A treadmill primarily works the muscles of your lower body: your quadriceps, hamstrings, glutes, calves, and the muscles along your shins. Your core also plays a supporting role in keeping you balanced and upright. How much each muscle group works depends on your speed, incline, and whether you hold onto the handrails.
Quadriceps: Your Front Thigh Muscles
Your quadriceps, the large muscle group on the front of your thighs, do a significant share of the work on a treadmill. They control your knee as your foot strikes the belt and absorb the impact of each step. During the middle of your stride, when your knee bends deepest, the quads fire hard to keep you from collapsing downward and then extend your leg to push you forward.
At faster speeds, your knee bends more dramatically during each stride cycle. Research on treadmill running found that knee flexion at midstance nearly tripled when comparing walking to running, which means your quads have to generate substantially more force as you pick up the pace. Walking on a treadmill still activates the quads, but running and jogging demand noticeably more from them.
Glutes and Hamstrings
Your glutes and hamstrings make up the “posterior chain,” the powerhouse muscles along the back of your hips and thighs. On a flat treadmill, these muscles work during the push-off phase of each stride and help control your leg as it swings forward. They’re active, but they don’t dominate the movement the way your quads and calves do on a level surface.
Incline changes that picture dramatically. When researchers measured muscle activity across increasing grades, hamstring activation increased significantly, with both the inner and outer hamstrings staying active for a longer portion of each stride as the incline rose. The glutes responded even more. Hip power generation (the work your glutes do to drive your body uphill) increased an average of 163% between a flat surface and a 10% incline. If your goal is to target your glutes and hamstrings more, cranking up the incline is the single most effective adjustment you can make on a treadmill.
Calf Muscles: Soleus and Gastrocnemius
Two calf muscles share the work of pushing off the belt with each step, but they contribute differently depending on speed. The soleus, the deeper calf muscle, becomes the dominant force producer during running and jogging. Because it only crosses the ankle joint, it’s unaffected by how much your knee bends, making it especially efficient when your knee is deeply flexed at higher speeds. Studies found that soleus activity was significantly higher during running and jogging compared to every other lower leg muscle measured.
The gastrocnemius, the more visible outer calf muscle, works from the moment your foot is loaded through the late push-off phase. It handles the final burst of ankle extension that propels you forward. Interestingly, on an incline the gastrocnemius actually activates for a shorter duration during each stride, even though overall calf demand increases. The soleus picks up more of the sustained workload as the grade rises.
Shin Muscles
The tibialis anterior, the muscle running along the front of your shin, has a specific job: it pulls your toes up toward your knee so your foot clears the ground during each swing and controls how your foot lands. On a treadmill, this muscle contracts either eccentrically or isometrically right before and after your foot hits the belt, preventing your foot from slapping down.
This muscle works harder at faster speeds. Treadmill research showed tibialis anterior activity was significantly higher during running and jogging than walking because the foot needs to be controlled more quickly as stride speed increases. Walking on an incline, however, doesn’t increase tibialis anterior activation the way it does for nearly every other lower body muscle. It’s one of the few muscles that actually decreases in activity as the grade goes up.
Core and Stabilizer Muscles
Your abdominals, obliques, and lower back muscles work throughout a treadmill session to keep your torso upright and your pelvis stable. They aren’t the primary movers, but they fire continuously to prevent you from swaying side to side or pitching forward. This stabilizing role increases at higher speeds and inclines, when the forces acting on your body are greater.
Holding onto the handrails significantly reduces this demand. Research confirms that gripping the rails lightens the overall muscular workload and forces an unnatural walking pattern. Your core doesn’t have to stabilize as much when your hands are anchored, and your legs carry less of your bodyweight. Walking or running hands-free is the better option if you want full engagement from your stabilizers and a more natural stride.
How Speed Changes Muscle Recruitment
Walking, jogging, and running on a treadmill don’t just work the same muscles harder. They recruit different types of muscle fibers. At a comfortable walking pace, your body relies mostly on slow-twitch fibers, which are built for endurance and fatigue resistance. As you increase speed toward a hard run, your body progressively recruits fast-twitch fibers, which generate more force and power but tire more quickly.
This shift happens gradually, but sustained high-intensity running accelerates it. A study on middle-distance runners found that after 40 minutes of running at a challenging pace, the estimated recruitment of fast-twitch fibers increased by roughly 29%. This happened partly because the slow-twitch fibers became fatigued and depleted their fuel stores, forcing the nervous system to call on more powerful fibers. For practical purposes, this means interval training and faster treadmill runs build more muscular strength and power than steady walking, even in the same muscles.
Incline vs. Flat: Where the Work Shifts
On a flat treadmill, the effort is relatively balanced between the front and back of your legs, with the quads and calves doing slightly more. Increasing the incline shifts the workload decisively toward the posterior chain. Here’s how the major muscle groups respond as you go from flat to a 10% grade:
- Glutes: Hip extension power increases by an average of 163%, making them the biggest beneficiary of incline walking.
- Hamstrings: Both the inner and outer hamstrings activate for a longer portion of each stride, turning off later in the gait cycle.
- Quadriceps: The rectus femoris and the vastus muscles both show increased activity, though less dramatically than the posterior chain.
- Calves: The soleus works significantly harder overall, though the gastrocnemius shortens its active window.
- Tibialis anterior: The one exception. Shin muscle activity tends to decrease with incline.
If you’re using a treadmill specifically to strengthen your glutes, walking at a steep incline (sometimes called the “12-3-30” approach) will target them far more effectively than running on a flat surface.
Treadmill vs. Running Outside
A treadmill works the same muscles as outdoor running, but not quite to the same degree in every muscle group. The moving belt assists your leg during the push-off phase, which means your hamstrings and glutes don’t have to work as hard to propel you forward. Research comparing treadmill and overground running found that glute and hamstring activation was lower on the treadmill during the stance phase, while the tibialis anterior (shin muscle) actually worked harder on the treadmill at faster speeds.
The practical difference is modest. EMG studies show that overall muscle activation profiles between treadmill and overground running are similar, especially for the thigh muscles. Setting your treadmill to a 1% to 2% incline closely mimics the muscular demands of running on flat ground outdoors. At 1% grade, thigh muscle activation most closely matches overground running. At 2%, the lower leg muscles align best.
Curved, non-motorized treadmills close this gap further. Because you power the belt yourself rather than keeping up with a motor, your hamstrings and glutes engage more naturally throughout the stride, similar to how they work when running outside.