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  • Writer's pictureJesse McMeekin

Cardio and Hypertrophy—Part 2

Does cardio kill your gains? Or can it actually boost them? And how does HIIT (High Intensity Interval Training) effect muscle growth? In part one we looked at how lower level, steady state cardio can help you build more muscle. In part two, we'll take a look at more intense, interval-driven methods and their impact on your workouts.

A Quick Refresh

Lower level cardio—what I grew up calling "steady state" cardio—has a number of potentially positive effects on the body's ability to build muscle. Namely:

  • Improved interworkout recovery

  • Improved intraworkout recovery

  • Better delivery and removal of substrates/waste products during exercise

  • Increased parasympathetic tone

  • Better sleep

  • More anabolic hormonal state

I'm not arguing in favor of marathon training—the law of diminishing returns certainly applies here—but two or even three days a week of conditioning work can absolutely help you get bigger and stronger. Experience and a good coach can help you dial in the right amount for you and your goals, but that's a pretty good starting point.

Turning Up the Intensity

I am a huge proponent of moderate doses of low level work—and always a little disappointed by the "no cardio" club (you're missing out guys)—but interval based methods have their place in the muscle building arsenal as well.

Despite some fairly broad, inaccurate terminology ("metcon", "tabata" etc.), there's a pretty wide range of training methods that fall under the broader heading of "interval training". Since a complete breakdown is a little much for our purposes here, I'll focus on the commonalities between the different methods. If you're interested in more specifics, and particularly in how to apply these to your own workouts, drop me a line.

For now, let's dig into what defines interval training, regardless of the specifics.

No Replacing

All interval training methods revolve a round a central idea: the work being performed during the work intervals is too intense to maintain indefinitely. That's it. If you could do it for an hour or so, you're not really using interval training, you're just taking a break every now and then.

While it may seem simple (and honestly, it is), this shifts the conversation around the effects of interval training pretty radically. Up until now I've referred to is as "cardio" to keep things simple, but that's a bit misleading. Sure, the cardiovascular system is being trained, but the term "cardio" is often used interchangeably with "aerobic", and interval work is not aerobic.

The reason the work effort can't be sustained indefinitely is because the energy demands are high enough that the body turns to short-term, quick energy production; either the lactic energy system or the creatine phosphate system. Without getting into detail, both systems are able to produce energy quickly and without oxygen, but they churn out some waste products that lead to some pretty real discomfort and fatigue.

What this means for us is that while interval work offers some benefits to anyone looking to gain a few pounds of muscle, it does not replace lower level, aerobic work. Instead, it builds on those effects.

Interval based training methods offer us a more targeted set of benefits and adaptations, and the smarter the application the bigger the benefits. Here are the three biggest players:

Fiber Type Specific Endurance

Muscle fibers are broadly split into two categories (and from there, into further subcategories): Type I and Type II fibers. Type II fibers are the "fast-twitch" fibers, so called for their higher contraction rate. These guys are (relatively) low endurance, but more prone to getting bigger (hypertrophy) than the Type I fibers.

Type II muscle fibers are typically reserved for high force production situations: that nice uphill walk gets your heart rate up and indirectly may help you build more muscle, but an all-out uphill sprint uses the same fast-twitch fibers we're trying to build. The result? More endurance in the same muscle fibers we use on leg day, and over time, that may mean more reps, more sets, more weight, and more muscle.


When we think of HIIT, or any of the other interval-based training methods out there, we don't often think of them as a means of recovery. In reality though, some of these methods do just that, particularly with intelligent exercise selection.

Most of the muscle soreness you experience after a tough lift (DOMS—Delayed Onset Muscle Soreness) can be attributed to the eccentric phase of exercise, that is, the part of a movement where muscle lengthens against resistance. It's the down phase of a squat or a bench press that makes you sore.

A few of my favorite interval protocols work best with exercises that eliminate the eccentric portion of a movement. These include sled pushes, loaded step ups, heavy-resistance spin bike or assault bike work, and both the rowing machine and the ski ergometer. Each of these exercises involves a hard, heavily resisted concentric (shortening) muscle action, but eliminates the resistance for the eccentric phase. The result is that we get some additional blood flow to the muscles—and the specific muscle fibers—that need some TLC without beating them up any further.

(For anyone wanting to nerd out a bit, High Intensity Continuous Training or HICT, High Resistance Intervals or HRI, and to a lesser extent Alactic Intervals are my favorite applications of this idea.)


The final benefit we'll cover is a fairly broad adaptation I've begun thinking of as "tolerance". It's best explained with a bit of a story:

About a year and a half ago I started a training program that involved weekly competition. More of these competitions than I care to remember were based around the rowing machine or the assault bike, and an awful lot of them were variations on how far you could go in a certain time.

This kind of work led to a bit of a running joke: the "heart rate PR". Every few weeks I managed to hit a new high for my heart rate, each one substantially above my predicted max. As this was happening, I realized that as awful as 189 beats per minute felt (particularly when it was held for a few minutes), it wasn't scary anymore. Those first few workouts had me almost convinced I was gonna die... but after a while, I built up a tolerance. Tolerance to what? To the waste products, to the shortage of oxygen, but mostly to the feeling of absolute maximum effort.

I'm convinced that, physiological distinctions and differences aside, that tolerance translated into an awful lot of extra reps, extra sets, and extra stimulus. Put more colorfully, I learned to eat sh*t, and to go back for seconds.

Wrapping It Up

There's a lot of science here, even if some of it was glossed over. There's also a lot of terminology, and an official buttload of acronyms (I think I need an acronym to keep track of my acronyms). In order to make sure the good stuff hasn't gotten lost in the mix, here are your takeaways:

  • Aerobic work is still pretty awesome (if you haven't yet, go read about it)

  • Intervals are cool, but they don't replace aerobic work

  • Intervals need to be hard... how hard kinda depends, but hard

  • Intervals target the same muscle fibers strength training does

  • Intervals can actually speed recovery when used properly

  • Intervals can probably help you do other terrible stuff, leading to more muscle

As usual, shares, likes, follows, comments, and all the rest are appreciated.

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