Concurrent Training and the Interference Effect: Will Cardio Kill Your Gains?

“Cardio will ruin your gains!” We’ve all heard some version of this phrase from a veteran of the iron game. Right or wrong, the idea that cardio will negatively affect strength and hypertrophy adaptations from lifting has persisted for the better part of a century and has been dubbed the interference effect. This idea is so pervasive in strength and bodybuilding communities that many lifters are missing out on the many health benefits and fitness adaptations properly dosed conditioning can provide.

All the while, the “hybrid athlete” has emerged. Many popular training programs are delivering robust results while combining endurance and strength training together. This hybrid form of training has become the new gold standard in the fitness industry. CrossFit and Hyrox are as popular as ever, and seemingly every influencer has become a shill for “Big Zone-2 Cardio.”

Cardio is cool again, which is cool! But, little attention is being paid to the interference effect – how strength and endurance training affect one another. This article will cover what the interference effect is, what the science says about combining lifting and cardio together, and how to practically apply these concepts to your own training to get the most fitness out of your training.

The interference effect is defined as reduced strength and/or hypertrophy gains when resistance and endurance training are combined together in the same exercise program. The first use of the term “interference effect” in this context dates back to 1980 when Dr. Robert Hickson showed that untrained men had reduced strength development with concurrent training, or combined strength and endurance training, when compared to resistance training alone.1

In this view, endurance training “interferes” with the adaptations from resistance training when both forms of exercise are done in the same program.

What explains Dr. Hickson’s findings? And, have those findings been repeated over the last 45 years? Let’s explore some potential mechanisms that explain this effect.

First, some definitions. Exercise is a type of physical activity that is planned, repetitive, and structured with the goal of improving or maintaining health or fitness. Conditioning is an umbrella term that refers to exercise designed to improve endurance performance and cardiorespiratory fitness, both of which rely on the ability of the heart, lungs, and circulatory system (blood vessels) to support muscular function at a given level for a relatively long period of time. In contrast, resistance training is a form of physical activity where muscles create force via contraction against a load, which may be external (barbell) or internal (bodyweight) to the individual to create increases in strength and/or muscular hypertrophy.