HIIT (high-intensity interval training) is the best way to get the most benefit for your effort. The best bang for your buck. It’s been said that though it’s tougher than steady pace cardio, it’s much more enjoyable.
And it induces great health benefits, physique changes and just how the body works.
What we’ll look at:
- How HIIT works
- HIIT for Fat loss
- HIIT and hypertrophy
- HIIT for Increasing fitness
- Does HIIT deplete Muscle glycogen
- Does HIIT induce Muscle damage
How does it work?
It basically consists of some time spent sprinting and then some of the time actively recovering or resting between sprints. This is repeated for a number of intervals for a certain duration of time.
The longer the rest is following the sprint, the better you recover for the next sprint, and the more effort you can put into the sprint, for example against a high resistance. So this type is more for developing explosiveness and speed and power, when you put in maximal effect (100-170% VO2max) against a high resistance for a short duration (15-30sec). When you rest shorter, your ability to keep cycling at the highest intensity and resistance diminishes.
A sprint rest ratio can be adjusted to your personal needs in every situation.
You can do HIIT with basically anything, such as sprinting, weights, spinning, swimming, rope jumping etc.
The general public can do 15-20min of HIIT 3 times a week and the similar and even greater results than people who do 1 hour of steady pace cardio 5 times a week.
I advise doing spinning, sprinting or rowing for HIIT as its benefits will translate directly over to your lifts in weightlifting such as squats and deadlifts.
I don’t sprint a lot as it’s really hard on your body and can interfere with your performance and progression of your squats and deadlifts.
HIIT boosts Fat loss
First off, as a natural bodybuilder, or just someone who want to lean down, might want to do some sort of cardio to induce fat-loss. It’s been shown in studies that doing too much cardio will interfere with your workouts and decrease performance.
Whereas HIIT can actually aid in strength and hypertrophy gains rather than inhibit it, plus burn fat at the same time.
But again, doing HIIT too much will also affect your workouts negatively.
So when done properly:
HIIT results in significant elevations in catecholamines, growth hormone and testosterone during short or long sprints, whereas steady pace cardio results in a negligible increase in catecholamines, growth hormone, or testosterone.
Catecholamines and growth hormone increase lipolysis, which in turn increase fat release from adipose tissue. There is also significantly more β-adrenergic receptors in abdominal compared to subcutaneous fat, and therefore is HIIT very effective at lowering abdominal fat as catecholamines bind to these receptors and increase fat-loss specific in that area.
As seen in multiple studies (1, 2, 3, 4), HIIT is very effecting at lowering abdominal fat. As a matter of a fact, the energy cost, including the time spent doing HIIT can be less than a steady pace workout, but the amount of fat burned is so much greater (5).
There are two things you need to keep in mind:
1) The resistance
Going to a too high resistance will start to mimic resistance training. We don’t want that. We want to a moderate resistance where we won’t experience any possibility of muscle damage or using muscle glycogen. But the resistance shouldn’t be too light either as it will then not put enough stress on the legs in order to increase VO2max or heart rate.
2) The intensity
There is a balance between intensities and fatty acid utilization and oxidation. From 0-65% of your VO2max intensity, your body uses only fatty acids for energy. As you go higher than 65% VO2max, your body decreases fatty acid oxidation and start using muscle glycogen for energy.
If fat-loss is the goal, it’s advised to stay below or at 65% VO2max.
But how can you do that with HIIT you might ask?
Well as you sprint, you work up to 100% VO2max. That takes about 1-2 minutes depending on your fitness level. You can test this yourself. Get on a spinning bike for example and sprint full out until you feel that you’re about to pass out. That time is your VO2 max. Now you only want to work up to 65% of your VO2max during each sprint. So let’s say it takes you 1 minute to reach VO2max. Then the duration of each sprint to get to 65% of your VO2max will take 39 sec.
If you’re a beginner, I advise doing a 1:2 sprint rest ratio. Then as you become more fit, and you are able to sprint longer and to decrease your sprint rest ratio to 1:1. That would be the best ratio to follow for fat-loss.
Do maximal effort sprint up to 65% VO2max at a moderate resistance and then spin for that same duration at 20% VO2max for your rest interval. Resulting in a 1:1 sprint rest ratio.
This method will ensure that you use only fatty acids as energy and not your muscle glycogen.
You might also think that with each sprint, you might not recover enough and that after a while your body will switch to muscle glycogen because VO2max and heart just keep on increasing.
Well during repeated high resistance sprinting such as Wingate, the body switches mainly to fatty acids after the 4 sprints.
In the case of a lower resistance full effort sprints, your muscles start with using fatty acids and finish with using fatty acids. It will not use muscle glycogen as the resistance is not high enough.
When do your muscles start using muscle glycogen?
