Is HIIT harmful? Can it damage your health and fitness? It all depends how you use it…
- High Intensity Interval Training (HIIT) has become a popular training method for squeezing in a workout in a small amount of time.
- Failure to balance high and low intensity training methods can hurt your fitness and your health. HIIT is a powerful tool to be respected.
- HIIT, performed properly, can improve several markers of fitness, both aerobic and anaerobic, heart rate recovery and peak power output. (1)
High Intensity Training has infiltrated almost every corner of the fitness landscape. But is HIIT actually improving, or sabotaging your health and fitness level?
What is HIIT?
High Intensity Interval Training refers to any workout in which you are:
- Working out at greater than 90% of your max heart rate
- Using maximum-intensity periods of work followed by rest periods
- Examples: Interval Sprints, High Intensity Circuits done for 3 minutes with a short rest period between sets.
The Allure of HIIT
HIIT is the preferred method of many with limited time to work out. The rationale is, you can squeeze a quick HIIT workout in 20 minutes and get the same benefit as an hour of a normal workout. For the majority of us with crazy busy lives, HIIT offers a convenient solution.
Many also believe that performing such high intensity training simply must be more beneficial. After all, it’s harder and requires you to really push yourself when compared to an hour of lower intensity training. It’s the widely accepted misconception that more is better.
I hate to be the one to kill a good vibe.. but I feel like it’s my duty to expose this problem so you can hopefully have a better understanding of what you’re actually doing in the gym and to your body and mind.
First off, I have to be clear that I’m not saying that HIIT training is “Bad.” There are actually plenty of studies that show some really amazing results, and I’ll cover that later. But….
To understand the problem with too much HIIT training, you have to understand a few things about your body and how it responds to stress:
- Stress, from anything.. be it work stress, being stuck in traffic, relationship stress, or really intense physical demands.. is registered by the body in a similar way.
- The sympathetic nervous system (fight or flight) is activated in response to stress, dumping stress hormones like adrenaline and cortisol, into the body.
- Keeping your body in a state of constant, chronic stress is the opposite of promoting health and fitness. The body cannot recover, build muscle or improve any markers of fitness when it is in a chronic state of stress.
The main take-away here is that HIIT training is INTENSE… and it takes your body a lot longer than you think to actually fully recover from it. A really high intensity trainings session can take several days to recover from.
What You’re doing to Your Body
Overdosing yourself with too much high intensity training is causing you to accumulate an ever increasing recovery debt. Your body is never fully recovered, and is being forced back into a sympathetic, stress dominant state over and over. Sooner or later, something will give.
The massive amounts of energy being demanded by constant stress will be recruited from other areas.. your immune system, your cognitive function.. and a host of other functions you probably don’t want to mess with. The end result is a reduction in actual fitness and health.
New Research into HIIT and Overtraining
A new study by Les Mills Lab found that 40 minutes a week of HIIT training was optimal for most people. Beyond this amount of high intensity training, most people will not reap additional benefits and will in fact become more fatigued. Read about the study by Les Mills here.
How Much HIIT is Optimal?
The answer to this question depends on your current level of fitness and the nature of the training. Remember that HIIT refers to training at maximum intensity (at least 90% of max heart rate).
For 90% of people out there, there is absolutely no need for more than 2 true high intensity training days per week. Elite athletes rarely ever go beyond 3 days per week, and even then it is only for a short time.
So how much is optimal? Well, if you’re in pretty good shape, 2 days per week would be optimal. If you’re out of shape, 1-2 days per week is where I’d recommend starting. If you feel like you’re in great shape, you can utilize HIIT up to 3 times per week, but definitely keep an eye on your fatigue, and if you’re that serious about your fitness, you can use things like heart rate variability to check in on your body and see if it’s being overstressed.
Check out my article on Heart Rate Variability: https://supastrong.net/2020/01/22/tracking-your-heart-rate-variability-can-change-your-life/
What are The Benefits of Using HIIT?
Now that I’ve sufficiently killed the HIIT vibe, let me backpedal a bit.. To be clear, HIIT is not a bad thing. It is actually a well researched, highly productive form of training.. Studies have shown that you can improve aerobic and anaerobic conditioning, get leaner and stronger utilizing high intensity interval training.
A 2017 study by Frontiers in Physiology found that high intensity interval training was superior to lower intensity training, showing improved:
- Anaerobic power
- Heart rate recovery
- Neuromuscular status (peak power)
So what are we to make of all this? HIIT can cause big improvements in fitness, but can also damage fitness.. what do we do?
Knowledge really is power. HIIT is a powerful tool in your arsenal, with the capacity to dramatically improve your conditioning and fitness. But it is to be respected…
Your training week should include a lower intensity training day for each high intensity day. This way you can reap the benefits of both types of training, without putting your body in a constant state of stress. Lower intensity training promotes recovery and parasympathetic function.
Play the long game. There is no magic pill. Balance high and low intensity training and you’ll make much bigger gains in the long run.
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(1) Jabbal, Arjun & Baxter-Jones, Adam. (2017). Does High Intensity Interval Training Improve Aerobic Power Development More Than Endurance Training?. USURJ: University of Saskatchewan Undergraduate Research Journal. 3. 10.32396/usurj.v3i1.211. https://www.researchgate.net/publication/328590929_Does_High_Intensity_Interval_Training_Improve_Aerobic_Power_Development_More_Than_Endurance_Training
(2) Frontiers in Physiology 02 Aug 2017. “Hiit leads to greater improvements in acute heart rate recovery and anaerobic power as high volume low intensity training.” https://www.frontiersin.org/articles/10.3389/fphys.2017.00562/full