So the age-old myth; Does cardio kill your progress in the gym? And if so, how?
The mechanism basically breaks down to how the enzymes and the protein transcription within the muscle are influenced by our training and environment.
There are two competing pathways the mTOR (or Mechanistic Targeting of Rapamycin) and the AMPK (AMP-Activated Protein Kinase). The mTOR pathway is heavily linked to our anabolic pathways which are growth and repair of our tissue. That's when we take energy (from food) and create new cells or repair and fortify existing damaged cells. Meanwhile, the AMPK pathway is the opposite it plays a role in catabolism or breakdown. Taking glucose, fatty acids and amino acids breaking them down and making energy or ATP. Basically when you think mTOR think strength and hypertrophy and when you think AMPK think endurance and mitochondrial biogenesis.
mTOR is basically up-regulated by a state of rest and digest. High ATP levels, caloric intake (in particular protein, but carbs and fats also play a role), heavy resistance training or explosive power training, rest and, high insulin levels, (that comes back to the food again), and then high anabolic hormone levels (growth hormone in particular but also IGF-1 and Testosterone). These factors all trigger mTOR to fire up.
Which is basically the life of a bodybuilder, powerlifter or sprinter. Train at a super high intensity (in short bursts), get plenty of rest, eat like a horse, and sleep really well.
On the other hand, AMPK is activated basically by one thing, an increased level of AMP in your muscle.
ATP is the stuff that our body uses for energy when you break that ATP (adenosine triphosphate) down you get energy for muscular contraction on one side and you get a byproduct molecule called ADP (adenosine diphosphate) on the other. When you do moderate intensity, high volume training (that is endurance training or cardio), ADP can be broken down further into more energy, just not as much this time. You might have guessed but now you have energy and a further reduced byproduct AMP (adenosine monophosphate). That AMP, that's the molecule that triggers your AMPK pathway and starts the body on this more aerobic, type I, mitochondrial biogenesis pathway.
Back to the big question.
The answer is basically yes, to a degree; If you spend too much time in the middle zone for intensity (often classified as 65-80% of max heart rate). In this zone of interference, you're going to up-regulate AMPK by having high levels of AMP in the muscle. Which will inhibit and block your mTOR which therefore interferes with muscle growth.
There are a bunch of negative effects of this mTOR downregulation that will influence and affect athletic performance.
- Number one: It inhibits your mTOR and hypertrophy signaling, your ability to promote muscle growth.
- Number two: It reduces strength, speed, and power but blocking your Type II adaptation and making you more Type I muscle fibre dominant (this is both a muscular and a neural adaptation).
- Number three: It causes neural fatigue and it's going to exhaust the nervous system. (high-intensity training also causes neural fatigue, but at least you are getting a fitness benefit as a result)
- Number four: It drains your glycogen stores in both the muscles and the liver which is a big factor when it comes to power athletes and power sports. One of the leading theories about what causes overtraining syndrome is chronically low glycogen.
- Number five: The last and most important thing is it's not going to help you with your sporting fitness. Doing a lot of low to moderate intensity work it doesn't actually get you ready for the explosiveness and the aggressiveness that you need to exhibit on a court, field or on the track.
A much better option is to do your active recovery work in a low enough intensity to avoid any interference with your strength and power gains. And then do your high-intensity work with sprints and intervals at a high enough intensity to help you on the court or the field. Read more about our approach to sports specific fitness here.