Combining endurance and strength training has always been tricky, because of the “interference effect” between the two types of workouts:
The classic study on concurrent strength and endurance training was published by Robert Hickson in 1980. After ten weeks of seriously intense endurance training, strength training, or both, the verdict was that strength training didn’t hinder endurance gains but endurance training did hinder strength gains. Here’s what the strength changes in the three groups looked like (from a graph redrawn by University of California Davis researcher Keith Baar in this paper):
One explanation of the interference effect goes something like this: Resistance training activates a protein called mTOR that (through a cascade of molecular signals) results in bigger muscles. Endurance training activates a protein called AMPK that (though a different signaling cascade) produces endurance adaptations like increased mitochondrial mass. AMPK can inhibit mTOR, so endurance training blocks muscle growth from strength training.
[...]
There’s now emerging evidence of an alternate molecular-signaling pathway in which metabolic stress — not just from endurance training but also from other triggers like caloric deficit, oxidative stress, and aging — hinders muscle growth. This alternate pathway involves a complex series of links between a “tumor suppressor” protein, another protein called sestrin, and various other obscure acronyms. But details aside, what’s particularly intriguing about the hypothesis is that it’s highly sensitive to the presence of the amino acid leucine, which binds to sestrin and triggers the synthesis of new muscle protein. And this, Baar says, suggests some strategies to beat (or at least minimize) the interference effect.
[...]
Does workout order matter? In the traditional mTOR versus AMPK picture, you’re better off doing endurance before strength training. That’s because the endurance signals only stay elevated for about an hour following exercise, while strength signals stay on for 18 to 24 hours. But in the revised picture, where metabolic stress is the key, the order of workouts is less important than your energy balance.
Finally, it’s important to point out that you shouldn’t stress about this unless you’re training pretty damn hard. As a general rule of thumb, if you’re not doing endurance training four or more times a week, or pushing your workouts (i.e., sustaining above 80 percent of VO2max), you’re unlikely to be hurting your strength gains.