By Christian C. Evans

From Fall 2017 of USA Triathlon Magazine


HIGH-INTENSITY INTERVAL TRAINING (HIIT) is a technique that has been used extensively to train athletes as well as for improving fitness and outcomes for people with medical conditions ranging from diabetes to heart disease. HIIT typically involves six to 10 short duration (10-60 seconds), super-high-intensity efforts (all-out or near 100 percent heart rate or VO2 Max) with a longer rest bout in between. Overall, HIIT is considered safe and effective, but is it better compared to moderate-intensity training for improving fitness and triathlon race performance?

As triathletes and consumers of triathlon products and media, we rely mainly on the word of manufacturers, athletes, and coaches to make decisions about the intensity and frequency of training. If there were no scientific literature available, then those sources would be appropriate. But over the past 30-40 years, there has been a lot of research on moderate intensity aerobic training and HIIT. The purpose of this article is to provide a brief overview of the scientific literature evaluating HIIT and advice about its use.

Fortunately, several systematic reviews (studies that objectively evaluate many individual studies and draw a conclusion on the overall effectiveness of a technique) have already been performed that have examined HIIT. Five recent reviews were identified that addressed the question “is HIIT effective in improving fitness?” Four compared HIIT to moderate – intensity exercise for fitness and one compared the two techniques on body composition.

Overall, these systematic reviews included subjects who were both adolescents and adults (range 11- 45 years old), who ranged from healthy, untrained to recreational and competitive athletes. The modes of exercise included running and cycling. The intensity of the HIIT varied from 70 percent to 175 percent of the pre-training max work or VO2 or “all-out” effort.

As expected, VO2 Max increased robustly after HIIT training in each of the reviews. Compared to moderate-intensity exercise, HIIT led to further gains, ranging from 8-20 percent greater with HIIT. Based on just these findings, HIIT would seem to have an advantage in terms of effectiveness. However, in many of these studies, the HIIT protocol overlapped with moderate-intensity aerobic workouts performed on the off days. Most of the authors concluded that an important moderator of the effect of HIIT was the overall volume of exercise (i.e., HIIT added to the total volume or the HIIT protocol was itself a high volume training stimulus). Therefore, it is possible that the increased VO2 Max with HIIT was due to increased training volume and not the specificity of the training protocol. In terms of changing body composition, only one study addressed this (Costigan et al., 2015). This study suggested a marginally greater effectiveness of HIIT on BMI compared to a review of moderate-intensity aerobic exercise for weight-loss in adolescents.

A few words of caution are worth mentioning about HIIT. First, the age of the subjects in these studies was skewed toward younger people. Second, the long-term feasibility of participating in HIIT has not been determined. The Costigan et al. study reported 40-100 percent retention rate; whereas, Sloth et al. reported 95-100 percent compliance and a low dropout rate. A third issue is a misconception about time savings with HIIT. While time spent doing HIIT intervals may yield greater improvement in fitness compared to the time spent performing an aerobic exercise, the full HIIT protocol time (intervals plus rest plus additional aerobic training to get the full effect), may actually be greater. A fourth issue is the actual effectiveness of HIIT on triathlon race performance. Many HIIT protocols involve very short intervals (10-30 seconds), whereas triathlons are longer events (even a sprint is typically greater than one hour). No studies have actually investigated the effectiveness of HIIT in increasing performance at a long duration race.

While HIIT offers a potential way to increase cardiovascular fitness above moderate-intensity aerobic exercise, realizing these gains may require additional time and the potential for injury, especially in older athletes, is not well established. Furthermore, the effectiveness of HIIT for improving triathlon race performance is unknown.

CHRISTIAN C. EVANS, PT, PH.D., is a licensed physical therapist. Evans has been a competitive triathlete for over 35 years, has competed in races ranging from sprints to IRONMAN and has finished in the top 10 in his age group at USA Triathlon Age Group Nationals.


Bacon, A. P., Carter, R. E., Ogle, E. A., & Joyner, M. J. (2013). VO2Max trainability and high intensity interval training in humans: a meta-analysis. PloS one, 8(9), e73182.

Costigan, S. A., Eather, N., Plotnikoff, R. C., Taaffe, D. R., & Lubans, D. R. (2015). High-intensity interval training for improving health-related fitness in adolescents: a systematic review and meta-analysis. Br J Sports Med, bjsports-2014.

Gist, N. H., Fedewa, M. V., Dishman, R. K., & Cureton, K. J. (2014). Sprint interval training effects on aerobic capacity: a systematic review and meta-analysis. Sports Medicine, 44(2), 269-279.

Milanovic, Z., Sporiš, G., & Weston, M. (2015). Effectiveness of high-intensity interval training (HIT) and continuous endurance training for VO2Max improvements: a systematic review and meta-analysis of controlled trials. Sports Medicine, 45(10), 1469-1481.

Sloth, M., Sloth, D., Overgaard, K., & Dalgas, U. (2013). Effects of sprint interval training on VO2max and aerobic exercise performance: a systematic review and meta-analysis. Scandinavian Journal of Medicine & Science in Sports, 23(6).

Visit www.usatriathlon.org/quicktraining for more from Lisi Bratcher, a USA Triathlon Level I Certified Coach.

About the Author Michael Lantz (Big Papa)

The Wellness Warrior™; Health & Leadership/Business Coach, Speaker, Blogger, Author, Ironman Triathlete Helping others live with more health and joy, paying for their dreams and make a difference in the world! Learn more: http://HealthIsAHabit.live