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Metabolism, in the mind’s eye

In my experience, most high-intensity training (HIT) practitioners don’t focus on metabolism. It’s hard to blame them. Discussion of force, velocity, muscle tension, and momentary muscle failure have real-world applications that clients can knowingly experience. Metabolism is practically invisible and lacks easily accessible realtime metrics that we can share with our clients. However, we would be remise if we didn’t share the truly amazing adaptations that occur within seconds of finishing a HIT session.

The energy systems

Before we jump into the adaptations that occur from HIT, a basic understanding of metabolism is necessary. Most people are familiar with the term cellular respiration. This is the cycle that breaks down nutrients (fat and carbohydrates) into the energy currency of our bodies, adenosine triphosphate (ATP).

“The Immediate System” – The Phosphocreatine System

This energy system delivers ATP quickly to the working muscle and is heavily relied upon during high-intensity muscle contractions. This system can provide energy for all-out effort lasting between 6-10 seconds.

“The Short-Term System” -Glycolysis

The system breaks down glucose and glycogen. Glycogen is a polymer of glucose stored within the muscle (and liver). Glycolysis can work at two speeds: fast and slow. During slow glycolysis oxygen is used in breakdown of glucose. Fast glycolysis does not require oxygen and therefore generates a great deal of lactate. This system is called the short-term system because it can supply the majority of ATP for short (2-3 minute) bout of intense exercise.

“The Long-Term System”- The Aerobic System

The aerobic system (aka oxidative phosphorylation) provides energy for long-term sustained exercise. This system has the capacity to deliver large amounts of ATP to the working muscles for extended periods of time. However, the aerobic system works at a relatively slow rate when compared to glycolysis. Because of this it can become overwhelmed during high-intensity exercise.

It is important to recognize that these systems are working all the time to provide ATP to meet the demands of the body-not one or the other.

Practically, we can measure metabolism by measuring some of the by products of burning fat or carbohydrate; oxygen and carbon dioxide. Fat and carbohydrates are chemically different from one another. Therefore, these nutrients require different amounts of oxygen to breakdown into ATP. This also effects how much CO2 is produced as a by product. What cannot be disputed is that fat requires more O2 in relation to CO2 produced. Measuring metabolic rate can provide fundamental information on a person’s metabolic heath. Just as a blood test can measure levels of glucose or cholesterol a metabolic test can measure rate of calories burned in a day and how much of those calories are derived from fat. Estimation of RMR can be woefully inaccurate especially in obese individuals. Overestimation of RMR by 200 calories could amount to 21lbs (9.5 kg) of fat in just one year (1).


Instant gratification

Technology pushes the boundaries of complexity and efficiency. What once was slow is now fast. Exercise is comparative in that we once understood that meaningful exercise takes place only in the “steady-state” where the magical “fat-burning zone” resides. Here’s the thought process.

fat burning thought process simplification
Gross over simplification


Jumping the gun on the fat-burning paradigm was not all bad. Countless studies have documented the positive healthy benefits (increased VO2max, improved blood lipid profile, and reduced body fat) of steady-state exercise. Although, many of us in the HIT community fail to acknowledge this fact. However, it is possible that the substantial time commitment to exercising in the steady-state may have isolated people intimidated by such a time commitment. Also, the emphasis on burring fat has ignores the importance of glucose metabolism.


Burning sugar

After a bout of HIT (whether resistance or interval) we experience numerous instantaneous benefits. One of the most commonly cited acute benefits of HIT is improved insulin sensitivity and glucose metabolism. As mentioned previously muscle contains a large amount stored glucose called glycogen. HIT uses primarily muscle glycogen to make ATP. Reduced glycogen levels send signals throughout the body to replenish muscle glycogen storage after exercise. Although most forms of exercise improve insulin sensitivity, intense exercise recruits a larger pool of muscle fibers which sensitizes those fibers to bring in more glucose to make more glycogen.

Most of us are familiar with the concept of blood glucose. Biologically, the control of blood glucose levels is highly regulated. If your blood sugar goes too high or too low we often feel sick, irritable, confused, or we can loose consciousness. Insulin is the hormone that works primarily on the two organs that store the most glucose; liver and muscles. When we consume carbohydrates our blood sugar levels start to rise, which causes the release of insulin from our pancreas. The faster blood sugar returns to the fasting level, the greater the insulin sensitivity.

