HIITing the gym soon?

Here's what you should know about HIIT training.

While it is widely known that the majority of Americans do not meet the physical activity guidelines of 150 minutes per week of moderate intensity activity, those that do exercise have a host of training styles to choose from. It can be incredibly overwhelming to know what the “best” way to train might be. Plus, this is dependent on one’s access to a safe space to exercise, equipment, schedule, training experience, and personal interests. 

Exercise can generally be grouped into two types: high intensity interval training (HIIT) and moderate intensity continuous training (MICT). The National Strength and Conditioning Association defines HIIT as “repeated high intensity exercise bouts interspersed with brief recovery periods” (NSCA, 2021, p. 568). Moderate intensity continuous training is what most would think of when they think about standard physical activity: walking, bicycling, and other activities that are typically aerobic in nature.

HIIT came on the scene in the early 2000s and according to the ACSM (Thompson, 2021) has consistently ranked in the top 10 fitness trends since 2014. It characteristically consists of a short bout of high intensity movement (obviously), but think close to maximum effort) followed by a recovery period. NSCA recommends a work/rest ratio of 1:1 for near maximal intervals. For intervals at maximal effort, guidelines indicate a work/rest of 1:5 (NSCA, 2021). 

HIIT has been sold as a way to maximize your time in the gym, for those with little of it, and coincides with the whole “no pain, no gain” mentality that we often expect from the gym elite. However, more is not always more.

The fitness collective has seemingly over-hyped HIIT as the best way to train, but we have failed to educate clients on how to appropriately incorporate it into a well-balanced training program.

HIIT is tough, physiologically taxing and demands much from the body. This is why it’s only recommended to be used 1-2x/week (NSCA, 2021). Read that again. 1-2x / week. Not 3, 5, or 7. Cortisol levels in the body increase as exercise intensity increases (Hill et al., 2008). Increased cortisol levels, coupled with continuously taxing training sessions lead to overtraining. If you note changes in your sleep, mood, immune system, and levels of fatigue, you may be experiencing symptoms of overtraining.

While all of that may seem scary, the benefits of correctly utilized HIIT exercise are plentiful. HIIT is promoted for its “afterburn” or excess post-exercise oxygen consumption. What’s even more interesting is that the effects of HIIT last longer than MICT even when a break in training occurs, which is common for virtually every human dealing with life’s stressors (Gripp et al., 2021). 

Despite its name, HIIT is not reserved for elite athletes, fitness gurus, and highly trained individuals. It’s safe for children and adolescents and has been demonstrated in numerous studies to improve heart health in youth (Cao et al., 2019) . It’s also demonstrated efficacy in improving the health of overweight and obese adults and those diagnosed with metabolic syndrome (Poon et al., 2024). For those living with chronic disease, it improves many cardiometabolic markers, including waist circumference, blood pressure, cholesterol,  fasting blood glucose (Poon et al., 2024) and VO2max, as demonstrated when training intervals were greater than two minutes (Su et al., 2019).

Since its inception, there have been nuanced definitions of HIIT. Intervals as a whole can be submaximal (77+% max heart rate MHR), near maximal (90%+  MHR), or maximal (>95% MHR). HIIT intervals involve work that’s above the moderate intensity threshold. It also includes a subset of training known as Sprint Interval Training (SIT) which is defined as “all out”, maximal effort (Coates et al., 2023). For those working on general fitness and less concerned about various thresholds, all of these would be considered vigorous effort. 

Short SIT, where bouts of exercise are <10 seconds, have demonstrated improvements in a matter of weeks. What’s more exciting is that this still applies when non-conventional modes of exercise like boxing, paddling, and functional exercises are used (Boullosa, 2022). So, if you hate running, or you’re simply not a sprinter, you can utilize another mode of training to reap these benefits. 

Despite my love for running, I still prefer to train at my highest intensities on a stationary bike.

1. It’s a safer, more controlled environment that reduces my risk for injury because it’s non-weight bearing.

2. I can track my metrics.

3. Form is less of an issue for me. I, personally, have much stronger training sessions on the bike because I’ve been teaching cycle classes for 10+ years.  

Of note, Bonafiglia et al. (2022) published a meta-analysis highlighting these issues specific to VO2max related research. One commonly cited issue with these types of training studies is the short duration of the actual intervention. An eight or twelve week study in chronically diseased or unfit individuals would likely show significant improvements, given their untrained status. 

For a longitudinal examination in older adults, the Generation 100 study (Stensvold et al., 2020) followed approximately 1500 older adults (70-77yo) over the course of five years. Subjects were assigned to one of three groups where they either: followed the standard PA guidelines, performed MICT, or  incorporated 2 sessions of HIIT per week. The HIIT group demonstrated a nearly 3% reduction in all-cause mortality compared to their MICT counterparts. Protective effects were shown in all groups due to the volume of  physical activity sustained over five years, but perhaps the most important takeaway from this study is that older adults can, and should, incorporate HIIT into their exercise regimen.

Want to get started with HIIT training?

Let’s simplify this for practical application. 

