Key Points:

• For sedentary populations, HIIT is the most time efficient – not necessarily the best – training type to increase aerobic and anaerobic capacity
• Elite athletes can include HIIT training to improve their lactate response and improve performance even after hitting a VO₂max plateau
• There remain many questions about how HIIT works, best practices, and its real-world applicability

High-intensity interval training (HIIT) consists of short, repeated bursts of intense exercise alternating with rest or low-intensity breaks. It is a style of training that has been used for centuries to maximize athletic performance, but has gained immense popularity in recent years as a more time-efficient avenue to improved health and fitness. Although the details are not as simple as that, HIIT research findings have implications for all, from the sedentary population to elite athletes.

Researchers have compared the outcomes of HIIT exercise to those of conventional moderate-intensity continuous training (MICT) in two ways. The idea of a matched-work set-up is to “match” HIIT and MICT exercise protocol factors like average intensity, duration, and total distance so that the estimated energy expenditure is the same among both the HIIT and MICT groups. On this basis, HIIT is effective as an alternative to MICT, producing comparable, or even superior, performance enhancements in both healthy and diseased populations. Studies have also looked at low-volume HIIT versus MICT. Involving significantly less time and exercise volume than the MICT counterpart, low-volume HIIT is an area that is especially interesting for its application to the general population. Although there is less known about low-volume HIIT, emerging evidence indicates that it, too, is successful in stimulating physiological changes similar to those induced by MICT.

General Population

Lack of time is consistently listed as one of the most common barriers to engaging in regular exercise. As such, the finding that low-volume HIIT can induce similar cardiovascular and skeletal muscle improvement as MICT, with 90% less training volume, is exceptional. It means that, in theory, exercise and its associated health benefits are more accessible to a largely inactive population. In sedentary adults, HIIT is the most time efficient in increasing aerobic and anaerobic capacity – although it should be noted that it is not necessarily superior to traditional exercise training. Beyond being an interesting variation on a daily workout, HIIT could become a key therapy for both preventing and treating diseases linked to obesity and inactivity.

Endurance Athletes

For well-trained endurance athletes, HIIT can be used to enhance performance. Even though individual HIIT intervals are so short that they principally use anaerobic energy systems, multiple studies show that HIIT improves aerobic capabilities, and thus performance, in elite endurance athletes. In a study investigating the differences between HIIT and high-volume long slow distance training (LSD) on rowing performance changes, the HIIT-trained group had notably higher improvements in maximal oxygen uptake (VO₂max), in their work rate at lactate threshold, and in their VO₂ at lactate threshold. Endurance performance success is highly associated with an individual’s lactate threshold – more so than one’s VO₂max. The development of the athlete’s lactate response and the improved work rate at VO₂max suggests that HIIT training could be a valuable tool for elite athletes who have hit a VO₂max plateau: it is possible to keep increasing maximal work rate even if the VO₂max remains unchanged. Rather than switch over to exclusively using HIIT, it is likely optimal to combine endurance training and HIIT to achieve the best aerobic improvements, including VO₂max, time to exhaustion, and peak power output.

While there are tremendous positive results affiliated with HIIT, it may not always be the most practical exercise modality to implement. Adherence is one of the biggest questions: is HIIT a long-term solution and does it have real-world applicability? Individuals need to be highly motivated to put in the intense effort required for HIIT to be effective.

It is also important to keep in mind that there remains much to be understood. With variables like frequency, duration of intervals, intensity, recovery time, and duration of the exercise program, there is yet to be a standardized HIIT protocol that yields the best results. The precise agents responsible for the performance improvements are also unknown. Researchers have suggested increased mitochondrial enzyme activity, reduced lactate accumulation, improved ventilatory threshold, and increased buffering capacity, among multiple other mechanisms. As well, previous studies exploring the impact of HIIT have only been short-term interventions, but long-term ones are necessary to gain a greater understanding on its applicability to clinical cohorts, as well as its sustainability as part of a regular exercise training regimen.

McMaster University’s Dr. Martin Gibala, an expert in HIIT research, likens HIIT to an emerging drug on the market and the currently prescribed 150 minutes of moderate-to-vigourous aerobic exercise per week as the accepted drug of choice. Like in the world of pharmacology, there needs to be extensive randomized clinical trials, comparing a multitude of factors, to conclusively establish the extent of HIIT benefits and suitability.

Related reads from SIRC:

Building training intensity with HIIT (Blog)

All About CrossFit – A Beginner’s Guide (Blog)

High Intensity Interval Training (Blog)

Sources:
Foster C, Farland CV, Guidotti F, Harbin M, Roberts B, Schuette J, Tuuri A, Doberstein ST, Porcari JP. The effects of high intensity interval training vs steady state training on aerobic and anaerobic capacity. Journal of Sports Science and Medicine. 2015; 14(4): 747-755.

Gibala M. (2014). High-intensity interval training: Is it really the holy grail of exercise? The Globe and Mail.

Gibala MJ, Little JP, MacDonald MJ, Hawley JA. Physiological adaptations to low-volume, high-intensity interval training in health and disease. Journal of Physiology. 2012; 590(5): 1077-1084.

Gillen JB, Gibala MJ. Is high-intensity interval training a time-efficient exercise strategy to improve health and fitness? Applied Physiology, Nutrition, and Metabolism. 2014; 39(3): 409-412.

Hutchinson A. Lactate Threshold and VO2 Max Explained. Active.

Ni Chéilleachair NJ, Harrison AJ, Warrington GD. HIIT enhances endurance performance and aerobic characteristics more than high-volume training in trained rowers. Journal of Sport Sciences. 2017; 35(11): 1052-1058.

Nicolò A, Girardi M. The physiology of interval training: a new target to HIIT. Journal of Physiology. 2016; 594(24): 7169-7170.

Stöggl T, Sperlich B. Polarized training has greater impact on key endurance variables than threshold, high intensity, or high volume training. Frontiers in Physiology. 2014; 5(33): 1-9.

About the Author: Lily is a fourth-year student in the kinesiology program at Western University. With a background in synchronized swimming, she continues to be actively involved in the sport as a coach and varsity athlete.

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The information presented in SIRC blogs and SIRCuit articles is accurate and reliable as of the date of publication. Developments that occur after the date of publication may impact the current accuracy of the information presented in a previously published blog or article.