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The virality of HIIT over the last 10 years has made it a staple in many fitness lovers' regimes. But what’s the science behind this high-energy, high-stress, exercise form? And is it suited to everyone?

Underground Gym is here to answer all your HIIT questions and more. Let’s get started:

What is HIIT?

High-Intensity Interval Training (HIIT) is an approach to exercise that uses short bursts of intense activity performed at >75% of an individual’s maximal power. These high-intensity sessions are broken up by 10-second to 1-minute periods of rest or low-intensity exercise.

HIIT-format workouts typically last between 20-30 minutes and are designed to increase cardiovascular endurance, boost metabolism, and burn calories. 

The intensity of the workout is what sets HIIT apart from other forms of exercise, as it pushes your body to its limits and forces it to adapt to the stress. HIIT can be done with running, cycling, bodyweight exercises, and weightlifting, making it highly adaptable.

Benefits of HIIT

HIIT has been in the headlines for a decade because of its near-endless list of benefits. Here are the most important benefits.

The benefits of HIIT include:

  • Simultaneous fat loss and muscle gain.
  • Improved cardiovascular performance.
  • A short-time requirement makes it easier to fit a workout into your schedule and stick to the long-term.
  • Little to no equipment is needed as HIIT can be achieved when relying solely on body weight
  • As HIIT is performed at an individual’s maximum intensity, it can be modified for those with illnesses. It has even been approved for conditions including lung disease, heart disease, and chronic kidney disease
people doing military hiit class

The Science Behind HIIT

HIIT works because cycling between spiking your heart rate and periods of rest causes physiological adaptations in your body as it battles to handle the external stimulus. According to a scientific study in 2021, these adaptations improve your: 

  • cardiovascular and metabolic function
  • motor unit recruitment enhanced SV
  • maximal cardiac output
  • blood flow
  • skeletal muscle mitochondrial content
  • And your capillary density

HIIT also raises something called Excess Post-Exercise Oxygen Consumption EPOC which means it continues to burn more calories over the next 24 hours.

It’s important to note, that the extent of these adaptations depends on the exercise’s intensity, duration, and frequency. Meaning, you won’t see these benefits unless you stick to a training regime. 

HIIT vs. Traditional Cardio

Traditional cardio involves steady-state, moderate-intensity exercise over a longer duration. HIIT alternates between high and low intensity, often in a shorter time frame. 

The advantages of HIIT over traditional cardio include a more significant calorie burn because of EPOC (Excess Post-Exercise Oxygen Consumption). Users also benefit from greater cardiovascular and metabolic function from a shorter exercise window. One study also found HIIT activates more AMP-activated protein kinase (AMPK) compared to 30 minutes of continuous exercise. Better activation ultimately improves your blood oxidation when your energy is low.

These benefits make HIIT a time-efficient and effective option for those looking to maximize their workout results.

Why is HIIT Better Than Cardio or Steady State?

High-intensity interval exercise has been found to be enjoyable leading to greater physical improvements compared to steady-state or moderate-intensity continuous training (MICT). These findings make HIIT more impactful in a regular exercise routine than steady state.

A woman uses a kettlebell and leans on a benchpress in the gym. A man is pushing a weight sled behind her

How to Incorporate HIIT – Getting Started

For beginners interested in starting HIIT workouts, we recommend starting with a warm-up to ensure you perform your best during a HIIT session and avoid injury. 

At Underground Gym we offer a variety of HIIT classes, including standard HIIT, HIIT Strength, HIIT Box, and Military HIIT, led by experienced trainers and suitable for all fitness levels. 

Using a class to develop regular practice can make HIIT an enjoyable way to improve your fitness levels and your capacity for everyday tasks.

Are There Any Specific Dietary Recommendations for HIIT Practitioners?

HIIT athletes should eat nutrient-dense foods to fuel their high-intensity workouts. A nutrient-dense diet includes a balanced diet of carbohydrates, protein, and healthy fats and staying hydrated before, during, and after workouts. You can also eat or drink a high-protein post-workout snack to aid muscle recovery and replenish energy stores. But in general, there is no HIIT-specific diet you must follow to engage in the activity.

people doing military hiit

How Effective is HIIT for Weight Loss?

HIIT is highly effective for weight loss because, as well as the calories burned during the session, individuals experience Excess Post-Exercise Oxygen Consumption. EPOC is where you burn more calories than usual over the next 24 hours following a HIIT workout.

How Long Does It Take to See Results With HIIT?

You can feel results from HIIT in as little as two weeks, and you’ll see changes anywhere from the one to three-month mark. How long it takes to see results depends on your starting point, training intensity, and genetics.


Underground Gym is a friendly accessible workout space that isn’t afraid to push you to your personal limits, wherever they lie. Drop us a message to speak to a trainer about planning a tailored HIIT workout in our gym or suited to your home set-up.