In the late 19th century, it was widely assumed that there was an ideal athletic body — a physique that would provide optimal performance in every sport.

Now, of course, we know that this is nowhere near true. The ideal physiques for powerlifting, swimming, soccer, and endurance running are all very different from each other.    

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It’s clear that you can’t have the ideal body type for every possible activity, but how do different types of training interfere with each other? And more specifically, what is the trade-off between cardio and strength training, and how drastic is it? What — if anything — can you do to lift like a champion while still running and climbing mountains like a Spartan Pro?

Is the Interference Effect a Real Thing?

Intuitively, you've probably already subconsciously realized that there must be some interference effect. After all, it's often quite obvious that strictly strength athletes and endurance-only athletes generally have vastly different body types.

Studies indeed show that combining endurance and strength training will reduce the benefits of both. Furthermore, despite the common advice to do weights before cardio, the exact sequence of exercises within a workout doesn’t actually make much of a difference in terms of the interference effect.

Related: Stop Choosing Strength Over Stretching: How to Hit Both (Hard)

What does matter, however, is frequency and duration. The longer and more frequent your cardio sessions are, the more detrimental the effect is on your strength. Presumably the reverse is true as well, with more strength training sessions creating more interference with cardio training.

That said, it may take awhile for the interference effect to show up, particularly if you’re untrained. In a 16-week study of untrained men, the interference effect was not visible at week 8, but it was prevalent at week 16.

The Science Behind the Interference Effect

The body — as well as individual muscles — will adapt to the specific demands placed on them. Different demands will produce different adaptations, and the adaptations demanded by strength and endurance training largely conflict with each other.

There are several mechanisms behind the interference effect. For starters, strength, power, and endurance all require different neuromuscular adaptations.

Gene activation is a vital component of muscular adaptation. Strength training activates a genetic signaling pathway called mTOR, which makes muscles grow bigger and triggers the conversion of type 1 (slow-twitch) to type 2 (fast-twitch) muscle fibers. Endurance training activates an entirely different signaling pathway called AMPK. While this enhances cellular energy metabolism, it simultaneously inhibits muscle growth.

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Competition between AMPK and mTOR is, therefore, a major mechanism behind the interference effect. (Plus, another enzyme, PKB, inhibits muscle tissue breakdown, and like mTOR, it both inhibits and is inhibited by AMPK.)

That said, studies on untrained individuals usually find a weaker interference effect (or even none at all), particularly if the duration of the study is short. That’s because, early on, it is quite easy to get better at everything you try, as most athletes notice when they start training for the very first time. The more progress you make, however, the more you have to specialize to continue seeing gains in the desired direction.

How to Minimize the Interference Effect and Make Concurrent Training Work

There are a few strategies you can use to minimize the interference effect.

First, don’t over-rely on cardio to burn fat. Instead, rely more on sound nutrition and monitoring your caloric intake. (This tends to be more effective, anyway.)

Second, whenever possible, separate cardio and strength training into different workouts instead of doing them back to back. Cycle calories accordingly, eating more around your strength sessions. 

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Third, because some of the mechanisms behind the interference effect are localized to specific muscles, you can separate cardio and strength training by body part. For instance, you can time your running sessions relatively close to your upper-body strength sessions, but separate running and leg day by 24-48 hours. Likewise, rowing or rock climbing can be separated from upper-body strength sessions, but timed closer to leg day.  

Finally, the magnitude of the interference effect depends on the difference in adaptations required. Heavy weight training will interfere with endurance adaptations more than low-weight, high-rep training. Similarly, sprinting will interfere with muscle growth less than long-distance jogging.

Related: 4 Rules for Getting as Strong and Fast as Possible

If you’re training for a Spartan race, for instance, you need to be able to run relatively long distances. And while obstacles like the Atlas Carry, Sandbag Carry, and Hercules Hoist may have you moving some serious weight, you won’t be lifting especially big loads nearly as much as you'll be relying on your endurance levels.

In that case, Spartan racers will benefit most from training with low-weight, high-rep exercises in the gym, while continuing to run several miles in endurance workouts. And of course, separating running days from lower-body lifting days is ideal.