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HIIT Controversies, Unraveled and Explained
Len Kravitz, PhD

High-intensity interval training (HIIT) involves alternating work bouts (8 seconds to 8 minutes) of high-intensity movement with low-intensity recovery periods (variable time length). HIIT has become a stalwart exercise programs in the fitness industry to improve a person's cardiorespiratory fitness level, effectively burn a lot of calories in a relatively short amount of time, and improve metabolic markers of health (Wewege et al, 2018). However, along with the popularity of HIIT are also some controversies surrounding this method of training. This research column will take an evidence-based approach to bring clarity to three of the current controversies surrounding HIIT. The specific method used to answer these debates is referred to as a 'Proof to 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 Study:
Wewege, M.A., Ahn, D., Yu, J. 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:e009305. DOI: 10.1161/JAHA.118.009305

Wewege et al. (2018) note that cardiovascular disease (CVD) includes persons with 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, safety concerns have been expressed by fitness professionals and exercise physiologists about the efficacy of using this high-intensity exercise stimulus with the CVD population.

The Study:
Wewege et al. (2018) completed a systematic review, specifically investigating whether HIIT is a safe exercise program for persons with either heart failure or coronary heart disease. Their review included 23 studies with a total of 1,117 participants comparing HIIT to MICT. The HIIT programs included studies with high intensity work bout durations up to 4 minutes, with an intensity of &Mac179;85% of heart rate (HR) peak or &Mac179;80% peak aerobic capacity, or a ratings of perceived exertion (RPE) &Mac179;15 (on a 6 to 20 RPE scale). The MICT programs included continuous aerobic exercise at 60% to 75% of HR peak, or 50%-65% peak aerobic capacity, or 12-15 ratings on the RPE scale.
Results: There was only one major cardiovascular-related event among participants with HIIT in this review. This was calculated to be 1 event per 11,333 training hours, which is very low. After reviewing the data a multitude of ways, the researchers concluded that the “rewards” of applying HIIT with patients with CVD are reasonably compelling, and the accumulated evidence suggests it is an acceptable option to traditional MICT. Additionally. there is less risk for persons who do not have complex CVD health issues, and for those who are deemed to be low cardiovascular risk. Furthermore, the researchers concluded there is no evidence to suggest that HIIT is inherently unsafe for any specific CVD patient characteristics (including older age persons).

Take Home Message
For persons with cardiovascular disease, it is fine to do HIIT at a suitable level to their health status. Wewege et al. (2018) recommend fitness pros regularly monitor the client's physiological responses during (with HR and RPE) and immediately after (i.e., blood pressure) a HIIT session.

Controversy #2: HIIT Impairs Mitochondrial Function and Decreases Glucose Tolerance
Proof of Concept Study
Flockhart, M., Nilsson, L.C., Tais, S. et al. (2021). Excessive exercise training causes mitochondrial functional impairment and decreases glucose tolerance in healthy volunteers. Cell Metabolism, 33, 1-14.

Cardiovascular exercise positively affects metabolic health through improved glucose regulation and increased oxidative capacity (Flockhart et al. 2021). Mitochondria are small organelles found within some of the body's cells that are primary sources of ATP synthesis, providing the necessary fuel for exercise and activities of daily living. HIIT training has been shown to increase the content of mitochondria (Jacobs et al., 2013), as well as improve insulin sensitivity (Richards et al., 2010), resulting in better glucose regulation and prevention of type 2 diabetes. However the upper limit to where these positive benefits may occur has been a controversial source of argument, and was a primary purpose of this 4-week HIIT study by Flockhart et al.

