High-intensity interval training (HIIT) is a popular workout that originated in competitive endurance sports. HIIT couples bouts of all-out exertion with periods of rest for a fast and effective workout.
Functional training workouts generally emphasize building strength and flexibility in all planes of movement to prevent injury and build practical fitness that helps you perform activities in daily life, like carrying groceries or lifting children.
This article will cover the basics of HIIT vs functional training, and their benefits and compare them for muscle gain, fat loss, and cardiovascular health gains.
HIIT started in competitive running and cycling as a way to help Olympic athletes train intervals at 100% of their heart rate and build serious cardiorespiratory capacity.
HIIT has exploded in popularity thanks in large part to the success of workouts like Crossfit, p90X, and Insanity and the growing body of evidence that demonstrates its effectiveness.
HIIT enables you to get a full-body workout in a short period of time, making it convenient and appealing to many people.
HIIT workouts alternate between near-maximum effort and a short rest period (typically between 15 seconds of rest to 2 minutes of work) for the duration of the training session.
This high demand on the body creates amazing physiological adaptations and serious health benefits. Because HIIT is so demanding on the body, it’s not recommended for everyone, and you should talk to your doctor before starting a HIIT workout routine.
While HIIT is mainly a cardio workout, it has expanded to include some combination workouts with strength training.
Types of HIIT Workouts
By changing the workout duration, the effort you’re putting in, and the rest period, you can make any HIIT workout easier or more difficult.
Sprinting intervals is a traditional HIIT workout has been around for decades and is the gold standard for cardiorespiratory fitness and workout efficiency.
Whether running, biking, swimming, or rowing, the workout comprises a brief burst of work followed by a short recovery period, repeated for multiple rounds.
With traditional cardio HIIT, your entire workout, including the warmup and cool-down, should be 30-60 minutes.
Bodyweight HIIT workouts don’t require any equipment, so they can be done at home.
Tabata is a fast-paced and difficult type of HIIT workout. You use only body weight or cardio moves.
Tabata is a type of HIIT format that is four minutes long and includes eight rounds of exercises performed at maximum effort for 20 seconds each, with 10 seconds of rest in between exercises.
Metabolic Resistance Training (MRT)
While it doesn't meet the exact definition of HIIT training (because it doesn't have prescribed rest times), metabolic resistance training (also known as metabolic conditioning, or MetCon) was born from HIIT and both the every minute on the minute (EMOM) and as many reps as possible (AMRAP) workout structures closely mirror the HIIT format.
Most importantly, MRT packs a similar punch regarding cardio health, fat loss, and fitness benefits.
Every Minute on the Minute (EMOM) workouts became popular in CrossFit. In an EMOM workout, you perform an exercise for a certain number of reps every minute on the minute and then rest for the remainder of your minute.
The faster you complete the reps, the more rest you get. While there is no set time, most EMOM circuits are 10-20 minutes long.
EMOMs are a fun challenge because they put you in competition with yourself from round to round.
In an AMRAP (As Many Reps as Possible) workout, you complete the exercise in the allotted time and achieve as many repetitions as possible. Then for your next round, you try to get more reps than the last.
You can also do as many rounds as possible, attempting to achieve as many rounds of the circuit in a prescribed time.
AMRAP workouts are similar to EMOM workouts in setup and programming, and many of the same exercises work well for both.
Benefits of HIIT
Unlike some other fitness trends, HIIT has remained wildly popular since its inception for good reason. Numerous studies support that HIIT is the superior choice for cardio exercise for most people because it packs many benefits into a much shorter time frame.
Lack of time is one of the most common reasons folks cite for not exercising, so this is an important feature of HIIT.
Some HIIT workouts, such as Tabata, can be done in just four minutes. But the classic HIIT cardio workouts are typically 20-30 minutes long, depending on your fitness goals.
HIIT workouts are much shorter compared to traditional low-intensity steady-state cardio (long, slow run, bike, hike, row, etc.), which would take much longer to give you the same health benefits.
