High Intensity Interval Training: Foundations

High Intensity Interval Training: Background and Foundations

     In the 1930’s the idea of alternating periods of high intensity training with lower intensity training was first codified as “Fartlek” training or “speed play” by Gosta Holmer in Sweden.[1] I remember that this was a big part of physical training in the early 90’s even, when I was suffering through Marine Corps bootcamp and, later, Officer Candidate School (OCS) at Quantico, VA.

Later in the 90’s Meyer et al showed that patients recovering from coronary artery bypass grafting (CABG) showed more improvement from interval training than from sustained exercise at a prescribed intensity.[2]

Today, High Intensity Interval Training (HIIT) has shown marked results, in less time, than its lower intensity, sustained, counterpart. The performance of crossfit athletes (Crossfit being defined as constantly varied, high intensity, functional movement) is a massive empiric case-in-point. In fact, page 20 of the Crossfit level 1 training guide, in reference to the lack of ability of long distance athletes to do anything but long distance athletics, it states: “The key to developing the cardiovascular system without an unacceptable loss of strength, speed, and power is interval training”.

Governing bodies such as the American College of Sports Medicine (ACSM) recommend at least 150 minutes of moderate-intensity exercise per week to prevent chronic disease. The ACSM recommends 75 minutes of vigorous intensity exercise per week as an alternative. Since the majority of Americans do not meet these recommendations,[3] and time is the leading barrier to meeting these exercise recommendations,[4] it makes sense that HIIT should be a focus of our workout programming: more effective in less time. It certainly appears more mentally engaging than just running in a straight line for long periods of time and this was born out in a recent publication.[5]

Exercise adaptation in high intensity interval training is largely dependent on intensity and duration of the work interval, but it is always important to remember that to make the exercise both safe and transferrable to every day life, proper form must be followed (practice does not make perfect in movement, exercise makes permanent). HIIT improves VO2max, energy capacity, resting muscle glycogen content, and time to exhaustion but it also achieves greater total and abdominal fat loss than its less intense counterpart.[6]

If someone is just beginning an interval training program with a below average level of fitness then a moderate intensity of exercise can be used first before proceeding to HIIT. For the purposes of our discussion, moderate intensity is 60-80% of heart rate reserve (HRR) whereas HIIT is normally conducted at 90% of an individual’s HRR. HRR is simply the difference between your normal resting heart rate and your maximum heart rate. Maximum heart rate is 220-age.

For me, at age 44 and a resting HR usually around 60:

Max Heart Rate         176

Heart Rate Res           176-60 = 116

HIIT for me                116×0.90 + 60 = 164

Long intervals are normally greater than 3 minutes

Medium intervals 1-3 minutes

Short intervals 10 seconds to 1 minute

The higher the intensity (and shorter the interval by default) the greater the recruitment of Type II muscle fibers (the larger fibers that most benefit glucose metabolism).

There are four widely known models of HIIT: Tabata, Wingate, Conventional, and Clinical. The most widely utilized is the Tabata interval. It was developed by Dr. Izumi Tabata in his work on Olympic Speed Skaters. It entailed 8 rounds of 20 seconds of high intensity exercise interspersed with 10 seconds of recovery.[7]


Name Interval Rest Repeated… Modality Total Time
Tabata 20 sec all out 10 sec 8 times Any 4 minutes
Wingate 30 sec all out 4 min active 4-6 times Cycle ergometer 18-27 min
Conventional 60 sec 90% HRR 60 sec 10 times Any 20 minutes
Clinical 4 min 90% HRR 3 min active 4 times Cycle or treadmill 25 minutes


Ring dips Crossfit
Intervals with functional movements are the best way to train – get off the machines!

Metabolic Pathways and Interval training in Crossfit

 Crossfit offers a slightly different version of interval training that is both more functional, more enjoyable (due to the competitive atmosphere or due to group agony), and simpler. The focus is no longer on achieving a certain VO2max or HRR but on quantifying work done and then trying to better your last benchmark.

