How effective are HIIT workouts?
To the surprise of nobody, brief search engine research shows a steady climb in high-intensity interval training (HIIT) over recent decades. This type of training, characterized by short bursts of intense activity followed by brief periods of rest or low-intensity recovery, offers a plethora of research-backed benefits for both body and mind.
Understanding HIIT
HIIT involves alternating between short bursts of maximum effort exercise and brief periods of rest or low-intensity activity. These intervals can vary in duration, typically lasting anywhere from 20 seconds to several minutes, depending on fitness level and goals. The workout itself lasts between 30 and 60 minutes.
Whether you do your HIIT workouts from online programs, at group classes, or on your own, you’ll find they often include a mix of aerobic and resistance training. “High-intensity” is a relative term, with the intensity of each workout adapted to your aerobic capacity. The key is to push yourself to near your max capacity during the high-intensity phases, followed by adequate recovery periods to allow your heart rate and fatigue to briefly recover for the next burst of effort.
Ask a variety of trainers, fitness enthusiasts, and even scientists what the definition of an HIIT workout is, and you’ll probably get a myriad of conflicting responses. For the sake of a clear-ish definition, we’ll stick to the research when discussing HIIT and refer to popular discourse to supplement as we see beneficial.
Sprint-interval training vs. high-intensity interval training
Much of the research on HIIT actually includes two categories — high intensity interval training (HIIT) as well as sprint interval training (SIT). According to studies, these categories are defined in the following manner:
HIIT consists of brief, intermittent bursts of vigorous activity (less than VO2peak but usually involves < 100% [70%-90%] of VO2peak* or 85%-95% of the peak heart rate) interspersed with active rest periods, whereas SIT is classically a Wingate-type protocol (all-out, vigorous-intensity exercise involving approximately 350% of VO2peak) interspersed with longer complete rest periods…
The difference between HIIT and SIT is that SIT refers to anaerobic supramaximal VO2max (all-out) intensity and HIIT refers to aerobic submaximal VO2max intensity. The peak power output (PPO) of SIT is about 350% of the power output at VO2max. Meanwhile, the common elements between the two protocols are the high work intensity adapted to the current aerobic capacity of the individual, and the aim of improving both aerobic capacity (VO2peak) and metabolic capacity.
To break down the above: SIT and HIIT methods carry the same principals — high-intensity intervals with brief rest. But where HIIT aims for between 70% to 90% of your V02 max, SIT involves an all-out effort to surpass V02 max as much as possible.
Both methods carry many similarities, but there are a few differences:
Fomat: HIIT involves multiple efforts lasting roughly 30 seconds to a few minutes with equal or less rest periods interspersed. SIT efforts are typically less than 30 seconds with several minutes of interspersed rest.
Muscular adaptations: SIT primarily targets fast-twitch muscle fibers due to its emphasis on maximal sprinting efforts. This can lead to improvements in explosive power and speed. HIIT, while also targeting fast-twitch fibers to some extent, may have a greater focus on overall cardiovascular fitness and endurance.
Adaptability: SIT may be more challenging for beginners or individuals with certain physical limitations due to maximal sprint effort intensity. HIIT workouts can be more adaptable to various fitness levels since they allow for adjustments in intensity and duration of intervals.
Fat oxidation: While both are effective methods for increased fat oxidation (fat loss), SIT showed some greater effects.
*V02 peak, also called v02 max, is the maximum amount and rate of oxygen your body can absorb and use during exercise. Basically, it measures your aerobic fitness; it’s super common to see these tests completed with endurance athletes, such as cyclists or rowers.
Benefits of HIIT workouts
1. Enhanced V02 max & endurance
A higher VO2 max, or maximal oxygen uptake, is indicative of your body's ability to transport and utilize oxygen during exercise. Oxygen is essential for aerobic energy production, such as for running, cycling, rowing, etc., particularly during long-sustained efforts.
While an improved V02 max benefits nearly anyone, it’s especially important for athletes or others with a focus on performance. A higher VO2 reflects better coordination between your cardiovascular system and respiratory system, which means your lungs can more effectively take in oxygen, and your heart becomes more efficient at delivering oxygen-rich blood to your muscles.
In molecular terms, your body can more efficiently produce adenosine triphosphate (ATP), a nucleotide that provides the energy to drive muscular contractions and other bodily functions. This effect allows your muscles to sustain aerobic metabolism for longer periods, perform better, and delay the onset of lactic acid buildup and subsequent fatigue during endurance activities.
2. Improved strength & power
CMT typically stimulates type 1 muscle fibers, which are slow-twitch muscle fibers associated with endurance efforts. HIIT, in comparison, stimulates mostly type 2, AKA fast-twitch fibers associated with brief activities that require a lot of power, such as jumping, heavy weightlifting, and sprinting.
This type of training can translate not to just better performance for those specific brief efforts but can also benefit middle to long-distance endurance. Imagine cycling a long distance, for example, but suddenly needing to power up a hill or sprint past an opponent — that’s where fast-twitch power can make a big difference.
2. Efficiency
Time (or lack of) is one of the biggest barriers of health and fitness in general, and getting to the gym is no exception. HIIT workouts trade off duration for intensity, and studies show they can demonstrate comparable if not better results than continuous moderate-intensity training (CMT) in a fraction of the time.
