Long distance running does not build significant muscle mass. In fact, the primary adaptation your body makes to endurance running is metabolic, not structural. Your muscles become more efficient at using oxygen and fuel, but they don’t grow larger the way they would from lifting weights. That said, the answer has some important nuances depending on your age, training history, and how you fuel your runs.
What Running Actually Does to Your Muscles
When you run long distances consistently, your muscles adapt by improving their internal machinery rather than increasing in size. Endurance training does not increase muscle fiber number or cross-sectional area. In some cases, the cross-sectional area of individual muscle fibers actually decreases, which shortens the distance oxygen and carbon dioxide need to travel within the cell. This makes your muscles better endurance machines, but not bigger ones.
The reason comes down to competing signals inside your cells. Your body has two key regulatory systems that respond to exercise. One promotes muscle growth by driving protein synthesis, and it responds primarily to heavy mechanical loading like resistance training. The other activates during endurance exercise to manage energy demands, essentially telling your cells to become more efficient rather than bigger. Research in muscle cells confirms that higher-intensity sustained loading suppresses the growth signal through activation of the energy-management pathway. In plain terms, the longer and harder you run, the more your body prioritizes endurance efficiency over muscle building.
The Exception: Beginners and Older Adults
If you’re sedentary or deconditioned, starting a running program can actually produce measurable muscle growth, at least temporarily. A landmark study found that older men (around age 68) who walked and jogged five times per week for six months experienced a 9% increase in thigh cross-sectional area. A later study comparing young men (age 20) and older men (age 74) who were non-exercisers found that both groups gained muscle from aerobic training, with the younger group seeing about a 7% increase and the older group about 6%. Interestingly, the older men produced more muscle growth per unit of work completed, suggesting they may be more sensitive to the anabolic stimulus of regular exercise.
This doesn’t last, though. Once your muscles adapt to the demands of running, further growth plateaus. For young, already-active individuals, running offers little to no additional muscle-building stimulus. The initial gains in beginners likely come from the simple fact that any new physical challenge forces adaptation when your baseline is low.
The Hormonal Shift During Long Runs
Long distance running creates a hormonal environment that actively works against muscle growth. During prolonged runs, cortisol (your body’s primary stress hormone) rises to roughly 1.5 times its resting level, while testosterone, which supports muscle repair and growth, drops gradually and stays suppressed into the evening. The ratio between these two hormones tips decisively toward a catabolic state, meaning your body is breaking tissue down rather than building it up. This shift is a normal part of endurance performance, but it’s one reason high-mileage runners tend to carry less muscle mass over time.
Running Combined With Strength Training
Many runners also lift weights, hoping to get the best of both worlds. This works, but running does introduce a small interference effect. A systematic review and meta-analysis found that when people combined aerobic and strength training, whole-muscle size wasn’t dramatically affected compared to strength training alone. However, at the individual fiber level, there was a measurable negative effect on fiber growth, and this interference was more pronounced when the aerobic exercise was running rather than cycling. Running’s high-impact, repetitive nature likely creates more muscle damage and recovery demand, which competes with the repair process that drives growth from lifting.
If your goal is to build muscle while still running, cycling may be a better cardio option for minimizing interference. If you’re committed to running, separating your runs and lifting sessions by at least several hours, and prioritizing nutrition around both, can help reduce the conflict.
Why Running Falls Short for Age-Related Muscle Loss
Running engages your slow-twitch muscle fibers almost exclusively. These fibers are built for endurance and fatigue resistance, not force production. The fast-twitch fibers responsible for strength, power, and the bulk of your muscle mass get relatively little stimulation from distance running. This is why aerobic exercise alone does not significantly improve muscle strength in older adults dealing with sarcopenia (age-related muscle loss). A meta-analysis of over 1,200 older adults found no statistically significant improvements in grip strength or knee extension strength from aerobic exercise alone. Resistance training and combined programs, on the other hand, produced significant strength gains.
Long-term endurance runners do show one advantage: older runners tend to have larger slow-twitch fibers than sedentary people their age, and even larger than younger adults matched for fitness. So running preserves the fibers it uses, but it won’t protect the fast-twitch fibers that decline most with aging.
Protein and Calorie Needs for Runners
Even if running won’t build muscle, what you eat determines whether it breaks muscle down. Endurance runners need about 1.2 to 1.4 grams of protein per kilogram of body weight daily. For a 150-pound (68 kg) runner, that’s roughly 82 to 95 grams of protein per day. During longer training runs, consuming about 0.25 grams of protein per kilogram per hour of exercise, combined with carbohydrates, helps offset muscle breakdown in real time.
Total calorie intake matters just as much as protein. If you’re not eating enough overall, your body will break down protein for energy instead of using it for muscle repair. This is a common pitfall for runners who undereat relative to their training volume, and it accelerates muscle loss over time. Carbohydrates are especially important here because they spare protein from being burned as fuel, keeping it available for its repair and maintenance roles.