When you start exercising over the intensity of 65% VO2max, your muscle start using muscle glycogen. If you stay below that point, your body will use only fatty acids, even during the sprints.
During a wingate test at very high resistance, your muscles can use up to 25% muscle glycogen after just the first sprint (6). That’s not a problem if your not worried about saving you muscle glycogen for you workout, but if you want to save it, don’t spin at a high resistance.
And as I’ve mentioned above, after doing HIIT for a few week, your body will ‘learn’ to rather use fatty acids as energy and not muscle glycogen as its fatty acid oxidation capacity also increase significantly.
HIIT increases muscle mass
HIIT does induce hypertrophy in untrained men, but not so in trained men.
The reason to do HIIT is that is has a complementary effect on your weightlifting, whereas steady pace cardio has an opposing effect.
It’s been shown that doing too much cardio will interfere with your strength and hypertrophy gains. Too much cardio is 45-60 min for >3 days a week.
That’s why we keep it short and sweet, where it will actually aid in our gym adaption and not oppose it.
HIIT improves muscle oxidative capacity – which makes it better at utilizing energy during sets and during rests, and increases resting glycogen stores – which will provide more energy during hard sets.
It also increases total GLUT4 transporter content in muscles that will help with faster recovery post workout.
It’s able to increase mTOR (mTOR is an anabolic signaling pathway that increases muscle protein synthesis that stimulates hypertrophy), whereas long steady pace cardio inhibits mTOR. It also reduces lactate production, which will help you recover faster during sets.
Doesn’t HIIT increase AMPK and blunt hypertrophy?
AMPK is a cellular signaling enzyme. If cellular ATP levels are decreased, AMPK is increased. A few basic functions of AMPK is that it stimulates fatty acid oxidation in the liver and muscles, and increase glucose uptake in the muscles. So it helps you burn fat. But because it inhibits mTOR (although not directly), could lead to blunted hypertrophy.
But that’s not the case. As seen in this study, exercise induces activation of AMPK doesn’t affect or inhibit the muscle hypertrophic response.
AMPK will just help your muscles adapt to the exercise and help you burn fat. It’s any way lowered to baseline after 1-2 hours after exercise.
HIIT boosts Fitness more than regular endurance training
Wingate tests (30sec sprints) 3 times a week for two weeks are able to increase VO2max by 13%, where are shorter sprints also increase VO2max but not by that much.
HIIT protocols are also able to increase anaerobic capacity by 5.4-8% in two weeks. The longer the sprints, the greater increase in anaerobic capacity.
Anaerobic capacity is the total amount of energy the muscles can generate from anaerobic sources such as creatine phosphate breakdown and anaerobic glycolysis. So the greater your anaerobic capacity, the longer you can spend time sprinting at a high resistance.
Sprinting for longer at maximal effort (such as the Wingate test) can also increase heart rate maximally after just one sprint whereas shorter sprints (6-12 sec) or with less resistance are only able to achieve maximal heart rate after about +-10 sprints.
Evidence suggests that time spent during interval training at maximal velocity or maximal power output is a key factor in improving VO2max.
If fitness is the goal, it’s advised to stay at a moderate to high resistance during sprints
HIIT induces Muscle damage. Is it a concern?
If building bigger better legs with weightlifting, we don’t want additional muscle damage and breakdown and blunted hypertrophy from cardio.
HIIT is able to induce muscle damage at high resistances, or when muscles have not yet adapted to HIIT.
Longer sprints at lower resistances don’t induce muscle damage such as shorter sprints at very high resistance (7).
Even as seen here, HIIT (in the study they did Wingate tests) vs steady pace cardio at 60% VO2max resulted in similar muscle damage.
HIIT, however, does increase inflammation a little after a session, but it’s nonsignificant (8).
So what we do is to keep it between a resistance that’s too low or too high and that won’t result in muscle damage. The speed you sprint at is more important than the resistance.
So yes, muscle damage can take place if you sprint at a too high resistance such as Wingate, but that’s not a problem if optimal weightlifting results are not the goal.
Further benefits of HIIT:
- Improves insulin sensitivity
- Increases testosterone levels
- Reduces appetite – especially good when doing fasted HIIT during your intermittent fasting
- Gets a good sweat going (which assists in detoxing the body)
- Improves blood vessel function and health
- Improves blood flow
- Release endorphins, that will make you feel kind of high and happy
- Releases dopamine
- Improves cognitive health
- Improves cardiovascular health
- Induces autophagy, which will improve muscle health by removing dead cell from the body to replace it with new cells.
I advise for fat-loss to do HIIT 3-4 times a week for about 20-30min a session at a 1:1-2 ratio.
It will definitely give you the results you want, and much faster than steady pace cardio.
HIIT has lots of benefits and the only downside is that you’ll just want to do it more regularly because it’s so much fun.