High-intensity interval training involves alternating work bouts (8 seconds to 8 minutes) of high-intensity movement with low-intensity recovery periods (of variable length). In our industry, HIIT has become a stalwart exercise program for increasing a person’s cardiorespiratory fitness level, effectively burning a lot of calories in a relatively short amount of time and improving metabolic markers of health (Wewege et al. 2018). However, while HIIT is popular, there are some controversies surrounding it. This column will take an evidence-based approach and look at HIIT research to bring clarity to three current controversies.

The specific method used to resolve these debates is referred to as a “proof-of-concept” approach, which means I have chosen the best study to answer the disputed question on HIIT.

Controversy #1: HIIT Is Safe for Persons With Cardiovascular Disease

Proof-of-Concept HIIT Research Study

Wewege, M.A., et al. 2018. High-intensity interval training for patients with cardiovascular disease—is it safe? A systematic review. Journal of the American Heart Association, 7 (21), e009305.

Introduction

Wewege et al. note that cardiovascular disease includes both heart failure and coronary heart disease. Women and men with CVD have traditionally completed low- to moderate-intensity continuous aerobic training (MICT) in cardiac rehabilitation programs. Although HIIT has been introduced and utilized in CVD populations, fitness professionals and exercise physiologists have expressed safety concerns about the efficacy of using this high-intensity exercise stimulus with the CVD population.

The Study

Wewege et al. completed a systematic review, specifically investigating whether HIIT is safe for persons with either heart failure or coronary heart disease. The review included 23 studies—with a total of 1,117 participants—that compared HIIT with MICT. HIIT programs in the studies used high-intensity work bouts lasting up to 4 minutes and intensities of ≥85% of heart rate peak, ≥80% of peak aerobic capacity or a rating of perceived exertion ≥15 (on a 6–20 RPE scale). The MICT programs included continuous aerobic exercise at 60%–75% of HR peak, 50%–65% of peak aerobic capacity or a rating of 12–15 on the RPE scale.

Results

There was only one major cardiovascular-related event among HIIT participants in the studies. Wewege et al. calculated that this was one event per 11,333 training hours, a very low incidence. After reviewing the data a multitude of ways, the researchers concluded that the “rewards” of applying HIIT with CVD patients are reasonably compelling, and the accumulated evidence suggests it is an acceptable alternative to traditional MICT.

Additionally, there is less risk for persons who do not have complex CVD health issues and for those deemed to have low cardiovascular risk. Furthermore, the researchers found no evidence to suggest that HIIT is inherently unsafe for any specific CVD patient characteristics (including older age).

Take-Home Message

For persons with cardiovascular disease, it’s fine to do HIIT at a level suitable for their health status. Wewege and colleagues recommend that fitness professionals regularly monitor a client’s physiological responses during the session (via HR and RPE) and immediately afterward (via blood pressure).

See also: HIIT Helps People With Asthma

Controversy #2: HIIT Impairs Mitochondrial Function and Decreases Glucose Tolerance

Group of people doing HIIT in a gym

Research has shown that HIIT training can increase the content of mitochondria.

Proof-of-Concept HIIT Research Study

Flockhart, M., et al. 2021. Excessive exercise training causes mitochondrial functional impairment and decreases glucose tolerance in healthy volunteers. Cell Metabolism, 33 (5), 957–70.

Introduction

Flockhart and colleagues began this study with the knowledge that cardiovascular exercise positively affects metabolic health by improving glucose regulation and increasing oxidative capacity. Mitochondria, they explained, are small organelles—found within some body cells—that are primary sources of ATP synthesis, providing necessary fuel for exercise and activities of daily living.

Other research has shown that HIIT training can increase the content of mitochondria (Jacobs et al. 2013) and boost insulin sensitivity (Richards et al. 2010), improving glucose regulation and helping to prevent type 2 diabetes. The controversy lies in where the upper limit of these positive benefits occurs; determining this threshold was a primary purpose of Flockhart et al.’s 4-week HIIT study.

The Study

Flockhart and colleagues recruited 11 physically active adults (six females ages 22–28 and five males ages 28–34). Participants were not doing HIIT in their exercise training and not exercising more than 5 hours a week. All study workout trials (completed on a cycle ergometer) were spread throughout the week and supervised by professionals with extensive experience in performance testing. At the end of each week’s trials, the researchers collected biopsy samples of the vastus lateralis muscle of the thigh (for mitochondrial analysis) and did an oral glucose tolerance test. Participants were instructed to perform all HIIT sessions at their highest possible average power output during all work intervals. Each workout began with a 10-minute warmup.

