burned during a resistance session. The interesting thing is that the amount of calories burned are not proportional to the intensity and the weight used.
For example, one study compared the calories burned using weights that were 80 percent of a one repetition max with weights that were 20 percent of a one repetition max. Each group performed only one rep with the weight load.
Enter the amount of weight you lifted (Lbs/Kg) and the number of reps you completed. Your One Rep Max (1 RM) will appear at the bottom left, and your various percentages of 1 RM will appear on the right side.
While the workload increased by a magnitude of four for the first group, the subjects in the 80 percent group used 12 times as much energy (calories) during their one repetition.
The number of calories you burn during the workout is greatly determined by your exercise selection, intensity and weights used.
Post-Workout Metabolic Effect
Intense weight training elevates your metabolism for up to 39 hours after your actual workout. In other words, because of the intense weight workout, your metabolism has been stimulated to where you are now burning more calories while you are doing nothing.
Carbohydrates power intense weight training sessions. The more intense the session, the more you deplete your carbohydrate stores and the more fat is burned during the recovery phase, i.e. after the workout during rest. As the intensity of the training increases there is a proportionate increase in fat burning after the workout.
One study showed that 15 exercise sessions per month (50 minute sessions at 50 percent of oxygen uptake) could lead to an extra 2 pounds per month of fat loss, strictly from the elevated metabolism and extra calories burned - while doing nothing! That's an extra 26 pounds , of fat burned per year.
Longer Term Metabolic Effect - The Addition of New Muscle
Another extremely important aspect of fat loss that occurs from training with weights is adding leas muscle to your body. Lean muscle is "metabolically active", i.e., muscle burns calories even while doing nothing. So, the more lean muscle you have, the higher your resting metabolism and the more calories you burn each day while doing nothing.
Studies have estimated that for each pound of muscle that you add to your body, you burn another 35 to 50 calories per day. So, an extra 10 pounds of muscle will burn approximately 350 to 500 calories a day, or an extra pound of fat every 7 to 10 days, without making any other changes.