HIT increases insulin sensitivity
As you can see in this graph only 2-weeks of HIT training increases insulin sensitivity. (Adapted from: http://www.ncbi.nlm.nih.gov/pubmed/19175906)


HIT training also increases your resting metabolic rate. Excess post-exercise oxygen consumption (EPOC) is a phenomenon were your body keeps burning energy (calories) after the exercise has ended and can remain increased for over 24 hrs. The size of the EPOC is determined by the degree of the anaerobic systems involvement during exercise and hormonal, thermal, and respiratory adjustment that are cause by intense exercise. Simply put, the more intense the exercise the greater the EPOC effect.

the more intense the exercise the greater the EPOC
The more intense the exercise the greater the EPOC effect.


Factors that increase EPOC

  • Respiratory
  • Circulatory
  • Hormonal
  • Ionic
  • Thermal

HIT that is of sufficient intensity perturbs our physiology in such a manner that is metabolically costly to return us to homeostasis. The EPOC effect can be even greater when there is a component of muscle damage as is the case with high-intensity resistance training. The metabolic cost of degrading and resynthesizing muscle proteins can account for as much as 20% of the resting metabolic rate (2). This adds to the metabolic burden of intense exercise and emphasizes the importance of proper recovery periods from HIT.


Long-term investments

The adaptations of consistent HIT training are substantial in that for the total time committed (~15-30 minutes) nearly all of the health related benefits of long duration (>60 minutes) steady- state exercise are accessible. A logical questions would be “how is something that is primarily anaerobic help the aerobic system and improve health?”. To answer that questions it is important to acknowledge that the aerobic and anaerobic systems do not work in isolation.

When we are exercising at peak intensity aerobic metabolism is still attempting to keep up. However, anaerobic metabolism (fast glycolysis) spins at an infinitely faster rate than the mitochondria’s aerobic machinery. Therefore, the anaerobic system dominates to deliver energy to the working muscle. After exercise, though, the aerobic system is left to clean up the mess. Metabolizing excess lactate and the resynthesis of energy storage is accomplished through increased aerobic metabolism.

Research out of several well respected exercise physiology laboratories has demonstrated that HIIT training increases the aerobic (fat-burning) system enzymes like citrate synthase, COX, and β-HAD. These results translated to whole-body measures of greater aerobic capacity (Increased VO2max) and greater fat burning during resting conditions (3).

Several studies have documented an increase in muscle oxidative enzymes as a result of HIT resistance training. Improvements in whole- body oxygen consumption has been less convincing with many studies showing no improvement.

However, the modality of assessing VO2max is highly specific (bike and treadmill) and without adequate training the lack of increase in VO2max as a result of HIT resistance training can be plausibly explained by the specificity  of training.

HIT stimulates greater EPOC
HIT stimulates a greater EPOC effect because it relies more heavily on anaerobic sources for energy. 


The beneficial glucose metabolism adaptations experience in the acute state appear to have long lasting effects. Long-term HIT significantly improves glucose tolerance, but has been shown to be more effective in the long term management of diabetes than several diabetic pharmaceuticals (4).


Long-term energy balance

Possibly the greatest long-term benefit to doing HIT is the sustained elevation in metabolic rate. Living in a modern society where calories are plentiful a strategy to prevent weight gain is equally as important as the strategy to lose weight. It is speculated that the “energy gap” created from the calories we eat to what we burn could be completed filled in by the increases in RMR demonstrated by minimal amounts of HIT. In other words, HIT could be the strategy to tip the scales in your metabolic favor. Leading to a longer and healthier life with more time on your hands.



  1. http://www.ncbi.nlm.nih.gov/pubmed/15883556
  2. http://www.ncbi.nlm.nih.gov/pubmed/19175906
  3. http://www.ncbi.nlm.nih.gov/pubmed/?term=10.1249%2FMSS.0b013e318193c64e
  4. http://www.ncbi.nlm.nih.gov/pubmed/22289907
  5. http://www.ncbi.nlm.nih.gov/pubmed/23997192


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