To HIIT or not to HIIT?
HIIT definitely had its time in the limelight. But is it really what’s best for your busy, high stress lifestyle? Unfortunately the answer is… it depends.!
What is HIIT?
High intensity interval training (HIIT) consists of periods of maximal intensity work followed by periods of active or passive recovery, repeated for a certain number of sets. It is popular because you can match or exceed the benefits gained from lower intensity continuous work in shorter amounts of time. HIIT can burn more calories and increase post-exercise fat oxidation and energy expenditure compared to steady state exercise. Because sessions are short and high intensity, it can be an effective way to make strength gains. It also has beneficial effects on cholesterol levels, cardiovascular fitness, blood pressure and blood sugar control.
So what’s the problem?
Sounds sweet as, right? All these health benefits and you’re only doing 20 minutes of work! Buzzing. The important thing is, although HIIT allows you to train for less time, it doesn’t mean that replacing all of your workouts with HIIT is going to see you get even better results. The recovery time in between sets and sessions is essential for you to see progression. It is during this time that your body repairs itself to come back even stronger than before. If you overtrain, your muscles won’t repair and grow and your cardiovascular fitness will actually decline.
Too much HIIT can cause:
Fatigue
Maladaptation to stimuli (reduced gains in response to exercise)
Reduced physical performance
Suppressed maximal attainable heart rate
Reduce mitochondrial respiration
Reduced glucose tolerance
Reduced immune function
Increased injury risk
HIIT increases stress hormones
Studies show that exercising at moderate to high intensities (above 60% VO2max) increases circulating levels of cortisol, which is a stress hormone. As the work intensity increases, so does the cortisol. Chronic high cortisol levels can lead to more fat storage around the waist, increased appetite, fatigue, inflammation, digestive issues and poor sleep. Which are obviously not the desired effects of working out. If you train too hard without sufficient recovery, are under higher levels of stress than normal, have anxiety, aren’t getting enough sleep, have been on the booze, smoke, or anything else that might put your nervous system under stress, your cortisol levels can remain spiked following a HIIT workout, making the workout counterproductive.
In contrast, lower intensity exercise (at 40% VO2max) actually reduces circulating cortisol levels. This is why restorative, gentle exercise should be an essential part of your lifestyle.
The recommendations:
Research suggests that detrimental effects begin with anything over 90 minutes of HIIT a week. These sessions work well split into a maximum of 3 x 20 minutes, with at least a day rest in between. HIIT should involve only ∼8–16 min of actual “work time” for vigorous-intensity exercise, and you need to be working HARD for the benefits to occur, at around 80-95% maximum heart rate. The rest period should be sufficient enough for your heart rate to come down to around 55-65% maximum heart rate, so for example, beginners could start at a work:rest ratio of 1:3, and as your fitness increases, the ratio can decrease.
If you are feeling stressed, unwell or dealing with any other of the stressors mentioned above, it may be beneficial for you to skip the HIIT and go for a gentle cycle, walk or do some yoga instead. Don’t punish your body any more by doing a million burpees!
How do I know if my body is up for HIIT or not?
Most health experts would say ‘listen to your body’, but how easy is this really? You may feel tired and stressed and a high intensity session in the gym is actually just what you need to boost your energy and release the stress.
This is where research grade tracking of your nervous system health comes in. You can directly measure your physiological stress and recovery state to understand when you have the resources to tackle a high intensity session, or if a Yoga class would be more beneficial.
Become part of The Wellness Revolution Programme to discover how your lifestyle is impacting your nervous system health and work with me to restore balance, create endless energy and optimal health. Book your Free Discovery Call here.
Flockhart, M., Nilsson, L.C., Tais, S., Ekblom, B., Apró, W. and Larsen, F.J., 2021. Excessive exercise training causes mitochondrial functional impairment and decreases glucose tolerance in healthy volunteers. Cell Metabolism.
Shiraev, T. and Barclay, G., 2012. Evidence based exercise: Clinical benefits of high intensity interval training. Australian family physician, 41(12), pp.960-962.
Hill, E.E., Zack, E., Battaglini, C., Viru, M., Viru, A. and Hackney, A.C., 2008. Exercise and circulating cortisol levels: the intensity threshold effect. Journal of endocrinological investigation, 31(7), pp.587-591.