Conditioning Confusion, All About HIIT | The Fit Facility

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Conditioning Confusion, HIIT

High Intensity Interval Training (HIIT) has never been more popular. For good reason too! HIIT workouts are an incredibly time efficient way to increase your metabolic rate, build muscle and/or lose fat (depending on one’s diet), and improve one’s work capacity.

High-intensity interval training (HIIT), also called high-intensity intermittent exercise (HIIE) or sprint interval training (SIT), 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. Though there is no universal HIIT session duration, these intense workouts typically last under 30 minutes, with times varying based on a participant's current fitness level.

However, with it’s popularity increasing, HIIT protocols have been shown to be a very misunderstood topic; which is understandable considering it’s definition doesn’t really put definitive parameters on the duration, exercise selection, etc.

However, it does give a term in it’s very name that will guide us throughout this article. That term? “High.” Armed with this one simple term, and a grasp on exercise physiology, understanding of energy system, pairing of exercises, etc. We can begin to dive down the rabbit hole and observe many of the short comings, and misunderstandings of HIIT programming. But first, we need a quick overview of bioenergetics.

Bioenergetics, Energy Systems

Without diving too deep into bioenergetics, just know that your body has a few different energy systems: phosphagen, glycolytic, and the oxidative energy system. They all differ in their energy supply, and subsequent ability to supply energy for the various activities, intensities and duration. Why is this incredibly important?

Exercise intensity is defined as a level of muscular activity that can be quantified in terms of power (work performed per unit of time) output.

Phosphagen, Glycolytic, and Oxidative Energy systems

Exercises that require a ton of strength and power require a rapid rate of energy supply, which is almost entirely supplied by the phosphagen system. If the duration of your lift is around 0-6 seconds, you’ll rely primarily on this energy system, as it’s used for extremely high bouts of strength and power. This puts the “high” in high intensity!

When the lift continues to 6-30 seconds, obviously the intensity level must be lower than it would be if the lift were around your maximum. While still high, the energy system begins to shift more towards the glycolytic energy system. If the lift continues in duration, a shift will begin, going through the full spectrum of the glycolytic energy system. From fast glycolysis. to slow, with durations of activity lasting 30s-2 minutes and finally 2-3 minutes.

Activities, resistance training, or whatever you choose to throw around obviously must be more a moderate intensity or load, for the simple fact that one cannot continue to use an extremely high intensity or load for an extended period of time. That requires more energy, and as stated above, we simply begin to deplete that at a rate that we cannot reproduce (which is why rest is beneficial).

This image can be found in the Essentials of Strength of Conditioning p.54

This image can be found in the Essentials of Strength of Conditioning p.54

The human body simply cannot sustain efforts at a high intensity or work load. The body doesn’t work like that. The longer you do something, or the heavier the load, the more tired you get. It’s really as simple as that.

Exercises that last longer than 3 minutes begin to use more of the oxidative energy system (aerobic). The intensity must be low. Jogging would be a good example of an exercise that utilizes primarily the oxidative energy system.

So to recap, high intensity exercises (HIIT, sprinting, etc) rely primarily on the phosphagen (anaerobic) energy system and fast glycolysis (anaerobic). As the duration increases, the intensity must decrease, thus slowly sliding down the spectrum of glycolysis and to the oxidative energy system.

Now that we’ve defined our terms, and actual energy systems we plan on targeting, it’s pretty easy to see how someone could quite easily butcher a “high intensity” interval workout (HIIT).

Many times, the work:rest ratio doesn’t reflect that of our previously defined term of “high intensity”. What it reflects is actually, low-moderate aerobic exercise.

“Rest is where the magic happens”

The National Strength and Conditioning Association (NSCA) defines rest as

The time dedicated to recovery between sets and exercises is called “rest period or interset rest”.

Rest isn’t a “dirty” word, and like coach Dos says, “rest is where the magic happens.” There are a ton of different metabolic reactions going on during “rest”, and just because you’ve stopped working, that doesn’t mean you’re body does too.

Why Do People Butcher HIIT workouts?

Actual footage of trainers and fitness enthusiasts butchering HIIT.

Actual footage of trainers and fitness enthusiasts butchering HIIT.

I honest believe it’s from a lack of understanding, and/or the incorrect belief that “more is always better.”

It’s human nature to assume that if 1 is good, 2 must be better. The same break down in logic can be applied in fitness. Many times people view rest as a weakness; if you’re not sprawled out on the floor, you’re not going to reap the maximum benefits of the workout. Or that if someone in the room is working harder than you, they’re going to get more “gainz" than you.

