Cardio: The Strength and Muscle Killer?

The Smart Lifter's Guide to Cardio

An Insane Practice?

You see it often in big gyms. After a hard weight training workout, lifters often hobble over to the treadmill for 30-60 minutes of cardio. Despite the easy pace, this isn't a smart move.

This double-time training session isn't uncommon. Maybe you've even done it. I know I've been guilty of it. My logic was that the strength session would help with getting stronger and add muscle, while the cardio session would help burn more fat. This was a bad idea and it still is.

Before we dig into the science, let's jump straight to the conclusion. Research has shown that doing your cardio right after your weight training causes you to build less muscle and less strength.

Here's what to do instead if your goal is to get big and strong, but you still want to do some cardio for health and fat loss:

  • Don't do cardio after lifting. You can cool down by walking out to your car.
  • Do cardio in the morning. Move your lower to moderate intensity cardio session to the morning, away from your strength training in the afternoon. This reduces the interference effect discussed below.
  • Limit cardio work in general. Keep your aerobic work moderate to about 3 days per week at 40 minutes max unless you're specifically focusing on increasing your aerobic capacity.
  • Fasted with aerobics and fed during strength training. Do your low intensity aerobic session fasted in the morning, but do your strength training sessions under the opposite condition by consuming a pre and post-workout nutrition with protein and carbs.

Why should you follow those rules? Let's jump to the science.

Let's take a simplistic view of your muscle fibers and divide them into two main categories:

  1. Big glorious fibers, a.k.a. anaerobic fibers.
  2. Smaller, non-glorious, weaker aerobic fibers. You know, the ones lots of skinny runners have.

One of the main reasons that aerobic (endurance or slow twitch) fibers are smaller is that they require oxygen to create ATP (cellular energy) and oxygen has to cross their cell membrane. If the aerobic fiber gets too big, oxygen can no longer passively cross the membrane.

The beautiful part of this process is that it's a passive one requiring very little energy since it relies on the concept of diffusion. What exactly is diffusion?

Picture this: You're locked in a small airplane on your way to the Arnold Sports Expo with some massive protein-consuming dudes. Out of nowhere, one of them has a massive protein fart. Even if you're sitting in the back of the plane, there's a good chance you'll know what just happened.

That process of those sulfur-containing compounds shooting out of a massive mammal's gluteal area and lofting their way to your nose is a passive diffusion process.

However, imagine the same situation in the massive hall of the Expo. The sheer volume of the room allows you to escape from the noxious crop dusting since the fumes have a much larger distance to travel.

Similarly, passive diffusion of oxygen across the cellular membrane in smaller aerobic fibers will limit how big the fiber can get since it must cross completely into the fiber. A larger cross-sectional area increases the diffusion distance for oxygen and substrates

On the other end of the spectrum, the wonderful thing about the bigger and stronger anaerobic fibers is that they're not limited by diffusion of oxygen since they run without it. Hence the term anaerobic or "without oxygen" fibers. This allows them to be much, much bigger in size.

Since you want to get bigger and stronger, you want to target these anaerobic fibers with training and avoid anything that takes away from that process. Aerobic training, by its very nature, is the polar opposite of anaerobic training.

Here's a quick comparison:

  Aerobic Work Anaerobic Work
Fuel: Fat Carbs
Size: Smaller muscle fibers Bigger muscle fibers
Oxygen: Needs it Doesn't need it

When you do aerobic training such as achieving 40-60% of your VO2max on a treadmill, you're signaling your body to target aerobic fibers. When you lift heavy shit, you signal your body to target anaerobic fibers. This is why you don't see pro-bodybuilders winning marathons and you don't see professional marathon runners winning powerlifting meets.

That is all well and good, but what you can learn from science will help guide you to more massive muscles.

In a series of experiments by Thomson, DM et al, scientists overloaded the plantaris muscle via removal of synergist muscles to enhance the strength training responses. What they did was surgically hack out the other helping muscles in the lower leg (of a mouse) so that the remaining muscle was left to do all the work.

The researchers found that markers for muscle strength and hypertrophy changes (mTOR phosphorylation and total protein) were increased in the rodent skeletal plantaris muscle from the massive amount of tension applied to it.

Other experiments used electrical stimulation to stimulate strength and endurance training to see what would happen. They used a higher-frequency intermittent zapping of mouse muscle to approximate strength training and they found significant increases in mTOR phosphorylation.

However, no change in mTOR activity was seen when they used a low-frequency electrical stimulation, which is closer to an endurance activity – further evidence that muscle is tension/pattern specific.

In short, electrical stimulation that mimicked strength training induced mTOR phosphorylation, which makes muscles grow. Conversely, electrical stimulation that mimicked endurance activity resulted in low mTOR phosphorylation and muscles didn't grow.

"That's great in mice, but what about humans?" you ask. Fair question. Recently, researchers observed an increase in mTOR phosphorylation (a good thing for bigger muscles) following an acute bout of resistance exercise of 8 weeks in human test subjects. It appears the mouse data above does apply to humans as well.

But what happens when you do a lifting workout followed by an aerobic workout? The fancy-pants term for this is the interference effect, or commonly referred to as "concurrent training."

The work of Hickson showed that endurance training may worsen adaptation to strength training when the two are performed in the same session, although subsequent studies have shown quite a mix of effects.

In a recent study from the European Journal of Applied Physiology, scientists trained 20 subjects over six weeks. Subjects were divided into two groups where one group performed only lower body power training (no aerobic work) while the other group did the exact same lower body workouts, but with an additional 30 minutes of low-intensity running at a moderate pace of 60-70% of max heart rate. This moderate aerobic session was done immediately after the strength session.

Performance measures were done (jump performance, leg press force, speed via rate-of-force develop­ment, and strength via a one-rep-max half squat). Researchers also jammed big-ass needles in participants' legs to grab eraser-size chunks of flesh to measure muscle fiber changes in size.

Even though both groups performed the exact same lower body power workouts, the group that added 30 minutes on the treadmill was worse off. Proof? They couldn't jump as high and their muscle fibers were smaller (less hypertrophy).

Researchers showed an interference effect from the concurrent training as the aerobic training impaired the strength/hypertrophy adaptations.

Not all studies in humans have reached the same conclusion, though. Recently, Kazior, Z. et al. found that resistance training plus endurance training did see a greater bump in muscle fiber size. That's puzzling because it's the opposite of what we've seen so far.

A possible problem is that the study was done in untrained subjects – which ain't you. Another problem is that partway through the study, the mode of endurance training shifted from 60% of VO2max to include intervals at 95% of VO2max which are much closer to resistance exercise, thus having less of an interference effect.

The good news is you can avoid this interference effect and maximize muscle hypertrophy/strength while still doing some aerobic training. Just remember the rules from above:

  1. Don't do cardio after lifting.
  2. Do cardio in the morning. Lift in the afternoon. Or do them on separate days if you can't manage two trips to the gym on the same day.
  3. Limit cardio work in general.
  4. Do your cardio fasted but not your weight training.
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Mike T Nelson, PhD, is a metabolism and fitness expert specializing in improving body composition and performance in athletes.  Dr Nelson is a university instructor and owner of Extreme Human Performance.   Follow Mike T Nelson on Facebook