A Primer on High Intensity Interval Training

The Broscience of what you need to know and workouts to do

I am in the midst of writing a very in-depth article on Cardiovascular Training, and the article got so long that including another section on HIIT would have made it a 30 minute reading experience.

So I elected to publish this guide on HIIT first.

---

Some Maintenance Updates as of December 1st, 2025

Elite Research USA is still having a black friday sale. If peptide research is something you engage in, use code AJAC10, stock up on your needed chems, and get a bigger discount

Click Here

If you know NOTHING about Peptides and want to learn, download the A-Z Guide

---

High Tech Health is having a Sale on their Saunas

Mention AJAC when you call to order, and you will get a deal.

---

Lets get into it HIIT

HIIT Stands for High Intensity Interval Training.

The technical definition of HIIT is a hybrid protocol that alternates between supramaximal anaerobic bursts and low-intensity aerobic recovery periods. Your heart rate goes really high, it comes back down, and then you repeat.

It is arguably the most time-efficient method for improving cardiovascular health, improving overall stamina, and improving metabolic flexibility (the ability to switch rapidly between burning fat and burning glucose).

There are multiple ways to do HIIT workouts, but lets cover mechanisms first.

The Mechanism of Action

HIIT works by repeatedly pushing the heart rate near its maximum (90%+), creating a significant oxygen deficit and depleting localized glycogen in the muscles being used.

During the rest intervals, the aerobic system must work in overdrive to clear metabolic waste (lactate/CO2) and replenish ATP stores.

HIIT workouts by definition are short and intense. They are not something you do daily, but 1-2 times a week. Its a powerful stimulus that should be done consistently, but not with excess frequency.

While all HIIT workouts require sprinting, not all sprint workouts are HIIT

This is an important distinction. During a HIIT workout, the goal is maximal output of power with accompanying fatigue. The goal is essentially make yourself as tired as possible in a short period of time.

During a pure SPRINT workout, sprinting meaning actual running on flat ground, the focus is NOT fatigue, but performance. The goal is getting faster with minimal fatigue. Sprint workouts you will perform full warmups and take extended rest periods.

Training to get faster and training to get metabolic benefits through power/fatigue synchrony are NOT the same thing.

Moving on…

The Metabolic Benefits of High Intensity Interval Training (HIIT)

The primary value of HIIT is not in the calories burned during the workout, but in the physiological adaptations triggered after its over.

1. EPOC: The “Afterburn” Effect

A distinct metabolic advantage of HIIT is Excess Post-exercise Oxygen Consumption (EPOC). Unlike steady-state aerobic exercise where the body meets oxygen demand in real-time, supramaximal HIIT intervals force the body into a severe oxygen deficit.

Post-workout, the body must work in overdrive to restore homeostasis. This restoration process involves replenishing ATP and Phosphocreatine stores, converting lactate back into glucose via the Cori-Cycle, and re-oxygenating the blood. This metabolic recovery is energy-intensive; research indicates that the metabolic rate can remain elevated for 12 to 24 hours post-workout, with an extra 3-4 calories per hour being burned.

While this increase in metabolism is small, its a positive indicator of the other metabolic adaptations taking place, such as

2. Improved Insulin Sensitivity and Glucose Disposal

HIIT is a potent intervention for managing blood glucose and preventing metabolic syndrome. Everyone with an elevated A1C or full blown type 2 diabetes should be doing HIIT.

The mechanism is driven by the rapid depletion of stored muscle glycogen during high-intensity bursts. This depletion signals GLUT4 (glucose transporter type 4) proteins to translocate to the surface of muscle cells. This reaction allows the muscles to act like a sponge, soaking up glucose from the bloodstream independent of high insulin concentrations.

Consequently, insulin sensitivity is significantly improved, and the likelihood of carbohydrates being stored as adipose tissue is also minimized.

3. Mitochondrial Biogenesis (PGC-1α)

The intense cellular stress of HIIT triggers the release of PGC-1α (Peroxisome proliferator-activated receptor gamma coactivator 1-alpha).

This protein is the “master regulator” of mitochondrial biogenesis. It signals the body to create new, more efficient mitochondria to handle future high-intensity demands. This increases the body’s overall oxidative capacity (VO2 max), as well as improving the mitochondria volume in the muscular tissue that was worked (which if you use an airbike like I recommend, its ALL of your major muscles)

4. Metabolic Flexibility

Metabolic flexibility is defined as the ability of the body to efficiently switch fuel sources based on availability and demand. The two main fuel sources being glucose (carbohydrates) or free fatty acids (fats).

HIIT forces the body to use all energy systems simultaneously; phosphagen, glycolytic, and oxidative, and makes the body switch rapidly between the anaerobic system (during the sprint) and the oxidative system (during the rest period to clear lactate).

The adaptations from this metabolic disturbance are powerful; you get power, speed, and hypertrophy of fast-twitch fibers, while the aerobic system becomes more efficient at waste clearance and fuel regeneration.

5. Lipolytic Hormonal Response

HIIT creates a unique hormonal environment that is highly conducive to fat loss. The “do or die” intensity of the intervals causes acute spikes in catecholamines (adrenaline and noradrenaline).

