High-intensity interval training (HIIT) workouts trains and enhances both the aerobic and anaerobic energy systems of the body. HIIT is a form of interval training, a cardiovascular exercise strategy alternating short periods of intense anaerobic exercise with less intense recovery periods, until too exhausted to continue.

Aerobic Aspects of HIIT

Most people have heard the term ‘aerobics’ and probably associate it with cardio-type exercises. During aerobic activity, the mitochondria are consuming oxygen and catabolically converting glycogen and fat to fuel- ATP. When HIIT is performed, the body is pushed to its limits so that mitochondrial biogenesis (creation of new mitochondria) is promoted and there is an overall improvement in their efficiency and ability to burn even more fat and remove greater levels of carbon dioxide and other cellular waste products.


With the increase in oxygen intake during HIIT activities and the resulting boost to the mitochondria, other chemical reactions in the body are enhanced. Human Growth Hormone (HGH) which is responsible for burning fat, building muscles and strengthening bones is produced at greater levels. This is an excellent way to help combat the effects of aging!
Catecholamine levels also rise significantly and this directly stimulates the mobilization of fat cells for the conversion into useable energy.
Aerobic, cardio and endurance exercises such as jogging, running (at a mild pace), swimming and biking tend to ‘fire’ slow twitch (Type I) muscle fibers which happen to contain the most mitochondria. The result is a stronger heart and lungs, more efficient fat burning, improved mood and lowered risk of diabetes.

Anaerobic Aspects of HIIT

Anaerobic activity is performed in the absence of oxygen. What this means is that ‘fast twitch’ (Type II A & B) muscles that depend solely on glycogen that is stored in the muscles for fuel fire in intense, relatively short bursts.

Whereas aerobic activity can occur over a long period of time as in distance running or moderately riding a bike, anaerobic activity such as heavy weight lifting, interval training, jumping rope or any type of sprinting is of much shorter duration but much more intense.

The benefits of anaerobic exercise center around building strength and muscle mass as opposed to significant cardio-vascular improvements but it does also improve your VO2max and endurance. The significance of oxygen consumption during anaerobic exercise is not in the performance of the activity itself but for better cellular response in the recovery period afterwards.


During intense exercise, lactic acid, which is a waste product of anaerobic energy production, can build up in the muscles and become quite uncomfortable. As your training advances and you continue to push your limits, your body will be better able to tolerate this build up and then remove it more efficiently so the onset of fatigue is delayed. In some cases, this ‘buffering’ of lactic acid and its effects can be improved by between 10 and 50%.

Due to the intense nature of anaerobic exercise in general but especially when performed along HIIT guidelines, it should not be performed by people who don’t already have a good degree of fitness. Even with proper preparation, it is important to warm up with aerobic activity first. When incorporating anaerobic elements in a HIIT workout, the critical fact to remember is to rest appropriately between intervals to remove lactic acid and replenish the muscle’s energy through oxygenation. Remember, muscle oxygenation takes longer than blood oxygenation or catching your
breath and is crucial for the proper functioning of cellular activity.

Overtraining Syndrome

Have you ever reached a plateau in your training or wondered why you are losing ground instead of making improvements? The reason this occurs is usually what is called ‘Overtraining Syndrome’. What this means is that a higher volume, intensity or frequency in training, if not handled properly with appropriate lower intensity intervals and rest periods, can result in Central Nervous System (CNS) Fatigue.

Stress, whether it is from an increased workout or a bad day at the office, causes the production of the hormone cortisol which raises blood pressure and blood sugar while lowering the body’s immune and inflammatory responses. Too much cortisol can also lead to a decrease in muscle tissue and an increase in abdominal fat.

The only way to combat the effects of cortisol release is to properly engage the relaxation response. This means adequate rest breaks during HIIT activities and the limitation of these workouts to just several times a week. Maintaining proper hydration and diet are also key factors in reducing the risk of this type of fatigue. (Other stress-causing factors need to be addressed and removed from your life, but that is another issue entirely.)

There are numerous physical and mental signals that point to over-training:

  • Overall lack of energy
  • Soreness in the legs along with other aches and pains
  • Increased resting heart rate
  • Inability to sleep
  • Decrease in appetite
  • Compulsive need to exercise
  • Mental slowness
  • Moodiness
  • Depression

Homeostasis, or the inherent tendency of the body to maintain stability, is an amazing adaptive function. The brain sends these signals of stress and fatigue so that you realize that something is wrong and stop what you are doing before it leads to real injury. For that reason, it is crucial to listen to your body and not focus on a prescribed routine or to keep shooting for the ‘high’ of dopamine flooding your system.