Combine this train of thought with certain workout protocols or Workout Of the Day (WODs), more often than not, it simply produces what many great strength coaches call “sloppy aerobics.”

Protocols with negative rest, As Many Rounds/Reps As Possible (AMRAP), and even poorly designed Every Minute On the Minute (EMOM) tend to be the guilty culprits.

The athlete(s) end up doing work, just for the sake of doing work.

If the intensity is where it should be, HIGH. Then the effort simply cannot be sustained. As covered earlier, the higher the intensity/load, the longer the rest/recovery needed.

If a rest/recovery isn’t needed, then the effort is sustainable and by definition, is not longer “high intensity”. It simply becomes aerobics with weights, or “aerobic-slop.”

When one’s ego and bravado are set aside, it’s actually an easy concept to understand.

Interval Training

The whole point of interval training is to strategically target specific energy systems. As stated in the essentials text.

Interval training is a method that emphasizes bioenergetic adaptions for a more efficient energy transfer within the metabolic pathways by using predetermined intervals of exercise and rest periods.

Those predetermined rest periods are also known as a “work to rest ratio” (from here on out ”work:rest”).

Why would someone use a predetermined work:rest?

Using a predetermined work:rest will allow you to actually target specific adaptions. Having predetermined rest will allow the exercise intensity to remain very high, while fatigue can be regulated at the same, or even less than what it would be if exercise were continuous with the same intensity.

Which, if we recall our previous overview of energy systems, makes sense. One cannot sustain exercise at a high intensity for very long. The human body doesn’t work like that.

So knowing this, if the efforts are sustained, then the intensity actually isn’t where it needs to be to elicit the desired training effect. The very definition of high intensity, is an effort that cannot be sustained. High intensity workouts are so effective because of this!

When one does a popular AMRAP (As Many Rounds/Reps As Possible) or even poorly designed EMOM (Every Minute On the Minute), and that effort lasts well into the spectrum of slow glycolysis and oxidative system (2-3minutes). Then the question must be asked “what exactly are we targeting?” high intensity workouts require rest, the energy systems being targeted are the phosphagen and the glycolytic energy systems (anaerobic).

These workouts are often times intending to target the anaerobic energy systems. However, without adequate rest, these thresholds can never be hit due to the sustained nature of those previously mentioned type of workouts.

As soon as the work time continues past 2 minutes without rest, the activity obviously become aerobic, and a different energy system begins to take over. The efforts cannot be high, because the work duration is so long.

Recommendations For Recovery

The importance of adequate rest can’t be stress enough. Use the following rule of thumb to ensure you get the appropriate rest!

For exercise that last 5-10s (90-100% of max power) use 1:12-1:20 (work:rest). For exercise that last 15-30s (75-90% of max power) use 1:3-1:5.

For instance, an effective HIIT timing protocol would look like 15s:45s (1 minute total), or even 30:30.

When prescribing exercise timing protocols, one must also account for exercise selection. For example exercises that tend to take longer to complete (step ups, lunges, etc.) would be great for 30:30.

Hopefully this brief article helps shed some light on how important it is to take the rest your body NEEDS. It’s near impossible to keep the intensity HIGH without taking an adequate rest. One of the biggest mistakes is that people don’t actually use a “high” intensity! So, if you’re going to be doing a HIIT style workout, just make sure that the intensity is high enough to elicit a favorable adaptation!

If all of this is way over your head, and you need all the benefits of these effective workouts, reach out and call: 256-284-2535, or fill out our form to request your free intro session! we would love to help!


Baechle, T. R. (2016). Essentials of strength training and conditioning. Champaign, IL: Human Kinetics.

Remedios, R. D. (2010). Mens Health cardio strength training. Emmaus, PA: Rodale.

Bourke, Robbie, and James FitzGerald. “OPEX Podcast.” OPEX Fitness, 3 Aug. 2018, #026 - Incorporating Strength Speed Work into Fitness Programs with James FitzGerald

Fleming, Wil, and Robert Dos Remedios. “The Performance Podcast | Strength Training, Olympic Weightlifting, Performance, Fitness, Speed | Wil Fleming and Coach Dos.” The Performance Podcast | Strength Training, Olympic Weightlifting, Performance, Fitness, Speed | Wil Fleming and Coach Dos, 6 Apr. 2014, Episode 4: Misunderstanding HIIT