The other day I saw a post about a HIIT challenge on social media.

Basically, it was all about doing a high-intensity interval training (HIIT) workout every day for a set number of days. 

And I really didn’t want to be Judge Judy …. but I did kind of slam down the gavel inside my head.

“Slow down, speed racer,” I muttered to myself. “Someone’s gonna blow a tire.”

Now, I know we all love challenges. We love to push ourselves. We love to feel like we’re badasses sometimes. And it’s nice to have a schedule, with a set beginning and end. There are a lot of amazing benefits when it comes to challenges.

And also if you know me, you know I HATE dogma, which is basically accepting a bunch of beliefs as somehow being true without ever being questioned – i.e., “rules” that are there just because they are “rules.”

But if you really are doing high-intensity training every day for 2 weeks (or however long), you’re asking for some trouble. Even if you think you’re immune to it.

How much HIIT is too much? Well, that depends. But every day? Yep. Too much. (And so is every other day, too.)

I want to get into a little bit of the “why,” because your workouts affect you right down to your cellular level. When you know how your body actually works, things make a lot more sense.

First, let’s take a quick peek at how your body makes fuel (hang with me, because it’s interesting!).

 

Muscle Fuel 101

First, you eat food. Through the digestion process, that food is converted into “substrates” (like proteins, fats, and carbs) that are THEN converted into fuel. That fuel is called adenosine triphosphate, or ATP, and it’s what powers your muscle contractions.

Here’s the thing about ATP: your muscles only store a teeny-tiny amount of it in your muscles – barely enough to fuel 3 seconds worth of movement!

That means in order to keep moving your muscles, your body has to quickly find a way to keep regenerating that ATP. It has three basic ways it can do that.

Those three energy pathways are:

  1. Phosphagen (aka ATP-CP)
  2. Anaerobic glycolysis
  3. Aerobic glycolysis

But as you’ll see, your body can hop back and forth between pathways, and maybe even straddle two of them, depending on what you’re doing.

Your Energy Pathways

The truth is, when I first started learning this stuff for my first personal training certification back in 2000, my eyes glazed over. So no worries, I am going to try to make this as not-glaze-worthy as possible.

Pathway 1: Phosphagen (aka ATP-CP)

This is the first path your body takes when it needs more ATP to fuel your muscles.

After your body burns up its stored ATP during your first 3 seconds of movement, it needs more instant energy, especially to fuel powerful movements like throwing, kicking, sprinting, and lifting heavy weights.

You’ve probably heard of creatine phosphate (CP), right?

Well, the creatine phosphate stored in your muscles quickly reconstitutes your ATP after it has released its energy. But because your muscle cells can only hold a small amount of both CP and ATP, there’s not a lot available (that’s why creatine phosphate supplements are so popular).

Anyway, that limited supply means this pathway runs out of gas after only about 10 seconds.

I like to think of this energy system as the fuel to light a match – it’s quick and burns bright, but it doesn’t last long.

Pathway 2: Anaerobic Glycolysis

Once you progress past the phosphagen (ATP-CP) phase, you’re into the anaerobic or “glycolytic” phase. Anaerobic literally translates into “without oxygen.”

When your body runs out of the creatine to fuel the creation of ATP, there’s another source of energy stored in pretty good quantities in your muscles: carbohydrates, in a form called “glycogen.”

Now, your cells require oxygen turn carbohydrates and fat into ATP.

But remember: this process doesn’t use oxygen. Instead, in your cells’ cytoplasm, glycogen is broken down into pyruvate and then into ATP after about a dozen chemical reactions. It’s a fairly fast process but it still takes some time, which is why it makes sense that your body first has to rely on the phosphagen stage, right?

That being said, there’s a little issue with this oxygen-free energy-creation situation. It has a byproduct: lactic acid. That’s what causes the deep muscle fatigue and burn that can get in the way of our performance.

This is the energy pathway that would (mostly) fuel you through an actual HIIT workout, during the hard push periods lasting between 30 seconds to 3 minutes, depending how hard you’re working. It’s also often used during weight training and longer “sprints.”

This second pathway makes a lot of sense logically, because it is (mostly) in control while your cardiorespiratory gets up to speed to take over the third pathway.

Pathway 3: Aerobic Glycolysis

After you’ve been working out for a couple minutes, your heart and lungs have had time to ramp up their output and supply your working muscles with oxygen.

That oxygen is important, because it fuels aerobic respiration, which – in a fairly long series of chemical reactions – breaks down glucose to make ATP.

Where does the glucose used by your muscles come from? It can come from glucose stored in your muscle cells, from food that’s in your system, glycogen from your liver, stored fat, and in very extreme cases, your body’s protein (or actual muscles).

All of this action occurs in your mitochondria, which your cells’ powerhouses. Because it’s a fairly lengthy process, aerobic glycolysis is the slowest form of ATP production – but it’s also sustainable for a lot longer, up to hours or more, and it’s your body’s comfort zone when it comes to producing energy. Which is why, as we’ve been learning through studies over the past decade or so, it might not always be tops when it comes to burning fat.

This is the path used to create energy for jogging, elliptical workouts, swimming laps, old-school aerobics, etc.

You Should Hit All Pathways

As you might have already noticed, during most workouts your body slides from one path to the next. In fact, even during a regular “easy” steady-state cardio workout,  you would travel through all phases of energy production before you settled into your stride.

And during some of my favorite kinds of workouts – interval training –  you go back and forth between the pathways.

Here are some of my favorites:

But here’s the thing: HIIT is tough.

It requires a LOT of your body, and it pushes you out of its “comfort” range when it comes to fueling your muscles. Lactic acid builds up and your muscles burn. That fatigue puts you at risk, which is why it’s a legit question to ask how much HIIT is too much.

You have to have a good base of fitness in order to maintain form through your movements in order to do them properly when you’re working at that level of intensity.

Post-Workout Burn

And when you’re done your HIIT workout (if you’re doing it right), you almost should feel you need a nap afterward. Your body enters the EPOC zone – excess post-exercise oxygen consumption – which means you are burning energy AFTER your workout to return your heart rate, blood pressure, muscles, etc., back to normal.

And because of all that, you need more recovery time between HIIT sessions so that you stay healthy and uninjured.

As a result, the American College of Sports Medicine recommends topping out at 2 HIIT workouts a week, on non-consecutive days – and tackling HIIT workouts only after you’ve built up a solid base of aerobic fitness.

I’d also argue that you need a good base of core fitness and strength. I’ve seen a lot of people tweak their backs, shoulders, and knees when they’ve pushed too hard too often during HIIT workouts.

How Much HIIT is Too Much?

The good news is, both kinds of workouts carry with them a strong list of health benefits.

Both help prevent disease, boost stamina, enhance longevity, help improve moods, and generally kick butt when it comes to quality of life. Some studies correlate an ability to work out hard with longevity, which makes sense.

That being said, HIIT workouts do seem to target fat – and especially belly fat – more than aerobic workouts. But that difference might be made up by just tacking a few minutes onto the end of a regular cardio workout.

And if you just can’t face a grind-it-out cardio workout that involves 45 minutes on the treadmill, HIIT is a great option.

But what if you’re having a meh kind of day? Or you’re a little tired or you’re still feeling the effects of yesterday’s workouts?

That’s when steady-state workouts rock. For me personally, I always have a huge energy and mood boost after a steady-state workout (your mileage may vary). I also notice a big increase in my overall stamina when I regularly (like 2 times a week) include steady-state workouts into my regimen.

My personal recommendation? Do both kinds of workouts, and keep at least two days between HIITs.

For more info: