HIIT vs Steady State – which one should I do during lockdown?
Tuesday, May 5, 2020. Author FitnessGenes
Tuesday, May 5, 2020. Author FitnessGenes
Although some countries are starting to ease their lockdown restrictions, many of us still face severe limitations on the amount and type of exercise we can do.
In France, for instance, if you fancy going for a walk or run, you’ll have to stay within 1km of your home and keep your outing to under an hour. At the time of writing, cycling for exercise is completely prohibited.
In Moscow, lockdown rules ban jogging and cycling altogether, with Muscovites permitted only to walk pets within 100 metres of their home. Here in the United Kingdom, we’re allowed out once per day for exercise, while gyms and sports clubs remain closed.
Given such constraints on time, space, and equipment, should we be focussing our valuable allotted time on any type of exercise in particular?
Say I’ve only got 20 minutes to spare and I want to burn some fat - should I go for a 20 minute continuous jog, or am I better off doing 10x1 min flat out sprints, with a 1 minute rest after each effort?
What if I’m stuck indoors, but I want to exercise to improve my blood sugar levels? Would it be better to walk up and down stairs continuously without resting, or would I benefit more from strenuous sets of burpees, taking a well-earned breather in between each set? (Presumably, both would equally annoy my downstairs neighbours).
In this article, we take a critical look at the scientific literature comparing two key types of cardio exercise:
As with the How do I boost my immune system? blog, we’ll mainly focus on large scale meta-analyses that pool and standardise the data from multiple controlled trials.
Many of us will have heard of HIIT (High Intensity Interval Training) before.
It involves short (typically less than 4 minutes) spurts of highly intense exercise which are separated by periods of complete rest or less-intense active recovery. A HIIT workout using a stationary bike, for example, might include 30 second intervals of high intensity (fast cadence and high resistance) spinning, each followed by 1 minute of easy (low resistance) cycling.
But, what exactly do we mean by “high intensity” exercise?
Generally speaking, HIIT intervals involve “near maximal” efforts that we perform at over 80-85% of our maximum heart rate (HRmax).
Another way of classifying the intensity of exercise is by comparing it to our body’s maximum rate of oxygen consumption (or VO2 max). High intensity intervals generally push us to over 75-80% of our VO2max.
Moderate-Intensity Continuous Training (MICT) involves a sustained effort, with no intervening rest periods.
An example may be cycling on a stationary bike continuously at a constant pace for 20 minutes.
In contrast to HIIT, MICT workouts are much less intense, with exercise performed at a moderate intensity. This typically corresponds to 55-75% of maximum heart rate (HRmax) or 40-65% of maximal oxygen consumption (VO2max).
If you’ve been anything like me during lockdown, you may have gained a bit of weight. And, unfortunately, I’m not talking about lean muscle mass!
A reduction in total energy expenditure (through less movement) coupled with increased energy intake (as a result of prolonged close proximity to the fridge and snacking out of boredom) will invariably lead to fat deposition and an increase in body fat percentage.
So, what type of workout is better for burning this newly accumulated fat: HIIT or MICT?
First things first: both HIIT and MICT are shown to elicit significant reductions in total absolute fat mass and body fat percentage. It’s an obvious point, but if you’re looking to shed fat and improve your body composition, doing any exercise (either HIIT of MICT) is better than doing nothing at all (or relying on dietary changes alone).
Now to HIIT vs MICT. A meta-analysis published in the British Journal of Sports Medicine (BJSM), which analysed the data from 1,012 subjects across 36 studies, found that HIIT was associated with a 28.5% greater reduction in absolute fat mass compared to MICT. People enrolled in HIIT programs lost 1.58 kg of fat mass on average, whereas those participating in MICT programs lost 1.13 kg.
Despite this finding, the meta-analysis found that, while both HIIT and MICT produce significant reductions in total body fat percentage, there was no significant difference between the groups. Those performing HIIT dropped their body fat percentage by 1.50 percentage points, which was similar to the 1.44% drop in those performing MICT.
Other meta-analyses of overweight and obese subjects also report no difference between HIIT and MICT in terms of improving body composition and body fat percentage. For example, a meta-analysis published in Obesity Reviews reported a 3 cm reduction in waist circumference in both HIIT and MICT groups, with exercise programs involving 3 days per week of exercise and lasting for 10 weeks on average.
Interestingly, however, the evidence seems to be clear that despite similar outcomes on body composition, HIIT requires less time. For instance, one meta-analysis of 22 studies found that MICT sessions lasted 40 minutes on average, compared to just 30 minutes for HIIT.
The BJSM meta-analysis crunched the numbers and concluded that while MICT gives you a 0.0026% reduction in body fat percentage per a minute of exercise, the equivalent figure for HIIT is 0.0050% per minute.
In other words, HIIT is a more time-efficient way of shedding fat.
There are good physiological reasons for this. Higher intensity exercise generally stimulates greater production of hormones such as adrenaline, noradrenaline, and growth hormone, all of which promote fat burning or fat oxidation.
Furthermore, HIIT causes more of something known as excess post-exercise oxygen consumption (EPOC). More commonly referred to as “afterburn,” EPOC occurs during recovery after exercise, when oxygen consumption is elevated above baseline resting levels as the body restores various metabolic processes back to normal. Such processes include the removal of lactate and H+ ions, rebuilding muscle glycogen stores, and oxidation of fat for fuel.
All in all, then, if you’ve only got a limited opportunity for exercise, say 30 minutes, and burning fat is your top priority, then HIIT represents a much better investment of your time.
Bear in mind, however, that by virtue of including high intensity, near-maximal efforts, HIIT workouts place more stress on your body. As such, the risk of injury is higher and you may require greater recovery time between HIIT workouts.
If you like endurance exercise (such as long distance running, cycling, rowing, cross-country skiing), then VO2 max is a parameter you may be monitoring and keen to improve.
Your VO2 max is your maximum rate of oxygen consumption and is a useful indicator of your cardiorespiratory fitness. It essentially is a measure of your body’s ability to take in oxygen in the lungs and distribute it to exercising muscles using the heart and cardiovascular system.
Generally speaking, a higher VO2 max means a better ability to supply working muscles with oxygen and greater aerobic fitness. A higher VO2 max, however, isn’t necessarily a guarantee of better endurance performance, as several other factors (e.g. lactate threshold, biomechanics) are important.
Nevertheless, if your main focus it to improve VO2 max, then the evidence suggests that HIIT workouts are likely to benefit you more than MICT.
For example, one meta-analysis of subjects with prediabetes and diabetes found that HIIT was linked to a 3.02 ml/kg/min higher increase in VO2 max compared to MICT.
More specifically, a meta-analysis of overweight and obese subjects suggested that high-intensity intervals of 2 minutes or longer are best for increasing VO2 max. This is because these longer bouts place greater demands on the heart and lungs and stimulate adaptations that increase the stroke volume of the heart: - the volume of blood ejected by the left ventricle with each contraction.
“Metabolic health” is a broad term that covers, among other things, how effectively your body regulates the metabolism of fat and sugar.
If you’re metabolically healthy, you’ll likely have healthy blood levels of sugar (glucose), fat (triglycerides), and cholesterol (including ‘bad’ LDL cholesterol and ‘good’ HDL cholesterol).
Good insulin function, a normal waist circumference and not carrying excessive amounts of visceral fat are also central to good metabolic health.
The above factors are all known to influence your risk of cardiovascular disease (e.g. stroke and heart attack) and Type II diabetes. The good news is that both HIIT and MICT have a beneficial effect on many these factors.
But, is one better than the other in terms of improving metabolic health?
Your blood lipid profile is a measure the amount of fat and cholesterol circulating in different particles in your bloodstream. (For a good overview of the transport and metabolism of fat, please visit your Blood Fat Levels (ApoA5) Trait).
Both HIIT and MICT have been shown to improve aspects of blood lipid profile, including blood triglycerides, total cholesterol, LDL cholesterol and HDL cholesterol.
In terms of which exercise program is better, most studies have found no significant difference between HIIT and MICT on lipid profile.
Fasting blood glucose
Your fasting blood glucose level is a measure of how well your body produces and responds to the hormone insulin. Elevated fasting blood glucose levels may indicate poor production and/or sensitivity to insulin. Poor sensitivity of tissues to insulin is also known as insulin resistance. (You can read more about this in your Fasting blood glucose level trait).
Exercise is an excellent means of improving the sensitivity of your tissues to insulin and therefore improving blood glucose levels. But, is HIIT or MICT better in this regard?
The scientific literature reveals mixed findings.
Some meta-analyses have found no significant difference between HIIT or MICT in terms of improving fasting blood glucose levels or related parameters such as insulin-resistance (HOMA score) and HbA1c (a measure of long-standing blood glucose levels and diabetic control).
By contrast, a 2015 meta-analysis of 50 studies found that HIIT was superior to MCIT in terms of improving insulin resistance scores. This finding may possibly stem from the greater effects of high intensity exercise on improving insulin sensitivity in skeletal muscles.
In this respect, it’s thought that HIIT typically recruits more muscle fibers and depletes muscle glycogen stores to a larger extent than MICT. In response, muscles may adapt by producing more glucose transporter proteins (GLUT4) that facilitate the entry of glucose into cells when stimulated by insulin. This improves the insulin sensitivity of muscles.
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