Strength + endurance training — killing your gains, or the optimal train?

On this episode of "What Does the Science Say?" I want to take a closer look at a very common misconception that I continue to hear in fitness circles: does endurance training diminish the effect of strength training?

Admittedly, earlier research once suggested a possible "interference effect," where concurrent training (strength plus endurance) would reduce the effectiveness of the other. This led to endurance athletes reducing strength training, and those focused on muscle building avoiding aerobic activities. But now, with more research done on the topic, we're starting to get a better picture of the situation.

I'm an endurance athlete — do I really need strength training?

Absolutely! Numerous studies have shown that sprinkling in some strength training can provide tremendous returns in terms of endurance sports.

Conceptually, it makes sense that building strength will increase power. With more power, you can increase your running economy and speeds! In fact, in a meta-analysis of 26 studies among endurance athletes, adding at least 5 weeks of strength training led to improvements in VO2 max, time trial performance, and running economy.⁽¹⁾

But if I'm focused on muscle building, isn't cardio just wasted energy?

It's true that early studies suggested this concept, but even elite bodybuilders include some cardio in their workouts.⁽²⁾ Not only does cardio help when cutting (and trying to emphasize muscle), we also know it's just as important for long-term health and wellness.

Now, I won't dismiss the "interference effect" in its entirety, because it does exist to an extent. Though recent research suggests that the effect is likely minimal and dependent on a wide number of factors.^(3,4)

Indeed, further studies offer even more nuance. Certainly, resistance training is a sure-fire way to build larger and stronger muscles, but this can be complemented by cardio, not suppressed by it. For example, when adding endurance activities to strength training, researchers found that additional anabolic (muscle growth) activity was attained by stimulating more muscle proteins than resistance training alone. This increase in muscle protein stimulation led to changes in multiple types of muscle fibers (compared to predominantly fast-twitch, type 2, fibers alone in resistance training). And by stimulating multiple types of muscle fibers, muscle growth was not impaired — instead, fiber area increased in the combined training group!⁽⁵⁾

As a reminder: type 2 muscle fibers are the fast-twitch fibers. These are helpful for quick, powerful bursts of energy, like sprinting — but also the same quick bursts required to lift weights. Type 1 muscle fibers are "slow-twitch" fibers. These are more fatigue-resistant, generating lower forces but over longer periods of time.

So what should my focus be?

When viewing the recent data broadly, it really supports the idea of concurrent training — not just for health, but for both muscle and performance goals as well.

The general guidance is to focus on your particular sport first. For example, if your goal is muscle strength, then perform your strength training first, giving a majority of your effort to that discipline. But consider adding lower-intensity cardio after a lifting session a few times per week. This will provide stimulus to other parts of your body left neglected by strength training alone.

And if your goal is marathon running, then structure your workouts so that you are getting your runs first, but supplement a few of these sessions with strength training and plyometrics.

As always, make sure you are eating the right diet for your needs. Make sure that you are getting enough protein to support your goals, and enough nutrient-dense calories to support the training in your plan. If you can meet these needs, your body will have the building blocks in place to capitalize on your training and help you reach your goals.

So, stay active, mix it up, and stay healthy!

Sources & further reading

1. Beattie K, Kenny IC, Lyons M, Carson BP. The Effect of Strength Training on Performance in Endurance Athletes. Sports Med. 2014 Jun 1;44(6):845–65.
2. Kistler BM, et al. Case Study: Natural Bodybuilding Contest Preparation. Int J Sport Nutr Exerc Metab. 2014 Dec 1;24(6):694–700.
3. Murlasits Z, Kneffel Z, Thalib L. The physiological effects of concurrent strength and endurance training sequence: A systematic review and meta-analysis. J Sports Sci. 2018 Jun 3;36(11):1212–9.
4. Huiberts RO, Wüst RCI, van der Zwaard S. Concurrent Strength and Endurance Training: A Systematic Review and Meta-Analysis on the Impact of Sex and Training Status. Sports Med. 2024 Feb 1;54(2):485–503.
5. Kazior Z, et al. Endurance Exercise Enhances the Effect of Strength Training on Muscle Fiber Size and Protein Expression of Akt and mTOR. PLoS ONE. 2016 Feb 17;11(2):e0149082.