Why Your Body Loves Intense Exercise (Even if You Don’t)

Relative intensity is measured by how challenging the activity feels while you’re doing it. If you would rate your effort as 7 or 8 on a 10-point scale, that counts as vigorous.

Or, if you want to keep it really simple, Gibala said, you can use the classic talk test. If you’d struggle to string more than a few words together while doing it, that’s vigorous. It doesn’t matter if you’re sprinting or just walking at a faster-than-normal pace for a minute at a time. If it’s challenging for you, it should work.

Now let’s return to what happens when you crank up the intensity.

Drain for Gain

In a 2016 review on interval training, Gibala and co-author Martin MacInnis noted that “higher intensities of exercise elicit a greater metabolic signal than moderate intensities.”

Gibala compares it to fuel efficiency when driving. If you give yourself plenty of time to reach your destination, you can drive at a steady speed and use a minimal amount of fuel.

But if you’re running late and drive like a maniac, your trip is a series of abrupt starts and stops. Each time you mash down on the gas pedal, you’re burning way more gas than you would typically need to get from one place to the next. 

Doing high-intensity intervals is the human equivalent of driving like a bat out of hell. You rapidly use up your muscles’ stores of adenosine triphosphate (ATP), which is their primary energy source – their “gasoline.” ATP depletion triggers a protein called adenosine monophosphate-activated protein kinase (AMPK). AMPK helps regulate cellular energy and, in this case, flips the “on” switch for another protein, PGC-1a. 

PGC-1a is kind of the final boss of metabolic signaling. It tells your muscles to create more mitochondria, the part of the cell that produces ATP. Having more mitochondria means more ATP, which means more energy the next time you need to push yourself. 

More ways your body adapts:

• Your muscles build and repair themselves faster. 
• Your blood vessels remodel themselves to become more resilient. 
• Your heart increases its stroke volume, which means it pumps more blood with each beat, and that blood reaches more places, thanks to an expanded capillary system. 

To be sure, your body makes those same changes to your metabolism and physiology in response to moderate-intensity exercise, as long as you do enough of it. 

They just happen faster when you work harder. 

The reason is as old as life itself. ...

Intensity and Your Muscles

“The only reason the body adapts is because there’s a threat to its survival,” said Brad Schoenfeld, PhD, a professor of exercise science at Lehman College in the Bronx, New York. 

Your body doesn’t know what year it is and that “threats” aren’t what they used to be. It just knows you’re pushing it harder, and it wants to survive. The goal, from an evolutionary perspective, is to survive a similar threat in the future. If that threat requires you to run 10 miles, then you’re going to improve your aerobic fitness. If the threat requires you to move something heavy, you’ll get stronger.

Let’s talk about strength training for a moment.

For Schoenfeld, a former competitive bodybuilder, lifting heavy things is both a personal and scientific pursuit. He’s published hundreds of studies on or related to resistance exercise over the past decade. 

Training for strength has a lot in common with interval training. Both are intermittent activities, which means you typically spend less time working than you do recovering from your work, while the intensity of that work determines how much you adapt to it.

You can describe strength training intensity two ways:

Intensity of load is the percentage of your one-repetition maximum (1RM) you’re working with on any given set. 

But let’s say you’re working with 80% of your 1RM. “It’s relatively heavy,” Schoenfeld said, “but if you just do that one time, it would not be that high an intensity of effort.”

Or let’s say you go to the opposite extreme, and you’re working with just 30% of your 1RM. ”It's a low intensity of load,” he said. But it would require “a very high intensity of effort“ to lift that weight to failure – the point at which your muscles are too fatigued to complete another repetition. (As he and his co-authors showed in a 2017 study, hard work with a relatively light load can still produce results.)

Schoenfeld said the best way to gauge intensity of effort is through repetitions in reserve, or RIR. It’s an estimation of how close you are to failure on any given set. “The closer you get to your RIR of zero, the higher the intensity of effort.”

Not every lifter needs to push themselves until they’re within one or two reps of failure, much less go all the way to failure.

Newer lifters will increase their strength and muscle mass simply by lifting, even if they never get close to their maximum load or effort.

But for more experienced lifters, getting close to zero RIR becomes more important if they want to continue making improvements in muscle size. (Training to failure is less important when strength is the primary goal, as long as the intensity of load is high enough.)

Intensity for Non-Weightlifters

But what if you aren’t especially interested in building bigger muscles, or in getting strong enough to use the weights at the far end of the dumbbell rack? How much does intensity matter to someone who’s pursuing health and fitness rather than strength and size?

In part, that’s because muscle size and strength are pretty much the only unique health benefits linked to resistance training. Almost everything else you gain from lifting weights – enhanced metabolic function; improved balance and mobility; a lower risk of death from any cause – is also linked to aerobic training.

Conversely, someone who only does aerobic exercise will get cardiovascular benefits but won’t maintain their strength and muscle mass without including some resistance training in their routine. 

“If you look at the data on lifelong exercisers who only pick a single approach, none of them are optimized,” said Andy Galpin, PhD, executive director of the Human Performance Center at Parker University in Dallas. 

Galpin saw this first-hand when he did every conceivable fitness test with a pair of middle-aged male identical twins. One twin, a lifelong endurance athlete, showed extraordinary metabolic health and cardiovascular fitness. But his brother, who didn’t exercise regularly, had superior muscle size and strength.

That’s where high-intensity training is perhaps most useful. With a small time commitment, someone who’d rather be doing something else can still get the most important benefits of their non-preferred form of exercise. 

“Because these things can be done quickly, it reduces the burden of not having enough time,” Galpin said. 

That should boost adherence – which, Schoenfeld said, “is the most important thing, hands down. It’s not even arguable. There has to be consistency.”

How Small a Time Commitment Are We Talking About?

In a 2021 study, Gibala and his colleagues showed improved cardiovascular fitness in healthy but inactive young adults who did circuits of body weight exercises for 11 minutes at a time three days a week.

In a classic study from 2006, Norwegian researchers showed that a single weekly bout of high-intensity exercise reduced the risk of death from any cause by about 40% in males and about 60% in females, compared to people in the study who didn’t exercise at all.