Picture yourself running for your life from a bear. You should’ve played dead, but here you are. You are running as fast as you can while at the same time looking for a getaway—or at least someplace to hide because you know your body can’t continue at this intensity much longer. As you approach a large factory, you duck behind a truck in the parking lot to catch your breath and let your heart rate slow. You can hear the bear’s breath as he inches closer to your hiding spot. Finally, your discovery becomes inevitable, so you jump up and then jolt like Usain Bolt. You climb up a nearby tree, and as you look down, you’re faced with the reminder that bears can also climb trees.

Once you climb to the right height, you jump onto the roof of the factory and leisurely stroll towards the fire escape, thinking you’ve outsmarted that bear. As you turn around to climb down, you hear the bear land firmly beside you. You run eight floors down the escape and collapse at the bottom. With the little bit of energy you have left, you maneuver to your knees and begin to crawl for the factory door. At this speed the bear easily catches up, just in time to politely return the bag you left behind after eating lunch and thank you for the exhilarating chase.

Tone back the intensity in this scenario by about fifteen percent, lose the bear, run the course a second time, and you have a session of High-Intensity Interval Training (HIIT). A HIIT workout can be any activity whereby you alternate between phases of high-intensity anaerobic exercise and low-intensity, or static, aerobic exercise.

To fully understand the exertion levels for phases one and two, let us examine the target heart rate during both. During the intense anaerobic phase, which we’ll call phase one, your goal is to reach eighty-five to ninety-five percent of your maximum heart rate (MHR). An estimated age-related MHR is calculated by subtracting your age from 220, or 200 for those classified as obese, and the resulting number is expressed in beats per minute. For example, a healthy 45-year-old would have a MHR of 175 beats per minute (220 – 45 = 175). As your heart rate rises and your body enters phase one, your energy requirements begin to change. It is these changes that facilitate the onset of some of the hidden benefits of HIIT.

Under normal circumstances, the oxygen we breathe is transported throughout our bodies and is sufficient to provide energy, as well as remove the by-products of our body’s energy production system. However, during anaerobic exercise, you’re pushing your body so hard that it cannot produce energy or remove the by-products fast enough. Lactate begins to build in the blood, causing the muscles to fatigue and eventually give out. At this level of intensity, the body can only push so hard for so long.

By the end of phase one, your body should be nearing this point. Then you shift to phase two, which can be a complete resting phase if your phase one was so intense that only a full rest can bring your heart rate back down to about fifty to sixty-five percent of your MHR. More commonly, phase two consists of an active rest activity such as walking, allowing your heart rate to slowly descend back into your aerobic range.

You might wonder who would want to put themselves through something this grueling. Because of its extreme intensity, HIIT is not for the faint of heart. Athletes at any level from Pee Wee Football to Major League Baseball are using HIIT to gain an edge over their opponents. Although they are the main demographic, athletes are not the only group benefiting from HIIT workouts. There is research being done with cardiovascular rehabilitation patients in controlled settings, and the results show promise. In a study by Ito Shigenori published in the World Journal of Cardiology, it is said that “aerobic capacity, which is expressed as peak oxygen consumption (VO2peak), is well-known to be an independent predictor of all-cause mortality and cardiovascular prognosis.”

VO2peak, or VO2max, is the maximum rate of oxygen consumption measured during exercise, and it has been used in the medical industry for years as an indicator of one’s cardiorespiratory fitness. The higher one’s VO2max, the more efficient their cardiorespiratory system is functioning. In other words, during strenuous activity, an individual with a higher VO2max can consume more oxygen than someone with a lower VO2max. Thus, they can produce more energy, allowing them to push harder for longer. This extra boost in VO2max might give someone with cardiovascular disease who can’t walk up a flight of stairs without becoming fatigued the extra push they need to make it to their bedroom. At the same time, the upsurge in VO2max can give a collegiate wide receiver the additional power he needs to beat the defending safety during an all-out sprint to the end zone.

Going hand-in-hand with VO2max is the lactic threshold. A high lactic threshold translates to a high level of muscular endurance or a delayed onset of muscle fatigue, which is crucial during those moments where you are being chased by a bear. As you exercise and expend energy through the use of your muscles, lactate starts to accumulate in the blood. Lactate is created during the body’s energy production process and plays many roles in the body’s adaptation to the stresses of exercise. An individual has reached their lactic threshold at the point where lactate is produced faster than it is removed, and it won’t be long before fatigue sets in.

