Intervals blunted strength gains 40%, not muscle growth
Brady Holmer says the cost of running hard alongside lifting shows up in performance, not in fiber size.
Runners who lift have heard the warning for years, that cardio kills your gains. Brady Holmer points to a 2026 study that says the tradeoff is real, just not where most people look for it.
So cardio didn't stop muscle growth, but it did cost max strength.
If endurance work is borrowing energy from quality strength work, you won’t optimize the latter (and vice-versa).
Two groups did the exact same lifting program for 16 weeks. One group also added four long-interval HIIT sessions a week. Muscle fibers grew about the same in both, roughly 19 percent versus 18 percent, and protein synthesis rose the same amount. What changed was strength. Leg press strength gains were blunted by about 40 percent in the group doing the intervals. That group also picked up an 11 percent bump in peak aerobic capacity.
What it actually means
The muscle still got bigger. The ability to express force in a max lift did not improve as much. Holmer's read is that the two kinds of work are competing for the same pool of quality, not that one shuts the other down at the tissue level.
"If endurance work is borrowing energy from quality strength work, you won't optimize the latter (and vice-versa)," Holmer writes. He says the tradeoff showing up at the level of functional performance makes sense.
Holmer was answering a question @hubermanlab tagged him in, asking whether you can get strong and fast at the same time. His short answer is probably, though not always, and not for the reason people usually give.
Why a marathoner should care
For anyone training for a road race, this is close to good news. The thing the study says you give up is max strength, the top end of a single heavy lift. The thing you gain is a double digit jump in aerobic capacity. If the lifting in your week exists to keep you durable rather than to add plates to the bar, the study does not flag a cost to muscle size or to the signalling that builds it.
The flip side matters for anyone chasing both at once. Four hard interval sessions a week is a heavy aerobic load to stack on a full lifting program, and in this study that load had a price. If a hard squat day and a track session are competing in the same week, one of them is likely getting a worse version of you.
One study, one population, one measure of strength. But it is a cleaner answer than the usual argument, because the lifting program was identical in both groups and the only variable added was the intervals.
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