These hormones then bind to adrenergic receptors on adipocytes (fat cells), triggering lipolysis (fatty acid breakdown for fuel usage). Additonally, sprints have been shown to cause acute spikes in Human Growth Hormone (HGH), which further increases fat oxidation. While these increases are short term, GH release is always beneficial.

The HIIT Menu: 8 Science-Based Protocols

All HIIT workouts will require sprinting in some form. Either running as fast as you can, peddling as fast as you can, rowing as fast as you can, or swimming as fast as you can.

Personally, I believe using an Airbike is the absolute BEST form to do HIIT. It gives you maximum safety, maximum power output, maximum measurability, its simply unmatched by any other modality.

The following Protocols are all intended to be done with an Airbike. They are grounded in scientific research and practice, and range from as short a few minutes (like the Tabata) all the way to 40 minutes (the Norwegian)

How to Use These Protocols.

• Frequency: Do NOT perform these everyday. 1–2 sessions per week is the maximum for most athletes lifting weights concurrently.

• Modality: USE THE AIRBIKE. Other safe modalities are cycling, rowing, uphill Walking or running. I would prefer you use the airbike over flat ground running and sprinting to minimize injury risk. You get hurt, its on you.

• Warm-up: All of the protocols require a 5–10 minute ramp-up in Zone 1/2 before starting the first interval. I do NOT suggest going right into a protocol with zero warmup.

1. The Tabata Protocol

Total Duration: 4 Minutes

• The “Micro-Dose”

• Work/Rest: 20 seconds work / 10 seconds rest.

• Rounds: 8 rounds.

• Goal: Maximum Anaerobic capacity in minimum time.

• Note: Extremely painful; requires 100% effort to be effective in this short timeframe.

2. The Meyer Protocol

• Total Duration: 15–20 Minutes

• Work/Rest: 15 seconds work / 15 seconds rest.

• Rounds: 30–40 repetitions.

• Goal: High aerobic/anaerobic output the entire session

• Note: This workout is more intensely aerobic versus anaerobic, and trains the lactate system. Your muscles will be burning rapidly Its arguably worse than the Tabata, because of the number of intervals, even though they go fast.

3. The Billat Protocol 30-30

• Total Duration: 10–20 Minutes (Variable)

• Work/Rest: 30 seconds work / 30 seconds rest.

• Rounds: Repeat until you can no longer hold the pace.

• Goal: Spending the maximum amount of time at your VO2 Max velocity.

• Note: Depending on your level of fitness, your workout may be quite short. The goal is sustaining the same pace across ALL intervals, versus trying to hit peak output in any one intervals.

4. Repeated Sprint Training (RST)

• Total Duration: ~20 Minutes

• Work/Rest: 6–10 sec sprint / <30 sec rest.

• Rounds: One “Set” is 2-3 sprints with 30 seconds rest between each, followed by 5 minutes rest, then repeat 2-3 times total

• Goal: This setup is intended to replicate sports where multiple sprints might be done in a short period of time, followed by a long rest period.

5. The Gibala Protocol “10-20-30”

• Total Duration: ~25 Minutes

• The Protocol: Rolling intensity (No stopping).

  • 30 seconds Low Intensity.

  • 20 seconds Moderate Intensity.

  • 10 seconds Max Intensity.

• Rounds: Repeat cycle continuously for 5 mins. Rest 2 mins. Do 3–4 rounds

• Goal: Aside from the metabolic benefits, this protocol is good for people who have NEVER done HIIT of any kind, and need to learn how to assess intensity.

6. The Wingate Protocol

• Total Duration: ~25 Minutes

• Work/Rest: 30 seconds MAX effort / 4 minutes very light recovery.

• Rounds: 4 to 6 rounds.

• Goal: Peak power output a

• Note: The long rest periods make the session longer, but actual “work” time is very low (2–3 mins). This is the easiest protocol in that it does not causing extreme EPOC and fatigue

7. The Little Method

• Total Duration: ~27 Minutes

• Work/Rest: 60 seconds work / 75 seconds rest.

• Rounds: 8 to 12 rounds.

• Goal: Probably the most challenging to due to the number of intervals, although the intensity of the intervals can be adjusted depending on your level of conditioning.

8. The Norwegian Protocol 4x4

• Total Duration: 40+ Minutes

• Work/Rest: 4 minutes work / 3 minutes active recovery.

• Rounds: 4 rounds (+ mandatory 10 min warm-up & 5 min cool-down).

• Goal: Deep structural heart health (VO2Max) and stroke volume.

• Note: This is the longest because it targets the aerobic system’s upper limits rather than pure anaerobic power.

What workouts do I Professionally Recommend?

If youre short on time, I recommend the Meyer or Billat Protocols. I personally modify these to 1 minute on, 1 minute off. I also will do 10/50 (work:rest) for 10-20 rounds as well.

If you have more time, the Little Method Protocol or Norweigan Protocol are both recommended.

Any HIIT is better than no HIIT. Find a protocol that works and stick with it weekly for best adaptations long term.