What The Heck Is HIIT, And Is It Bad For Me? Part 2

Jan 29, 2019

If you haven't read part 1 of my HIIT article, please view it here before reading any further. Part 1 explains what HIIT is, its benefits, and current exercise recommendations. Part 2 discusses the mental side of training. Specifically:

(1) Why you’re training – goals and motivation, (2) What your Needs are; including the adaptations that different types of training yield, (3) Your starting point, and (4) Your schedule

Let’s review a model to understand what’s best for you:

(1) Your Why: Goals & Motivation
Why are you doing cardio? Are you training for a particular endurance event? Are you trying to get yourself back in shape? Do you want to look and feel healthier? Are you training for a specific sport? Do you want to live a longer, healthier life? Do you want to be a role model for your kids? For your grand kids?

Your why is your motivation and starting point. You aren’t always going to want to train, but on those days, you need to remind yourself of your why and push through the laziness, fatigue etc.

How To Find Your Why:
What excites you? What stirs some emotion within you? It could be your family, it could be love, maybe your health, your career, money, food? Whatever it is, you need to discover your why, and find the thing that gives you a burning desire. When I think of myself, as soon as I discovered that my wife was pregnant, my already crazy work ethic went in to over drive. My motivation changed from self excellence and mastery, to provision for my family. You need to dig deep and find your why. Once you have it, nothing will stop you!

From my experience, not all of us have an inner burning desire, or external motivation. The true secret to success; especially in the absence of such things, is a love for the process. A love for accomplishing tasks, pushing yourself, learning and growing. If you can find that, you will become unstoppable at anything you put your mind to!

Setting Goals: Once you have figured out your underlying motivation, and / or have accepted to love the process, you’re ready to create goals and put them in writing! Visually seeing your goals on a regular basis keeps us on track. Of course, as you evolve and learn, don’t be afraid to adjust your goals. For more on the mental side, check out part 1 of my free e-book here.

(2) Your Needs:

Once you know why you’re doing cardio, you need to determine how you should train based on your age, training experience / current fitness levels, and the needs of your sport and / or activity. If you’re very serious about your training, and well conditioned, skip below to the Energy System image and the discussion that follows. For the rest of us, let’s figure this out.

Over weight and inactive
If completing HIIT, be extremely careful when selecting the exercises to minimize the risk of injury and negative associations with exercise. Generally speaking, extremely high intensity training will make you dislike exercise, and likely cause you to quit. Furthermore, your body is not ready to be pushed at maximum intensity. Chances of injury are unnecessarily high.

  • Recommendation: Avoid HIIT at the start, and begin more gradually with longer, steady state cardio, or slightly longer intervals at a lower intensity (e.g. 6 min intervals at 75% intensity x 5 reps).

Healthy weight but inactive:
Similar to the over weight an inactive category, I would avoid extremely high intensity intervals – higher risk of injury, and high likelihood of developing negative association with exercise, which means that you will likely quit!

  • Recommendation: Work at intensities below 85% of maximum heart rate (HR), and keep the intervals relatively long (in line with the slightly lower intensity of training). E.g. 3-4 minute intervals at 80% HR Max.

Healthy, but want to increase muscle:

  • Recommendation: Complete short HIIT workouts at high intensity if you are already conditioned. I would not complete HIIT training more than a couple of times per week though, as increased calorie expenditure will make it more difficult to put on muscle mass

Healthy, but want to lose some fat:

  • Recommendation: See how your body responds and make a decision. Reason being that HIIT relies on carbohydrate for fuel (see my blog post on nutrition and exercise for more details), and if you want to burn fat, you may want to complete slightly longer intervals at a lower intensity (e.g. 80% Max HR for 3-5 mins x 5-10 reps). Alternatively, given the intensity of HIIT training, you will burn lots of calories, which will also help promote fat loss. As noted, everyone’s body responds to training differently based on your diet and previous training habits, so please experiment with this and see what works best for your unique situation!

Healthy, but want to get ripped:

  • Recommendation: Go for HIIT – You’ll burn lots of calories, and when combined with strength training in the gym, you’ll put on lean muscle mass.

The Serious Fitness Enthusiast or Athlete:

As you’ve probably inferred from the image, depending on what you’re trying to achieve, different energy systems will dominate; although all will be active to some extent. When training, you need to make sure that you train according to the demands of your sport/needs. We’ll work through some scenarios comparing the 100m sprint vs. the 400m sprint vs. a 5km+ run.

(a) 100m sprint: this is dominated by the ATP-CP system. Depending on your speed (i.e. time to complete the race), the Anaerobic glycolysis system will kick in.

To excel at the 100m sprint, it is counter productive to complete long slow runs for endurance because such runs utilize the aerobic energy system. The “SAID” (specific adaptations to imposed demands) principle implies that if you’re training for 100m sprints, you need to complete short sprints of similar distance to have the appropriate physiological adaptations. Completing long and slow runs will not only have sub-optimal cardiovascular physiological adaptations, but you will also cause sub-optimal muscular adaptations – i.e. negatively impact fast twitch and slow twitch muscles!

In this case, HIIT of short (e.g. 10 sec), extremely high intensity (e.g. 95-98% HR max) intervals, with adequate recovery time (e.g. 1:6 work to rest ratio) between intervals will be ideal; especially if you’re also trying to develop your technique and speed!

(b) 400m sprint: this is dominated by the anaerobic glycolytic system. Starting power comes from the ATP-CP system, and some engagement (near the end of the race) of the aerobic system.