The Study: Flockhart and colleagues recruited 11 physically active adults (6 females age 25yr and 5 males age 31yr) who were not exercising more than 5hrs a week, nor were they doing HIIT in their exercise training. All study workout trials (completed on a cycle ergometer) were spread out throughout the week, and supervised by professionals with extensive backgrounds in performance testing. At the end of each week's of trials, the researchers collected biopsy samples of muscle (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. Weekly workouts progressed as follows:
Week 1 (called Light Training): Two Sessions
Both HIIT Workouts: 5 bouts of 4min work performed at 95%VO2max alternating with 3min of passive rest

Week 2 (called Moderate Training): Three Sessions
Two HIIT Workouts: 5 bouts of 8min work performed at 90%VO2max alternating with 3min of passive rest
One HIIT Workout: 5 bouts of 4min work performed at 95%VO2max alternating with 3min of passive rest

Week 3 (called Excessive Training): Five Sessions
Three HIIT Workouts: 5 bouts of 8min work performed at 90%VO2max alternating with 3min of passive rest
Two HIIT Workouts: 5 bouts of 4min work performed at 95%VO2max alternating with 3min of passive rest

Week 4 (called Recovery): Four Sessions
Three HIIT Workouts: 3 bouts of 8min work performed at 90%VO2max alternating with 3min of passive rest
One HIIT Workout: 5 bouts of 4min work performed at 95%VO2max alternating with 3min of passive rest
Results: Results of this study showed that following Week 3 (Excessive Training) there was a striking reduction in mitochondrial function that coincided with a negative disturbance in glucose tolerance (the ability of the body to absorb and use glucose) and insulin secretion. These data clearly confirm that extreme amounts of HIIT (i.e., 5 sessions/week at an all-out intensity) are clearly associated with negative physiological outcomes.

Take Home Message
The benefits of HIIT for improved cardiometabolic health are best achieved by completing moderate to high intensities of exercise, that are relative to the fitness level of the client. HIIT performed at extreme levels of training is contraindicated for meaningful health benefits to occur.

HIIT Controversy #3: HIIT is the only Cardiovascular Exercise a Person Needs to Do
Proof of Concept Study
Langan, S.P. and Grosicki, G.J. (2021). Exercise is medicine...and the dose matters. Frontiers in Physiology 12:660818. doi: 10.3389/fphys.2021.660818

The popularity of HIIT, for many people, has become more like an obsession in training, with these exercisers choosing not to do any other forms of cardiovascular training, such as MICT, lactate threshold training, or long, slow distance endurance exercise. However, from a physiological perspective, there are some physiological shortcomings if one only chooses to do only HIIT workouts (Langan and Grosicki, 2021).

The Study: Langan, S.P. and Grosicki (2021) compared the physiological adaptations of HIIT and Sprint Interval Training (SIT) versus MICT, to identify distinctive physiological differences. Initially, the researchers acknowledge that HIIT/SIT and MICT have many overlapping, mutual good benefits. This is particularly true when it comes to improving cardiorespiratory fitness. However, uniquely, low-to-moderate intensity training (40% to 70% of heart rate reserve) provides a more potent stimulus for peripheral physiological adaptation, which specifically will enhance maximal mitochondrial respiration (Langan and Grosicki). 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 greater increase in exercise economy, thus being able to exercise at steady state exercise intensities longer, and becoming more substrate (i.e., fuel) efficient (Langan and Grosicki). The researchers also note that a regimen consisting of only HIIT/SIT may promote an overtraining pathology, attaining a plateau or even decrement in physiological improvements. Langan and Grosicki continue that if improved health is the desired outcome, adopting a balanced training intensity distribution with varying intensities (i.e., doing HIIT and MICT) is likely the preferred approach to optimally develop all of the cardiometabolic benefits.

Take Home Message
HIIT is indeed 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 there is a diminishing of positive physiological outcomes, and a potential for overtraining. Fitness pros seeking the optimal health and performance benefits for clients should encourage clients to perform HIIT and MICT on a regular basis.
Final Thoughts on Controversies of HIIT: Lessons Learned
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.

Bio:Len Kravitz, PhD, CSCS, is the program coordinator of exercise science at the University of New Mexico, where he received the Outstanding Teacher of the Year and Presidential Award of Distinction.

Additional References:
Jacobs, R.A., Fluck, D., Bonne, T.C. (2013). Improvements in exercise performance with high-intensity interval training coincide with an increase in muscle mitochondrial content and function. Journal of Applied Physiology, 115, 785-793.

Richards, J.C., Johnson, T.K. Kuzma, J.N., et al. (2010). Short-term sprint interval training increases insulin sensitivity in healthy adults but does not affect the thermogenic response to beta-adrenergic stimulation, Journal of Physiology, 588: 2961-2972