The Tabata system is occasionally used with respect to time interval but the focus is on basic functional movements (not a treadmill where the earth is moving for you, or a cycle ergometer) and specific training of varying metabolic pathways. These pathways being the way that your muscles utilize energy resources to get work done. Other than the single rep max for a lift, which is a function of your neuromuscular efficiency at recruiting all available muscle fibers to complete a sudden high force movement, the energy pathways are:

  1. Phosphagen (or Phosphocreatine): used in the first 10-30 seconds of high force contraction
  2. Glycolytic (utilizes muscle glycogen stores): used in the next 30-120 seconds of high force contraction. A byproduct of this pathway is lactic acid.
  3. Oxidative (utilizes oxygen): Utilized after 120 seconds but does not lend to sustained high force work. This is aerobic work, obviously.

**These first two pathways are termed anaerobic because oxygen is not required for their energy production.

The energy currency of the muscle cell is ATP (adenosine triphosphate) which is broken down to ADP (Adenosine diphospate) which liberates energy when ATP loses a phosphate to become ADP. Creatine donates a Phospate back to ADP to reform ATP. This is why supplementing with Creatine in your diet will allow you to do a little more work in the 10-30 second max muscular output range. Glycogen is stored in muscle cells and is broken down to glucose for energy in the glycolytic pathway. This pathway does not yield much ATP per glucose molecule and an end product of this pathway is lactic acid or lactate. The oxidative pathway can also utilize glucose and is much more efficient in terms of ATP production but generally cannot achieve high force production.

Modified from pg 21 of the CF Level I training guide

Sprint Mid-Distance Distance
Energy System Phosphagen Glycolytic Oxidative
Work duration 10-30 sec 30-120 sec 120-300 sec
Recommended Recovery 30-90 sec 60-240 sec 120-300 sec
Load:Recovery Ratio 1:3 1:2 1:1
Repetitions 25-30 10-20 3-5


High intensity interval training can also be utilized in traditional bodybuilding as well where the aim is to maximize muscular size but not necessarily function. I am not sure why this would ever be your primary aim, but you can note that muscle size is a function of muscle fiber hypertrophy (Type II fibers) and muscle water content (increased by both supplementing with Creatine and increased glycogen content). Both creatine and glycogen are osmotically active (meaning they pull water into cells). By maximizing training at the junction of the Phosphagen and Glycolytic pathways, maximum hypertrophy can be achieved at the expense of developing the oxidative pathway and generally at the expense of functional movement and mobility. Thus, a set of 30-45 seconds (time under tension of 30-45 seconds) and a rest interval 2-3 times that should give the desired effect. Although program structure is much more complicated, this gives you a fundamental background on the energy systems involved.

Thus, for any athletic or fitness endeavor, High Intensity Interval Training is a critical part of your programming if you want maximal/optimal results. Start utilizing some of these ideas and let us know what you come up with!

-Lanny Littlejohn, MD

[1] Sears ES. Running Through the Ages. North Carolina: McFarland and Company, Inc., Publishers; 2001.

[2] Meyer K, Lehmann M, Sunder G, et al. Interval versus continuous exercise training after coronary bypass surgery: a comparison of training-induced acute reactions with respect to the effectiveness of the exercise methods. Clin Cardiol. 1990;13(12):851-861.

[3] U.S. Department of Health and Human Services. Summary of Health Statistics for U.S. Adults: National Health Interview Survey, 2010. Hyattsville (MD): Public Health Service; 2012.

[4] Salmon J, Owen N, Crawford D, et al. Physical activity and sedentary behavior: a population-based study of barriers, enjoyment, and preference. Health Psychol. 2003;22(2): 178-188.

[5] Bartlett JO, Close GL, MaClaren DPM, et al. High-intensity interval running is perceived to be more enjoyable than moderate-intensity continuous exercise: implications for exercise prescription. J Sports Sci. 2011;29:547-553.

[6] Trapp EG, Chisholm DJ, Freund J, et al. The effects of high-intensity intermittent exercise training on fat loss and fasting insulin levels of young women. Int J Obes. 2008;32(4):684-691.

[7] Tabata I, Nishimura K, Kouzaki M, et al. Effects of moderate-intensity endurance and high-intensity intermittent training on anaerobic capacity and VO2max. Med Sci Sports Exerc. 1996;28(10):1327-1330.

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