Endurance training can build muscle, but only under specific circumstances. Over a 12-week endurance program, sedentary or lightly active individuals have gained 7% to 11% more muscle mass in the muscles they trained. That’s a meaningful increase, though it comes with important caveats about who responds, which muscles grow, and how those gains compare to what resistance training delivers.
Who Actually Gains Muscle From Endurance Training
The muscle growth observed in endurance studies has come almost exclusively from people who were sedentary or had limited exercise experience before starting. For someone whose legs aren’t used to sustained effort, cycling three or four times a week provides enough of a novel stimulus to trigger growth, particularly in the quadriceps. The body doesn’t distinguish neatly between “cardio stimulus” and “strength stimulus” when the muscles involved haven’t been challenged before. Any sufficiently demanding new load can push them to adapt.
For trained individuals, the picture changes dramatically. If you already run, cycle, or lift weights regularly, endurance training alone is unlikely to add noticeable muscle. Your body has already adapted to the range of forces those activities produce, and the stimulus endurance work provides simply isn’t heavy enough to drive further growth. This is why elite marathon runners and competitive cyclists tend to carry less muscle mass than sprinters or strength athletes, not more.
Why Endurance and Strength Pull in Different Directions
Your muscles respond to exercise through competing molecular signals. Resistance training activates a pathway centered on a protein complex called mTOR, which ramps up muscle protein synthesis and drives growth. Endurance training activates a different sensor called AMPK, which responds to the energy drain of prolonged effort. AMPK improves your mitochondria (the energy-producing structures inside muscle cells), making you more efficient at burning fuel over long periods.
The problem is that AMPK directly inhibits mTOR through multiple mechanisms. It essentially tells the cell, “We’re in an energy deficit, now is not the time to build expensive new protein.” This is why very high volumes of endurance work can actively interfere with muscle growth, even when combined with strength training. The more energy you burn through long cardio sessions, the stronger that inhibitory signal becomes.
This doesn’t mean endurance training destroys muscle. It means that past the initial adaptation phase, the molecular environment it creates favors endurance adaptations (better oxygen delivery, more efficient energy use) over size increases. Your muscles get better at working longer without getting bigger.
How Different Activities Compare
Not all endurance exercise affects muscle the same way. Cycling involves only concentric contractions, where the muscle shortens under load as you push the pedal down. Running involves both concentric and eccentric contractions, where the muscle also lengthens under load during each foot strike. This distinction matters because eccentric contractions create more mechanical tension in muscle fibers and activate them at higher levels.
When researchers compared different endurance modalities combined with resistance training, high-intensity interval running paired with lifting produced the best results for muscle cross-sectional area in the legs. Moderate-intensity continuous cycling with lifting also performed well. Other combinations showed no clear advantage over resistance training alone.
Interestingly, running’s eccentric component may actually preserve muscle fiber integrity better than cycling, which could make it easier to pair with strength training without as much interference. If your goal is both endurance fitness and muscle maintenance, the type of cardio you choose matters more than most people realize.
High Intensity vs. Steady State
A common assumption is that high-intensity interval training builds more muscle than steady-state cardio because it recruits a broader range of muscle fibers, including the fast-twitch fibers typically associated with strength and power. The logic is sound on paper: harder efforts should demand more from the muscle and trigger a bigger adaptive response.
In practice, studies comparing HIIT protocols (like Tabata intervals) to steady-state training in untrained young adults found substantial equivalence across measures of both aerobic and anaerobic performance. All groups improved peak power output by 5% to 9% and mean power by 4% to 7%, with no significant differences between them. The untrained body responds broadly to any new demand, and the specific intensity matters less than simply being consistent with a challenging program.
For already-active individuals, HIIT likely has a slight edge for preserving or building muscle compared to long, slow efforts, simply because shorter, harder bouts produce less total AMPK activation and more mechanical tension per session. But the difference is modest, and neither approach substitutes for actual resistance training if muscle size is a priority.
Protein Needs for Endurance Athletes
If you do endurance training and want to support whatever muscle-building potential it offers, protein intake matters more than most endurance athletes assume. Current evidence recommends about 1.8 grams of protein per kilogram of body weight per day for endurance-trained individuals. That’s 50% higher than what sedentary adults need and comparable to what many strength athletes consume.
For a 70-kilogram (154-pound) person, that works out to roughly 126 grams of protein daily. During periods of hard training with restricted carbohydrate intake, or on rest days, needs climb even higher to around 2.0 grams per kilogram. Women in the luteal phase of their menstrual cycle (the two weeks before a period) may benefit from bumping intake slightly to about 1.9 grams per kilogram.
Endurance exercise increases protein turnover significantly. Your body breaks down and rebuilds muscle proteins at a higher rate during and after long efforts, and without adequate protein, it can’t fully replace what’s lost. Many endurance athletes under-eat protein because they focus on carbohydrates for fuel, which can accelerate muscle loss over time, particularly during high-volume training blocks.
The Practical Takeaway
If you’re currently inactive and start a cycling or running program, you will likely gain some muscle in your legs during the first few months, especially if you’re eating enough protein. Those gains are real and meaningful for overall health and function. But they plateau quickly, and endurance training alone won’t produce the kind of progressive muscle growth that resistance training delivers.
If you already exercise regularly and want to build muscle, endurance training is not the tool for the job. It can complement a strength program by improving cardiovascular health and recovery capacity, but the molecular signals it generates actively compete with muscle growth pathways. Keeping cardio sessions shorter or higher intensity, rather than long and moderate, minimizes that interference. And regardless of your primary training goal, hitting 1.8 grams of protein per kilogram of body weight gives your muscles the raw material they need to adapt to whatever you ask of them.