Similar to the 100m event, excellence in the 400m sprint will not come from long slow endurance runs that primarily utilize the aerobic system. Using the SAID principle, training and sprints around the 1 min mark at a high speed will be helpful (whether it’s weight training in the gym, or cardiovascular training). Of course, each of these races are very technical, and I’m only discussing the cardiovascular / energy system implications as I alluded to earlier. Acceleration, stride length, stride speed, body angle, arm and leg cadence etc. are all extremely important to excel at these events. A qualified coach can assist on those fronts!

So, to build your cardiovascular fitness for a 400m race, HIIT of moderate interval length (e.g. 1 min), moderate recovery at a 1:1 work to rest ratio (e.g. 1 min), and high intensity (90-95% of HR max) could work well. Again, this depends on your running technique, current level of conditioning and more.

(c) 5km+ run: this is dominated by the aerobic energy system. Remember though, that the ATP-CP system will be working for the first 7-10 seconds at the start of the race, after which the anaerobic system will kick in. Around the 2 min mark, the aerobic energy system will begin to provide a greater percentage of energy for the rest of the race.

This is where longer tempo runs will be most effective. Here, HIIT of longer work durations (e.g. 5 minutes), with moderate rest (1-2 minutes) at a 2:1 to 5:1 work to rest ratio, at moderately high intensity (e.g. 80-85% Max HR) will likely be ideal.

Of course, as alluded to earlier if your starting speed or running technique requires work, it needs to be completed separately. Remember, our discussion is just focused on the energy system/cardiovascular component of things. If you'd like more guidance around structuring your training around fitness and skill development, contact me here.

The Impact of Different Training Type:

Muscle Fiber Composition:
Research has shown that different muscles in the human body are predisposed to containing more (or less) of a certain muscle fiber type. For example, the quadriceps (on average) have more fast twitch muscle fibers, while the gluteus maximus (on average) has more slow twitch muscle fibers. Miller et al. (1993) (ref #7) showed that men had higher percentages of slow twitch muscle fibers in the biceps compared to women. Men also had higher percentage of fast twitch muscle fibers in the vastus lateralis (one of the quad muscles) than women.

A 2011 study (ref #5) from the Scandinavian journal Medicine and Science in Sports “investigated physiological and skeletal muscle adaptations in endurance runners subjected to 6 weeks HIIT”. The authors noted that “Cross‐sectional area of type II fibers tended to have decreased” post training in this group of eighteen well trained endurance athletes.

As you can see, there are several implications and discussion points related to fiber type, training, gender and more. An interesting book titled "The Sports Gene" by David Epstein (ref #6) digs in to the genetics behind sports performance. As noted in his book, The Institute of Sports Medicine in Copenhagen actually identifies levels of fast twitch and slow twitch fibers in athletes (via muscle biopsies), and then directs them to appropriate sports to maximize their genetic makeup!

For instance, someone that has predominantly slow twitch fibers in the calves, quads, hamstrings and glutes will not become a sprint champion, no matter how much sprint training they complete. Endurance training however will enhance the metabolic function of their slow twitch fibers and they will have an advantage there (relative to someone with higher proportions of fast twitch fibers). Talk about selection and conditioning!

Activity: Based on the images below, what differences do you notice between the athletes? Clue: muscle definition, size and event

(3) Your Availability and Starting Fitness:

The final factors to consider are (i) how much time you have to train, and (ii) what your starting fitness levels are. These are extremely important to consider because attempting something that your body either (a) cannot handle or (b) you don’t have the physical time to complete, can result in injury, or worse yet, negative mental associations with exercise which could stop you from ever exercising in the future. No one wants that!

Is this you? Limited time. Already fit. Can handle intense training.

HIIT is a great way to improve conditioning and burn lots of calories quickly! This becomes even better if you are training for a sport that has similar energy system demands and work to rest ratios (as described above).

Is this you? Limited time. Inactive. Cannot handle intense training.

Short fast intervals could potentially be a recipe for disaster because they’re too much of a shock to the sedentary body. Building the base fitness via longer slower intervals first (say 6-10 minutes of work and 1 min of rest x 2-10 sets) will prepare you for the harder more intense intervals.

Intermittent Fasting and HIIT:
Intermittent fasting is a concept that continues to grow in popularity. The basic premise is that you do not eat anything (ideally) after 6 or 7p.m each evening, and break your fast after anywhere from 12-16 hours (or longer). I will be writing a post specifically on this subject in the future. Stay tuned! That being said, I will say that there are positive weight loss benefits associated with this practice.

If you do engage in intermittent fasting, I would NOT recommend HIIT training. As mentioned earlier, given the intensity of HIIT, the body uses glucose (or carbohydrate) for energy. Intermittent fasting trains your body to rely on ketones (i.e. essentially fat stores) for energy.


  • Intermittent fasting: complete steady state, lower intensity cardio (or intervals). E.g. 6 min intervals at 75%-80% Max HR for several reps. Do not do HIIT.
  • Eating carbohydrate in the hours leading up to training: Complete HIIT or steady state lower intensity cardio.

Additional tips:
When doing HIIT or longer less intense interval training, I would highly recommend that you build your fitness on multiple pieces of equipment (e.g. rowing machine, bicycle, treadmill, elliptical. This is because you will challenge all of the muscles in your body, and you will not develop overuse injuries or any muscular imbalances associated with using only one piece of equipment.

The exception is obviously if you are training seriously for a specific sport or event. In this case, it is advisable to focus on that specific sport or equipment, with some cross training to ensure balance and variety. Too much of anything is bad.

As always, please post any questions or comments below. Happy training!


Ref #6: The Sports Gene by David Epstein
Ref #7: Miller, A.E.J., MacDougall, J.D., Tarnopolsky, M.A. et al. Europ. J. Appl. Physiol. (1993) 66: 254. https://doi.org/10.1007/BF00235103