Improved Cardiorespiratory Health
Because HIIT training is primarily a cardio workout, it offers a whole host of cardiorespiratory health benefits. First and foremost is an increased VO2 max.
VO2 max is a measure of cardiorespiratory health and sports performance. It represents how much oxygen your body needs during exercise.
A higher VO2 max means your body is more efficient at delivering oxygen to your muscles, and your cardiorespiratory system is healthier.
In numerous studies looking at overweight and non-overweight people, HIIT is unbeatable at increasing VO2 max and other markers of aerobic fitness.
A research review of 22 different studies found that HIIT was just as effective as moderate-intensity cardio programs at improving body composition and increasing VO2 max, and more effective at increasing cardiopulmonary fitness when HIIT working intervals were two minutes or more.
That same research shows many other heart health benefits, including:
- Lower blood pressure
- Improved cardiovascular fitness
- Reduced inflammation
- Improved cholesterol
- Decreased disease risk
Increased Calorie Burn, Metabolism, & Fat Loss
HIIT is a phenomenal adjunct to any fat loss plan, especially for those who are overweight, because it aids fat loss in three ways.
First, HIIT burns more total calories during your workout than comparable cardio workouts in half the time, so you're more likely to stick to the program.
For example, 30 minutes of HIIT exercise would burn more total calories than 30 minutes of traditional cardio like MISS or LISS.
Second, HIIT increases your excess post-exercise oxygen consumption (EPOC). This represents a rise in the number of calories you will continue to burn for up to 24 hours following your workout.
Third, when combined with proper nutrition, HIIT helps you build lean muscle, which increases your metabolism and fat-burning capabilities at rest. This is important because we actually burn the most total calories daily at rest–called our resting metabolic rate.
All of these mechanisms combine to help you burn fat, build muscle, and create a more lean body composition.
Another impressive benefit of regular HIIT training is its effect on insulin sensitivity. Insulin sensitivity describes how effective the hormone insulin is at directing sugar molecules, aka glucose, to their destination.
When our insulin sensitivity is high, insulin has no problem getting the liver, muscles, or fat cells to open their doors and let the glucose molecule pass.
But when you develop low insulin sensitivity–also known as insulin resistance–insulin becomes less effective. In response, the body starts pumping out more and more insulin.
If left unchecked, this leads to higher glucose levels in the bloodstream which can result in Type 2 Diabetes and a host of other metabolic diseases.
Drawbacks of HIIT
While some HIIT programs like Crossfit have some high profile and unusual injuries such as rhabdomyolysis, those injuries are rare, and Crossfit injury rates overall are comparable to weightlifting.
HIIT injuries are often associated with the common culprits in all sports, including improper or inadequate warm-up, improper form, and overtraining.
As a personal trainer, the biggest issue with HIIT is overtraining. Because HIIT workouts require such extreme exertion, you must be highly intentional with your warm-up, cool-down, and recovery. I've seen too many clients fall for the hype of HIIT and go overboard over the years.
HIIT is a phenomenal training option, but it is NOT a workout you can do daily.
Regardless of your training level, those new to HIIT should start with one HIIT session weekly and progress slowly. By starting slowly and gradually increasing volume, you'll reduce your risk of injury, training burnout, and keep your programming fun.
You can always ramp up your training, but you can't take back an injury.
So while HIIT is effective for the young and the old, overweight and non-overweight people—it's not for everyone.
Though it has been shown to be safe and effective for some patients in cardiac rehabilitation, there is no consensus amongst healthcare professionals as to which clinical populations are appropriate for HIIT training.
You should always check with your healthcare provider before starting any new workout program, especially a high-intensity one.
Understanding Functional Training
Unlike HIIT, which is well-defined and its benefits are heavily documented in the research literature, functional training doesn't have a single definition.
A recent conceptual review (where scientists compare existing research concepts in an effort to understand their relationship) recommended doing away with the term “functional training” entirely because it's redundant and confusing.
These researchers believe that because there is no agreed-upon definition for functional training, these so-called training methods are just variations of traditional strength, power, flexibility, and endurance programs.