Lactate threshold training, typically realized at eighty-five to ninety-five percent of MHR, trains the body to remove lactate more efficiently. This helps to delay the onset of the lactic threshold and allows a trained individual to outlast their opponent in intense endurance activities. Training at this threshold means that you repeatedly use and replenish the body’s stored muscle glycogen and in doing so, train your body to increase its muscle glycogen storage capacity. After a HIIT session, muscle glycogen stores are depleted. A combination of rest and diet will help replenish these stores and burn calories throughout the process.

Another very marketable benefit of HIIT is the phenomenon known as the afterburn effect. Besides sounding really cool, afterburn refers to the excess energy your body uses to replenish its depleted fuel stores, remove waste and other by-products, and repair your muscles after your workout. In a study published by Amy M. Knab, et al. in Medicine and Science in Sports and Exercise, it was shown that “[a] 45-minute vigorous exercise bout increases metabolic rate for 14 hours” post-exercise. That means that as your day goes on after a HIIT training session, your metabolism continues working in overdrive, providing a continued higher than normal calorie burn.

If these benefits appeal to you, you might be ready to start your first HIIT session. Because of the high level of intensity, it is a good idea to consult with a fitness professional who can create a program tailored to your needs. A qualified personal trainer or a good group exercise class instructor can monitor your intensity and heart rate during sessions and track your progress throughout the program.

A basic HIIT program uses a work-to-rest ratio of 1:2. For example, you sprint for thirty seconds and follow it with a sixty-second walk. Start your session with a three- to five-minute warm-up to increase your focus, get the blood flowing to your muscles, and loosen your joints and connective tissue. Afterwards, move directly into phase one, and run through the 1:2 cycle ten times. After cycle number ten, spend about three minutes cooling down with some light movement. Then take five to seven minutes to stretch the muscles you just finished working. Training sessions can be as short as fifteen minutes or as long as thirty. In thirty minutes or less, you can complete a versatile workout that combines aerobic, anaerobic, and resistance training. HIIT workouts should be limited to about two sessions per week and never on back-to-back days, if avoidable.

Exercise selection for HIIT is limited only by your imagination. Here are a few examples of exercises you can utilize with minimal equipment or none at all:

Without Equipment

  • Sprinting
  • Front kicks
  • Mountain climbers
  • Bicycle crunches
  • Squats
  • Push-ups
  • High-knee running
  • Squat jumps
  • Jumping jacks
  • Shadowboxing


With Minimal Equipment

  • Chest press
  • Overhead press
  • Bicep curls
  • Shoulder press
  • Squat press
  • Jumping rope
  • Treadmill sprints
  • Cycling
  • Box jumps
  • Battle rope

Exercise intensity can be the difference between success and frustration, so keep your intensity high when the moment calls for it. Intensity can be measured in real time using your heart and breathing rates. According to the Mayo Clinic’s Healthy Lifestyle Fitness column, vigorous exercise intensity causes one’s breathing to become fast and deep, and you shouldn’t be able to speak in full sentences during this time without pausing for breath. You should be at this level of intensity during phase one to realize the wide range of benefits of HIIT.

When you start to feel like your sessions are not hitting hard enough, there are countless ways to increase the intensity. Change the work-to-rest ratio. Instead of a 1:2 ratio, try 1:1, or if you are feeling brave, go all out with 2:1. Change the exercises, add some resistance with weights or elastic bands, increase the intensity of phase two, or increase the number of repetitions performed. Just keep the total HIIT time below thirty minutes to avoid overtraining.

Exercise should be personalized to each individual. Base your exercise choice on the behaviors, movements, and activities you enjoy so that you look forward to your session and succeed in meeting your goals. Stay focused on your training intensity, and nothing can stop you from achieving your goals. As always, consult with your physician before starting a HIIT or any other exercise program. Once you are cleared for exercise, explore the different options and decide if HIIT is right for you.

Ali Othman has been an NSCA certified personal trainer for the past thirteen years with specialization in weight loss, functional training, muscular hypertrophy, and human nutrition. He also works in the Technical Department at IFANCA and manages IFANCA business activities in South Korea.