Weekly workouts progressed as follows:

Week 1 (Light Training): Two Sessions

  • both HIIT workouts: five bouts of 4 minutes of work performed at 95% of VO2max, alternating with 3 minutes of passive rest

Week 2 (Moderate Training): Three Sessions

  • two HIIT workouts: five bouts of 8 minutes of work performed at 90% of VO2max, alternating with 3 minutes of passive rest
  • one HIIT workout: five bouts of 4 minutes of work performed at 95% of VO2max, alternating with 3 minutes of passive rest

Week 3 (Excessive Training): Five Sessions

  • three HIIT workouts: five bouts of 8 minutes of work performed at 90% of VO2max, alternating with 3 minutes of passive rest
  • two HIIT workouts: five bouts of 4 minutes of work performed at 95% of VO2max, alternating with 3 minutes of passive rest

Week 4 (Recovery): Four Sessions

  • three HIIT workouts: three bouts of 8 minutes of work performed at 90% of VO2max, alternating with 3 minutes of passive rest
  • one HIIT workout: three bouts of 4 minutes of work performed at 95% of VO2max, alternating with 3 minutes of passive rest

Results  

Results of this study showed that following Week 3 (Excessive Training) there was a striking reduction in mitochondrial function, and this coincided with a negative disturbance in glucose tolerance (the body’s ability to absorb and use glucose) and insulin secretion. These data confirm that extreme amounts of HIIT (i.e., five sessions per week at an all-out intensity) are clearly associated with negative physiological outcomes.

Take-Home Message

HIIT’s benefits for cardiometabolic health are best achieved by exercising at moderate to high intensities relative to the client’s fitness level. HIIT performed at extreme levels of training is contraindicated for meaningful health benefits to occur.

See also: HIIT Training for Fast, Efficient Fat Loss

Controversy #3: HIIT Is the Only Cardiovascular Exercise a Person Needs to Do

Women doing HIIT on bikes

From a physiological perspective, there are some shortcomings to doing only HIIT workouts.

Proof-of-Concept HIIT Research Study

Langan, S.P. & Grosicki, G.J. 2021. Exercise is medicine . . . and the dose matters. Frontiers in Physiology, 12 (660818).

Introduction

As HIIT has grown more popular, it has become an obsession for some exercisers—they choose not to do any other forms of cardiovascular training, such as MICT; lactate threshold training; or slow, long-distance endurance exercise. However, from a physiological perspective, Langan & Grosicki report that there are some physiological shortcomings to doing only HIIT workouts.

The Study

Langan & Grosicki compared the physiological adaptations of HIIT and sprint interval training (SIT) with MICT to identify distinctive physiological differences.

The researchers acknowledge that HIIT/SIT and MICT have many overlapping benefits—particularly when it comes to improving cardiorespiratory fitness. Compared with HIIT, however, low- to moderate-intensity training (40%–70% of heart rate reserve) provides a more potent stimulus for peripheral physiological adaptation, which specifically enhances maximal mitochondrial respiration. The lower metabolic byproduct accumulation during low- to moderate-intensity exercise allows for substantially greater exercise duration and reliance on oxidative energy production and mitochondria-derived ATP. This leads to a bigger improvement in exercise economy—meaning that people become more substrate (i.e., fuel) efficient and can maintain steady-state exercise intensities for longer.

The researchers also note that a regimen consisting of only HIIT/SIT may promote an overtraining pathology, resulting in a plateau or even decrement in physiological improvements. Langan & Grosicki suggest that if the desired outcome is better health, it’s wise to adopt a balanced approach that varies intensity by including HIIT and MICT. This is likely to optimize all of the cardiometabolic benefits of training.

Take-Home Message

HIIT is a time-efficient exercise program that elicits a powerful stimulus for heart and skeletal muscle adaptations. However, if it is the only cardiorespiratory stimulus a person does, it appears that positive physiological outcomes will diminish, plus the potential for overtraining will grow. Fit pros seeking optimal health and performance benefits for clients should encourage them to perform HIIT and MICT on a regular basis.

See also: HIIT and Memory in Older Adults

Final Thoughts on HIIT Research and Controversies

The idea of varying the exercise stimulus with HIIT and MICT appears to be an important overall take-home message for health that has strong evidence from physiological research in this area. Pushing exercise intensities to the extreme will likely result in some deleterious physiological outcomes. Finding the best exercise balance for every client is an ongoing goal of all fitness pros. This requires up-to-date awareness of scientific findings combined with creative, forward-thinking program design.