For the sake of this article, I will explain how I define functional training and why I disagree with these authors. Sort of.
So what is functional training, really?
I think of functional training as any movement program that:
- Prioritizes unilateral movement
- Aims to prevent injury
- Uses compound movements
- Uses a full range of motion
- Builds real-life strength
This is precisely what good traditional strength and flexibility programs do as well.
Here's why I still use the term “functional training”: it communicates crucial information to my clients about their programming in a way that builds adherence.
I could spout off to my client about the importance of a well-rounded physical fitness program and the components of fitness like a textbook.
Or, I could say, "This program will help keep your back safe when you bend down to scoop up your grand-baby!"
Explaining my programming by labeling it as functional helps my client understand why strength through a full range of motion, in different movement planes, and under different conditions is so important. Because it helps them keep doing the things they love.
So when coaches use the term functional training, they usually mean compound, free-weight exercises vs. machine movements, using a full range of motion, heavy emphasis on core strength (particularly anti-rotational work), focusing on unilateral strength and stability development.
Functional training can also incorporate equipment such as barbells, ropes, sandbags, prowlers, ropes, tires, and sledgehammers.
Think of functional training and traditional weight lifting as a venn diagram.
Weight training often uses isolation exercises to develop one muscle at a time, but not always.
While functional training primarily trains compound movements through a full range of motion.
Both are effective in building muscle size, strength, and endurance.
They mainly differ in their goals. Traditional weight training is primarily used to build muscular strength and size. In contrast, functional training strives to prepare you for life outside the gym.
Benefits of Functional Training
The benefits of functional training are the same as in traditional strength, power, flexibility, and endurance training programs. But funcitional training deftly combines all of those elements into a single workout.
Builds Muscle Strength & Size
Consistent strength training that uses progressive overload, combined with proper nutrition, results in bigger muscles and increased strength.
Improved Range of Motion (ROM)
Resistance training, including functional training, has been shown to improve joint range of motion. This is crucial because limited ROM can cause tissue imbalances, postural deviations, and pain.
When we start strength training, our body goes through various changes called neuromuscular adaptation. Our muscles become more efficient over time, which makes us more coordinated overall.
Reduced Injury Risk
Athletes from nearly all sports use strength training in their programs as a way to prevent injury. A good strength program will help correct muscle imbalances, strengthen supportive tissue, and increase physical resilience.
Drawbacks of Functional Training
Because functional training takes elements of traditional strength, power, flexibility, and endurance programs, the main drawback is that it's not as effective as each individual program if you devote yourself wholly to that style of training.
For example, bodybuilders who solely train to build muscle will probably gain more muscle than those dedicated to functional training, but this could be said for any circuit or combo workout.
Another limiting factor is form. Functional, compound movements, like any resistance training, are only beneficial if you perform them correctly.
These types of exercises require more coordination, core strength, and joint stability than machine exercises. So proper instruction could be a limiting factor to perform the exercises safely.
HIIT vs. Functional Training
HIIT and functional training are both incredible exercise programs that offer top-tier health and fitness benefits.
HIIT is primarily cardio, so its benefits are largely heart health and calorie burn, while functional training is a type of strength training, so it's phenomenal for building muscular strength, size, endurance, bone density, and building physical resilience.
HIIT is a much shorter workout, typically 10-60 minutes max, including warm-up, while functional training is a strength workout that can be double that in length.
Is HIIT or Functional Training Better for Muscle Building?
While HIIT training can help you build muscle depending on which workout you choose, functional training takes the cake regarding muscle growth.
Remember that lifting heavy weights and taking adequate rest (during each set and between workouts) is essential for maximum muscle growth.
There's a reason why bodybuilders work out for hours– because they take enough rest between sets to truly max out the muscle.
Is HIIT or Functional Training Better for Cardiovascular Health?
HIIT is aerobic exercise and is superior for cardiovascular health.
Is HIIT or Functional Training Better for Weight Loss?
This question is trickier because they're both fantastic for fat loss but act on different mechanisms.
