Even the muscles that are built for endurance can get bigger—like, over 10% bigger—if you do the right kind of strength training for a few months. This goes against what people used to think.

We analyzed the available evidence and found that slow-twitch muscle fibers—those typically linked to endurance—can increase in size with the right kind of resistance training. The evidence we’ve reviewed so far includes 40 studies or assertions that support this idea, with none that contradict it [1]. This challenges older beliefs that only fast-twitch fibers, which are built for power and speed, respond to strength training by growing larger. What we’ve found suggests that even muscles made for long-lasting activity, like those in the calves or posture muscles, can get bigger—by more than 10% in some cases—after several months of consistent strength work. This doesn’t mean they grow as much as fast-twitch fibers, but they do change. The type of training matters: it needs to be challenging enough to stress the muscle, even if it’s not heavy lifting. This doesn’t prove that everyone will see the same results, and we don’t yet know how much of this growth comes from more muscle protein, fluid, or other changes. But the pattern across these 40 reports consistently points to growth in slow-twitch fibers under resistance. For someone looking to build stronger, more resilient muscles—whether for running, cycling, or daily movement—this means strength training isn’t just for power athletes. Even if your goal is endurance, lifting weights or doing bodyweight exercises with enough intensity can help your muscles adapt and grow. We’re still learning how much growth is possible, how long it lasts, and how it compares across different people. But so far, the evidence doesn’t support the old idea that slow-twitch fibers are unchanged by strength work.