1. Before incorporating HIIT, you should work to meet the ACSM/CDC (2024) physical activity guidelines: 

   1. 150 minutes moderate activity per week ( i.e. 30 minutes of PA 5 days / week) OR 75 minutes of vigorous activity per week (i.e. 20 minutes of PA 3 days / week) PLUS

   2. 2 days of total body strength training per week

2. Once you’re consistently meeting the PA guidelines, switch one day of aerobic activity to HIIT. Make sure that you don’t have a heavy resistance training day the following day, and consider moving one of your weekly rest days to the day following your HIIT session

   1. After 2-3 months of adopting this training schedule, provided you’re feeling well and have no injuries or related training issues, you can replace another aerobic session (or a resistance training session) with a 2nd HIIT session. However, HIIT should only constitute 10-30% of your weekly training volume (Coates et al., 2023). 

   2. The key here is ensuring that you can adequately recover from the HIIT days, while maintaining the rest of your training volume, AND stay injury free. 

3. HIIT sessions can follow a 1:1 work rest ratio. The majority of us aren’t training in the severely high intensity realm. Most athletes don’t get close to that, either. 

   1. 30 second intervals are an easy place to start, and follow. 30 seconds of work at vigorous intensity, followed by 30 seconds of rest. Repeat 8x and that’s 1 set. Add in a 2-3 minute break between sets, and try for 2-3 sets in all. 

4. Recover well. Actually recover and resist the urge to train hard the following day. 

Everybody starts somewhere. The key is just starting. 

References

American College of Sports Medicine. (2024). Physical activity guidelines. https://www.acsm.org/education-resources/trending-topics-resources/physical-activity-guidelines

Bonafiglia, J. T., Islam, H., Preobrazenski, N., & Gurd, B. J. (2022). Risk of bias and reporting practices in studies comparing VO2max responses to sprint interval vs. continuous training: A systematic review and meta-analysis. Journal of Sport and Health Science, 11(5), 552–566. https://doi.org/10.1016/j.jshs.2021.03.005

Boullosa D, Dragutinovic B, Feuerbacher JF, Benítez-Flores S, Coyle EF, Schumann M. (2022). Effects of short sprint interval training on aerobic and anaerobic indices: A systematic review and meta-analysis. Scandinavian Journal of Medicine and Science in Sports, 32(5), 810-820. doi: 10.1111/sms.14133. 

Cao, M., Quan, M., & Zhuang, J. (2019). Effect of high-intensity interval training versus moderate-intensity continuous training on cardiorespiratory fitness in children and adolescents: A meta-analysis. International Journal of Environmental Research and Public Health, 16(9), 1533. https://doi.org/10.3390/ijerph16091533

Coates, A. M., Joyner, M. J., Little, J. P., Jones, A. M., & Gibala, M. J. (2023). A perspective on high-intensity interval training for performance and health. Sports Medicine, 53(1), 85–96. https://doi.org/10.1007/s40279-023-01938-6

Costa, E.C., Hey, J., Kehler, D., Boreskie, K., Arora, R., Umpierre, D., Scwajczer, A., Duhamel, T. (2018) Effects of high-intensity interval training versus moderate-intensity continuous training on blood pressure in adults with pre- to established hypertension: a systematic review and meta-analysis of randomized trials. Sports Medicine, 48, 2127–2142. https://doi.org.10.1007/s40279-018-0944-y

Gripp, F., Nava, R. C., Cassilhas, R. C., Esteves, E. A., Magalhães, C. O. D., Dias-Peixoto, M. F., de Castro Magalhães, F., & Amorim, F. T. (2021). HIIT is superior than MICT on cardiometabolic health during training and detraining. European Journal of Applied Physiology, 121(1), 159–172. https://doi.org/10.1007/s00421-020-04502-6

Hill, E.E., Zack, E., Battaglini, C., Viru, M., Viru, A., Hackney, A.C. (2008). Exercise and circulating Cortisol levels: The intensity threshold effect. Journal of Endocrinological Investigation, 31, 587–591 https://doi.org/10.1007/BF03345606

Poon, E. T., Wongpipit, W., Li, H. Y., Wong, S. H., Siu, P. M., Kong, A. P., & Johnson, N. A. (2024). High-intensity interval training for cardiometabolic health in adults with metabolic syndrome: a systematic review and meta-analysis of randomised controlled trials. British Journal of Sports Medicine. Online.  https://doi.org/10.1136/bjsports-2024-108481

Stensvold, D., Viken, H., Steinshamn, S. L., Dalen, H., Støylen, A., Loennechen, J. P., Reitlo, L. S., Zisko, N., Bækkerud, F. H., Tari, A. R., Sandbakk, S. B., Carlsen, T., Ingebrigtsen, J. E., Lydersen, S., Mattsson, E., Anderssen, S. A., Fiatarone Singh, M. A., Coombes, J. S., Skogvoll, E., Vatten, L. J., … Wisløff, U. (2020). Effect of exercise training for five years on all cause mortality in older adults-the Generation 100 study: randomised controlled trial. BMJ, 371(3485). https://doi.org/10.1136/bmj.m3485

Su, L., Fu, J., Sun, S., Zhao, G., Cheng, W., Dou, C., & Quan, M. (2019). Effects of HIIT and MICT on cardiovascular risk factors in adults with overweight and/or obesity: A meta-analysis. PloS one, 14(1), e0210644. https://doi.org/10.1371/journal.pone.0210644

Thompson, W.R. (2021). Worldwide survey of fitness trends for 2021. ACSM’s Health & Fitness Journal, 25(1), 10-19. https://doi.org/10.1249/FIT.0000000000000631