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Wendy Shafranski


March 21, 2024

The Interference Effect

Most of you have noticed that we perform strength and aerobic exercise on separate days. The main reason we do this is to avoid the interference effect, giving you optimal results for the workout being performed that day.

What the heck is the interference effect?

The interference effect states that endurance training signaling stunts muscle growth. And this inhibition in muscle growth leads to a decrease in muscle size and force capacity.

There is truth to this theory and it depends on things like exercise timing, selection, signaling and even your nutrition.

Most of us just want to be well-rounded and capable from an aerobic AND a strength standpoint. Both strength and VO2 Max are indicators of longevity. But, if you are a “specialist,” you must spend more time in whatever specific training will benefit your sport. For example:

  • An endurance competitor (think Ironman, marathons, etc.), must send more of a cardio signal and understands that will come at a cost to muscle.
  • A competitive powerlifter must spend most of their time building strength and they won’t be as aerobically fit as they can be.

Look at your goals and determine if your training is supporting them.

Now back to the interference effect and why we usually don’t program strength and cardio on the same day. That’s because it’s not optimal to do so when it comes to strength and muscle development. Yes, it is well-documented that strength training can be enhanced by appropriate aerobic work, just like aerobic capacity can be enhanced by strength training. But, again, the details matter.

Studies show that combining endurance and strength training will reduce the benefits of both. And, in general, the longer your cardio sessions, the more impact it will have on your strength. Running has been shown to have the most impact on strength.

Sure, it’s ok to blend cardio and strength sometimes, but make it a long-term habit and you can see decreases in muscle AND strength.

Now, if you must blend both from time to time, Research suggests performing resistance training before cardio is best to maximize your strength.

In the study linked above, scientists focused on 19 studies where participants performed strength and cardio during the same workout and analyzed VO2 max and lower-body strength. The studies lasted from 8 to 24 weeks, and the people exercised two to three times per week. When cardio was done before weights there was a significant drop-off in strength. But when VO2 max was the goal, the exercise order did not matter. So,  aerobic capacity might not be affected if you do weights before cardio compared to starting with cardio.

Here’s the takeaway: the type of exercise you predominately do and even the order of your exercise depends on your goals. If you can separate your aerobic and strength training sessions, that should give you the best results for both.  

Again, if you have limited time, you can do both in the same workout and make progress, but the prioritized exercises will improve faster. If you want to improve endurance, start with cardio. If you want to boost strength, begin with resistance training.

If you’re sticking to our training phases as written, you are good to go for strength and cardiovascular health. If you are adding a lot of work in addition to our program, that can come at a cost to your gains because of a lack of recovery and the “law of diminished returns.” For example, if you are coming to the gym five days a week, doing our strength and aerobic workouts and adding more cardio, it can have a negative effect. There’s a theory called Maximal Recoverable Volume (MRV) which is the maximal volume load an individual can accumulate, recover from, and respond to positively.

We spend a lot of time making sure we design training that can give you the best of both worlds. But, of course, your consistency and effort in the workouts matter.

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