Athletes come in all shapes and sizes. However, if we look at elite endurance cyclists and runners, we can see a pattern: Even when they have similar aerobic fitness, cyclists tend to have teardrop-shaped legs with large quads, whereas runners have leaner, more streamlined legs.
Why do cyclists’ and runners’ legs look so much different? The answer lies in how their muscles adapt to the demands of their sport. Let’s dive into the science.
Why Cyclists Have Bigger Quads
This is because cycling is a concentric-dominant activity, meaning the muscles shorten under load. Every pedal stroke primarily engages the quadriceps, especially the vastus lateralis and rectus femoris, to generate power. This repeated shortening under resistance stimulates muscle hypertrophy, leading to visibly larger quads but relatively smaller calves.
Research shows that elite cyclists can have up to 25% larger quadriceps cross-sectional area than runners with similar VO₂ max. In other words, even if a cyclist and a runner have the same aerobic capacity, the cyclist’s legs are built for sustained torque and power on the pedals.
Cyclists’ larger quads allow for higher force output with each pedal stroke, helping cyclists maintain speed over long distances or steep climbs.
By contrast, large quads would be a disadvantage for runners. More muscle means more mass to move with each stride. Leaner legs are more energy-efficient.
Key takeaway:
Cycling strengthens and grows the quads, optimizing them for power output.
Why Endurance Runners Have Leaner Legs
Running, by contrast, is largely eccentric-dominant. When your foot strikes the ground, the muscles lengthen under load, particularly in the quadriceps and calves. This type of loading doesn’t cause massive hypertrophy, which is why runners typically don’t have large quads like cyclists. Their leg muscles are more balanced in terms of size and strength.
The eccentric motion causes runners’ leg muscles to become stiffer. The stiffness is an advantage because the muscles act like springs, storing and releasing energy with each stride. A well-trained runner has such springy muscles and efficient movement that their feet don’t stay in contact with the ground long.
This “elastic recoil” can save up to 50% of the energy cost of running, making every step more efficient.
By contrast, cyclists don’t benefit from springy muscles. If their muscles are too springy, it can actually make it harder to transfer power from their upper body to the pedal. This is also why cyclists often suffer from pain in their calf muscles and Achilles tendon if they attempt running. Their muscles simply aren’t used to being stretched.
Key takeaway:
Running performance depends more on tendon elasticity and efficiency than brute leg size.
What about Triathletes?
Triathletes face a unique challenge: they need to be efficient both on the bike and on the run. Because of this dual demand, triathletes typically have legs that are moderately muscular, yet lean, combining enough quad strength for cycling with enough tendon elasticity for running.
However, achieving this balance can be difficult. If the quads become overdeveloped, it can reduce running efficiency and increase the risk of injuries such as IT band syndrome or knee pain. If the calves and tendons become too springy, they may not be able to adequately absorb impact from cycling, which can compromise pedaling efficiency and increase fatigue.
How triathletes manage this:
- Include strength training for hamstrings, glutes, and calves to maintain balanced muscle development.
- Focus on core stability and hip control to preserve proper running mechanics after cycling.
- Stretch and foam-roll quads regularly to reduce tightness and support posterior chain function.
Key takeaway:
Triathletes’ legs are optimized for both power and efficiency—strong enough for the bike, springy enough for the run.
Trail Runners Legs Are Hybrid
Interestingly, ultra-trail runners often develop leg muscles that more closely resemble those of cyclists than typical road runners. This is largely because trail running involves steep climbs, uneven terrain, and frequent bursts of power. Large quadriceps help trail runners generate the torque needed to push up hills and navigate technical terrain.
At the same time, trail running still requires efficient elastic energy use—stiff tendons and springy muscles help runners manage repeated strides downhill and absorb impact on rugged surfaces. The result is a combination of quad strength and tendon efficiency, giving them legs that are both powerful and resilient.
Key takeaway:
Trail runners’ legs are a hybrid—strong like a cyclist’s quads but retaining the springy efficiency needed for running.
Who Has Stronger Leg Muscles: Cyclists or Runners?
If “strength” means the ability to lift, push, or pull more weight, the answer isn’t straightforward. Cycling and running use the legs in very different ways, so it makes more sense to compare specific muscle groups rather than the legs as a whole.
Among athletes with similar aerobic fitness, cyclists usually develop stronger quadriceps, while runners tend to have stronger calves. Each sport builds strength where it’s needed most.
Glycogen: The Shared Fuel for All Endurance Athletes
Despite these differences in leg anatomy, both cyclists and runners—as well as all other endurance athletes—rely heavily on stored glycogen as a primary energy source during endurance exercise. Glycogen is the stored form of carbohydrates in your muscles, ready to be converted into energy when your body demands it.
- Cyclists use glycogen to maintain sustained power output during long rides or climbs.
- Runners use glycogen for repeated strides, push-offs, and high-intensity efforts.
When glycogen levels drop below 200 mmol/kg dry weight, muscle contractility decreases, calcium release inside muscle fibers is impaired, and legs start to feel “heavy.” The athletes will experience early fatigue, slower performance, and difficulty maintaining pace.
Carbohydrates Are Key
Glycogen is limited, so consuming carbohydrates can extend endurance, improve power, and delay fatigue. Key considerations include:
- Loading: Endurance athletes should eat a carbohydrate-rich meal the day before their session or race. This ensures muscle glycogen levels are topped off.
- Fueling: Athletes generally need 45-90 grams of carbohydrates per hour to sustain endurance exercise.
- Type: Fast-absorbing carbohydrates are crucial because they quickly restore glycogen, such as from energy gels or drinks.
Cyclists Have an Advantage with Glycogen Storage
While both cyclists and runners rely on glycogen in their leg muscles for fuel during long sessions, cyclists have an advantage: Research shows that cyclists can absorb and store glycogen more effectively than runners.
The reason? Muscle damage. Running is a high-impact, eccentric activity, especially during downhill or landing phases. Each step causes microscopic tearing and trauma to muscle fibers. This damage means muscle cells are less effective at two important tasks:
- Glucose uptake: Glucose transport proteins (GLUT-4) in cell membranes are responsible for moving glucose from the bloodstream into the muscles. But when muscle fibers are damaged, they can’t absorb carbohydrates as well.
- Glycogen replenishment: Once inside the muscle cell, glucose needs to be converted into glycogen so it can be stored for future use. The muscle damage from running disrupts the cellular machinery and enzymes responsible for this conversion, slowing down the entire process.
Normally, after glycogen depletion and proper recovery, muscles can store more glycogen than they had initially—a phenomenon called glycogen supercompensation. In runners, eccentric muscle damage can limit the magnitude of this effect.
By contrast, cycling is a low-impact, concentric exercise with minimal eccentric contractions. This means cyclists experience very little muscle damage during a ride. Without the need to repair damaged tissue, their muscles can focus entirely on absorbing and storing carbohydrates, and they can achieve more glycogen supercompensation.
What does this mean for runners? Runners will need to be even more mindful of recovery, making sure they get protein and carbohydrates within the recovery window.