3. Subtle metabolic boost
The intense nature of HIIT workouts can provide a subtle metabolic boost similar to other forms of anaerobic exercise, such as a standard weights or resistance-based workout, causing your body to burn calories not only during the workout but also in the hours afterward.
This phenomenon is known as excess post-exercise oxygen consumption (EPOC), which is the result of an increased rest energy expenditure as your body recovers in the hours following exercise. Some studies even evidence that HIIT, when compared to other conventional workouts, can increase caloric expenditure by up to 25% per minute while also keeping the perceived exertion relatively low.
4. Improved cardiovascular health
HIIT has been shown to improve cardiovascular health by increasing your heart rate and improving your body's ability to utilize oxygen, gradually driving your V02 max up. Over time, regular HIIT workouts can lead to reduced hypertension, improved cholesterol levels, and enhanced overall heart health, in some cases considerably more than CMT methods.
5. Preserves muscle mass
Unlike steady-state cardio, which can lead to muscle loss over time, HIIT workouts may help preserve lean muscle mass or even induce hypertrophy. This is because HIIT incorporates resistance training elements, such as bodyweight exercises or weights, which stimulate certain receptors responsible for muscle growth and maintenance. One study in particular observed the positive effects of HIIT training on maintaining or improving both muscle mass and strength in elderly populations with sarcopenia (normal age-related muscle wasting).
Research in this regard is less available or more obscure since most HIIT studies center around improvements related to V02 max, endurance, and body fat. It’s important to remember that, while a number of factors are involved, significant muscle atrophy in active people is more likely due to large caloric deficits. Both CMT and HIIT can likely contribute to muscle loss or gain depending on caloric consumption.
6. Versatility
HIIT workouts can be adapted to suit individuals of all fitness levels and preferences. Whether you prefer bodyweight exercises, cardio machines, or a combination of both, there are endless possibilities for creating effective HIIT routines. Additionally, HIIT can be performed virtually anywhere, making it a convenient option for those with busy schedules or limited access to gym equipment.
7. Mental benefits
Like nearly all forms of exercise, HIIT can also have positive effects on mental health. The intense bursts of activity followed by short rest periods can help improve focus, reduce stress, and boost mood by triggering the release of endorphins, the body's natural feel-good chemicals.
While there are mixed results across the board, many people report a high enjoyment of HIIT compared to CMT. This does kind of rely on preference rather than any objective advantage — do you get more satisfaction from short and intense efforts, or do you prefer the almost meditative state of moderate to low-intensity efforts? Maybe both?
8. Insulin sensitivity
Research has linked HIIT to improvements in insulin sensitivity, glucose regulation, and lipid profiles, which are important for reducing the risk of metabolic disorders such as type 2 diabetes and metabolic syndrome.
Examples of HIIT workouts
Want to incorporate HIIT into your routine? Here are some common formats:
1. Cardio intervals
Cardio intervals involve alternating between high-intensity cardio exercises, such as sprinting or cycling at maximum effort, and periods of active recovery, such as walking or jogging. For example, you might sprint for 30 seconds followed by 60 seconds of walking or jogging, repeating for a total of 15-20 minutes.
2. Circuit training
Circuit training involves moving from one exercise to the next with minimal rest in between. For a whole-body workout, a typical circuit might include exercises such as lunges, kettlebell swings, jump rope, and plank holds, performed for 30-60 seconds each with 15-30 seconds of rest between exercises.
3. Tabata
A classic Tabata workout consists of eight rounds of 20 seconds of high-intensity exercise followed by 10 seconds of rest, for a total of four minutes. Exercises can vary but often include movements like squats, burpees, push-ups, and mountain climbers.
4. AMRAP
In an AMRAP (as many rounds as possible) workout, often seen in CrossFit, you perform a series of exercises for a set amount of time, aiming to complete as many rounds as possible within that time frame. This format challenges both strength and endurance while allowing for scalability based on individual fitness levels. Example exercises include squats, push-ups, sit-ups, and box jumps, performed continuously for 10-20 minutes.
5. Pyramid intervals
Pyramid intervals involve gradually increasing and then decreasing the intensity or duration of intervals within a workout. For example, you might start with 20 seconds of high-intensity exercise followed by 10 seconds of rest, then increase to 30 seconds of exercise and 15 seconds of rest, before gradually decreasing back down. This format provides a challenging yet adaptable workout.
Considerations for HIIT workouts
Apply with balance
Even if HIIT proves to be a great workout, it doesn’t need to represent your entire routine. There are research gaps that can’t yet define the upper limit of HIIT effectiveness, and it’s possible that too much HIIT, as with any kind of training, can be harmful to progress and overall well-being. Instead, consider practicing HIIT with moderation, and complement your routine with a blend of strength training, steady-state aerobics, and high-intensity.
Form over intensity
During brief and intense intervals, it’s easy to get caught up in the sheer effort. Make sure to prioritize the form of your exercises, however — neglecting form can lead to a breakdown and injuries. Besides, balancing proper form with intensity helps to engrain that technique during big efforts, improving your efficiency and progression in the long-term.
Scale to your abilities
HIIT’s adaptability is one of its main perks. Plan your HIIT workouts around your personal abilities and goals. Refer to others for workout inspiration as needed, but don’t limit your progress by comparing yourself to another’s standard.