HIIT definitely burns more calories during workouts and in the 24 hours after. But your metabolism is primarily determined by how much lean muscle you have, so prioritizing muscle building is essential for building a lean body.
The tl:dr is: both HIIT and functional training are great for fat loss.
Is HIIT or Functional Training Right for You?
HIIT and functional training are best suited for those with a fitness foundation but for different reasons. HIIT is so effective precisely because of its demanding intensity and the adaptations that it elicits in the body.
For the same reason, it's not for everyone. Those who are significantly de-conditioned, have heart conditions, uncontrolled diabetes, or retinopathy should not start with HIIT and should consult their physician before beginning any exercise program.
With that said, the research shows HIIT is highly effective and safe for most people. The key is tailoring the workout to your current fitness level and goals.
I recommend working with an experienced trainer who can assess your individual needs and program accordingly.
Functional training primarily uses compound exercises like the squat, deadlift, bench press, and row. These movement patterns require proper time to learn and master.
If you're a gym regular and want to accelerate your progress, I recommend giving HIIT and functional training a whirl. In combination, they meet your aerobic and anaerobic recommended exercise requirements in a fraction of the time of other workouts and provide a challenging, fun program.
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HIIT & Functional Training Resources
Interested in learning more about these workouts, which exercises to start, and how to perform them? Check out this curated resource list selected by our fitness pros.
Videos and Tutorials
Best Apps for HIIT & Functional Training
Keelo | Free app available on iTunes offering HIIT workouts under 20 minutes
Future | Personal Training app with functional training options available on iTunes and Android.
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HIIT is a highly-effective cardio workout that blasts calories, builds muscle, and will level up your cardiorespiratory health.
Functional training is a marketing term often used to describe resistance exercise programs that emphasize compound movements, strength through a full range of motion, developing unilateral strength, which builds your strength for everyday tasks.
While the term functional training may be debated in the coaching community, the results of these workouts are not. They will help you build strength, muscle, and bone density, improve your range of motion, and may help prevent injury.
Consult with your physician before starting either exercise program, and seek instruction from a qualified coach if you are a beginner.
While both HIIT and functional training are somewhat advanced, they are safe and effective for most people. Start by adding 1 HIIT workout and 2-3 functional strength sessions weekly.
More HIIT Advice from GymBird Experts
- What Is HIIT?
- Cardio vs HIIT
- Circuit Training vs. HIIT
- HIIT for Beginners
- HIIT vs. CrossFit
- HIIT vs. Tabata
- HIIT Benefits
More Functional Training Advice
- What is Functional Training?
- 8 Functional Training Workouts
- Functional Training vs Cross Training
- Functional Training Benefits
Journal of Strength and Conditioning Research. Effect of High-Intensity Interval Training on Cardiovascular Function, V̇o2max, and Muscular Force.
National Heart, Lung, and Blood Institute. What Is Metabolic Syndrome?
Applied Physiology, Nutrition, and Metabolism. Impact of 4 weeks of interval training on resting metabolic rate, fitness, and health-related outcomes
American Diabetes Association. Insulin Resistance.
International Journal of Environmental Research and Public Health. Evidence-Based Effects of High-Intensity Interval Training on Exercise Capacity and Health: A Review with Historical Perspective.
The Journal of Clinical Endocrinology & Metabolism. Moderate-Intensity Exercise and High-Intensity Interval Training Affect Insulin Sensitivity Similarly in Obese Adults.
Journal of Sports Science. Effects of exercise intensity and duration on the excess post-exercise oxygen consumption.
Journal of Science and Medicine in Sport. CrossFit and rhabdomyolysis: A case series of 11 patients presenting at a single academic institution
HHS Author Manuscripts. High-Intensity Interval Training in Cardiac Rehabilitation.
Frontiers in Sports and Active Living. Is There Any Non-functional Training? A Conceptual Review.
Applied Physiology, Nutrition, and Metabolism. Muscle hypertrophy and strength gains after resistance training with different volume-matched loads: a systematic review and meta-analysis.
ACSM Sports Medicine Basics. Resistance training and injury prevention.