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Timing is Everything: Why the Duration and Order of Your Exercise Matters

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Reviewed for medical accuracy by:

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Sahand Rahnama,

Elizabeth Rust, Ph.D.


UM Medical School






Life doesn’t always seem fair. There are some people who lift weights for 30 minutes and then run for another 30 minutes only twice a week and still somehow manage to burn more fat than someone who runs half an hour a day, five days a week. While there are many variables to be considered like type of diet and intensity of exercise, in exercise, like many other endeavors, timing can be everything.

Learning the scientific basis of exercise metabolism can help the athlete become fit the smart way instead of the hard way. The order and duration of your exercises are important variables in influencing macromolecular metabolism. Simply put, working out for half an hour four times per week is not the same as working out for one hour twice a week, even though they add up to two hours total.

Without a doubt, aerobic exercise with the goal of decreasing body fat should be done for longer than half an hour. It must be done for longer than half an hour because there is a sequence of macromolecules that must be metabolized first before fat is predominately m etabolized. The first of these energy supplies depleted in exercise is creatine phosphate and glycogen1. These provide quick energy for short term, high-intensity exercise, or fuel for the very beginning of moderate intensity exercise. The type of exercise and how long it is performed will determine the primary macromolecule that is metabolized.

High intensity exercise is by definition anaerobic. The bulk of ATP for anaerobic exercise comes from glycolysis. The glycolytic process requires that muscle cells breakdown glycogen to glucose via the glycogen phosphorylase pathway2. However, in high intensity anaerobic exercise, the body initially uses up all of the glycogen in the skeletal muscle and the liver through the glycolysis pathway, creating buildup of lactic acid. There are two mechanisms of muscle fatigue that limit action in high intensity training: fatigue due to depletion of phosphocreatine (see article on creatine basics) and fatigue due to lactic acid induced muscle acidification3.

Lactic acid fatigue only occurs during high intensity training. The acid will lower the pH of the muscle from a resting value of approximately 7.1 to as low as 6.4. At a pH of 6.9 the function of the glycolytic enzyme phosphofructokinase is inhibited, slowing glycolytic ATP production. At pH of 6.4, all glycogen breakdown is inhibited. Moreover, H+ from lactic acid interferes with calcium binding to Troponin C and the subsequent shift of tropomyosin, preventing the actin-myosin crossbridges and thus decreasing contractile force3. More on fatigue will follow in a later article.

Meanwhile, to prevent muscle shut down, the lactate, acid, and pyruvate are transported out of the cells passively. But, the transfer and conversion is not fast enough to prevent the lactic acid buildup. Lactic acid shuts down the muscle in approximately 30 seconds of maximum intensity exercise, while the time scale of reestablishing pH takes place over approximately 15 minutes when not doing high intensity training. Consequently the duration of anaerobic exercise is short. Because the fast lactic acid buildup prevents the body from exercising longer, the body cannot exercise past its allotment of glycogen and then proceed to fatty acid metabolism. It is during a long duration of moderate intensity exercise, where lactic acid is not building up, that athletes get fat-burning benefits.

Exercising for longer than 30 minutes shifts the primary macromolecules that are metabolized from glucose to fatty acids. Shifting from glucose and glycogen supplies allows the body to efficiently mobilize and utilize free fatty acids (FFAs) derived from lipids in adipose tissue, which resides mainly under the skin. The body preferentially metabolizes FFAs in order for the glucose being produced in gluconeogenesis to be used by the brain. The brain only can metabolize glucose mainly and ketone bodies to a lesser extent; the brain cannot metabolize fatty acids.

Physiologically, lipolysis to utilize fatty acids is controlled by the pancreatic hormones insulin and glucagon and also by catecholamine hormonal regulation. Mechanistically, insulin will activate cAMP phosphodiesterase and reduce levels of cAMP in the body, deactivating lipolysis. This pathway is responsible for the storage of fat in obtaining energy. Glucagon mainly opposes insulin’s function to increase glycogen breakdown and increase gluconeogenesis 2.

After the first 30 minutes of exercise, the body runs out of its glycogen storage and then turns mainly to what is left of the glucose in the blood and then finally to fat and amino acids derived from muscle protein. Supporting evidence of fatty acid release comes from physiologic research where human gluteal fat cells isolated after 30 minutes of biking showed that cathecholamine induced lipolysis had increased between 35-50% 4. If exercise does not last until 30 minutes then fat burning is never achieved because all of the glycogen is not used u p. So while one may be able to prevent adding fat to the body, one is not metabolizing fat from the adipose tissues during the exercise. In short, exercises aerobically for less than 30 minutes, one is just maintaining the adipose tissue status quo and decreasing muscle mass.

Most importantly when concerning exercise duration, after 40 minutes of moderate intensity exercise the body burns primarily fatty acids for its energy. This is the time period of exercise that is most beneficial to health and a healthy appearance. Burning the fatty acids reduces the fat on the body, providing the rationale behind having a fat burning workout that lasts longer than 30 minutes. In addition, since the majority of the body fuel is FFAs, the muscle protein is used at a lesser rate.

Weightlifting (or any type of high intensity training) fits into a smarter schedule of exercise when it is the first component of the workout. As mentioned before, high intensity training is anaerobic and uses creatine phosphate and glucose as its fuel sources. Creatine phosphate is always the first source of energy in any type of exercise that is used up, as it replenishes ATP after the conversion to ADP (see article on Creatine Basics). Additionally, glycolysis extracts energy quickly from glucose that is derived from blood glucose or glucose extracted from glycogen phosphorylation. When the body uses all of the glycogen-derived glucose anaerobically, it must then rely on liver breakdown of proteins and lipolysis for the body’s energy. The transition to moderate-intensity exercise also allows the skeletal muscles to transfer the lactic acid and pyruvate to the liver so that the pH can return to normal so that the skeletal muscles can return to function.

If the body does not have creatine phosphate or muscle glycogen to burn quickly because aerobic exercise was performed first, then glycolysis for weightlifting is sub-optimized. If one does aerobic exercise first, then one will use their creatine and glycogen reserves without burning much fat. Then when one turns to anaerobic exercise afterwards he/she will be without his/her reserves of energy needed for glycolysis. When one does aerobics first and then high intensity training the workout is sub-optimized in all aspects.

Thus, MedFitness recommends that high intensity training is completed prior to aerobic exercise for individuals who are trying to maximize fat burning. This ensures that the energy needed for glycolysis is used in weightlifting. Then after 30 minutes of weightlifting, the glycogen and blood glucose have been used up and the body uses primarily fatty acids for its fuel. Switching to moderate intensity exercise lets the body consume FFA for metabolic energy and also gives the body time to remove the lactic acid so that aerobic exercise can be performed. Thus lifting weights first for 30 minutes and then doing aerobic exercises is the best way to maximize energy used in high intensity training while selectively burning fat efficiently


1. Widmaier, E., Raff, H., Strang, K. Vander, Sherman, & Luciano’s Human Physiology 9th ed., McGraw-Hill, 2004.

2. Nelson, D. L., Cox, M. Lehninger Principles of Biochemistry 3rd edition. Worth Publishing, 2000.

3. Wilmore, J., Costil, D.L., Physiology of Sport and Exercise, Human Kinetics Publishing, 2004.

4. Hargreaves, M. ed. Exercise Metabolism, Human Kinetics Publishing, 1995.



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