Why Am I Slower On The Treadmill? Why Pace and Perceived Effort Can Differ vs Outside

Running on a treadmill and running outdoors can feel surprisingly different—and that difference isn’t just in your head. Many runners notice their treadmill pace doesn’t quite match what they’re used to on the roads or trails. Often, it actually feels harder to hit the same speeds indoors, even when the overall effort seems similar.

In this post, I’ll break down what the research shows about why that happens. Drawing on peer-reviewed studies from the past 15 years, we’ll look at how treadmill vs. overground running impacts both pace and Rate of Perceived Exertion (RPE). We’ll explore the physiological and psychological factors at play—like biomechanics, air resistance, sensory feedback, and mental load—and we’ll wrap with some practical takeaways you can apply in your own training.

The Pace Dilemma: Why Does the Treadmill Feel Different?

If you've ever felt like running on the treadmill is harder than running outside at the same pace, you're not alone. Many runners report that their treadmill pace is slower for an equivalent effort compared to running outdoors. RPE – basically how hard an effort feels on a subjective scale – can vary between the two settings even if your actual speed or heart rate is the same. In simpler terms, a pace that feels "moderate" outside might feel "hard" on the treadmill.

Why does this happen? It turns out there are both physiological and perceptual reasons. Factors like biomechanics, oxygen uptake (VO₂), and heart rate play a role, as do mental aspects like boredom, motivation, and sensory input and environment. Let's break down the differences.

Physical Factors: Running Mechanics and Physiology

Energy Cost and Oxygen Uptake

A key question is whether running on a treadmill uses the same amount of energy as running outdoors. Intuitively, you might expect differences since outdoors you propel yourself through air, but studies show the energy cost (VO₂) is almost identical on a treadmill versus outside at the same pace​. In fact, a classic finding from the 1990s was that setting a treadmill at ~1% incline compensates for the lack of air resistance and makes the effort comparable to running on flat ground outside​.

However, newer studies indicate this 1% rule isn't a one-size-fits-all. A 2019 meta-analysis found that at most paces, running on a treadmill with 0% incline only slightly underestimates the oxygen cost (by a trivial amount) and the difference was not statistically significant​. In other words, you don't always need a 1% incline to mimic outdoor effor – for most recreational running speeds, a flat treadmill is energetically very close to outside running.

That said, at very fast speeds, air resistance becomes more important. Sprinters or fast runners might benefit from a slight incline to make treadmill running more comparable to the wind resistance they'd face outdoors. But for the average runner doing a comfortable pace, the lack of wind on the treadmill isn't hugely affecting energy burn. Purely in terms of calories burned or oxygen used, treadmill and road are pretty close cousins.

Heart Rate and Perceived Effort

What about heart rate and other internal responses? Interestingly, the differences here depend on how fast you're running. The same 2019 meta-analysis noted a pattern: at slower running speeds, runners’ heart rate and RPE were slightly lower on the treadmill than outside, but at faster submaximal speeds (think hard tempo runs), heart rate and RPE were higher on the treadmill​. In other words, an easy jog on a treadmill might feel a tad easier on the body than the same jog outside, but a hard run on the treadmill might feel tougher than pushing the pace outdoors.

Why the flip? One theory is thermoregulation – indoors on a treadmill you don't get the cooling effect of air rushing past you. At higher efforts, your body heats up more and your heart rate climbs to send blood to the skin for cooling. Outdoors, even a small breeze can help cool you down. So a fast run on the treadmill could drive your heart rate up more for the same pace, simply due to warmth and lack of airflow.

Physiologically, running at the same speed on a treadmill vs outside is mostly similar, but treadmill runs can lead to a higher heart rate and perceived effort at faster paces. These differences likely come from the environment (heat, cooling, etc.) rather than any fundamental change in how your body produces energy.

Running Form and Biomechanics

How does your running form change (or not change) on the treadmill? Some runners feel their stride is different on a belt – maybe shorter or bouncier – and there are common beliefs that the treadmill might be doing some of the work for you. Let's address that myth first: you might have heard people say "the treadmill pulls your leg back, so you don't have to push off as hard." In reality, biomechanists confirm that you still have to push off with essentially the same force – otherwise you'd drift backward off the machine​. Whether the ground is moving under you (treadmill) or you're moving over the ground (outdoors), the physics for your legs are very similar at a constant speed. A 2020 review concluded that most biomechanical measures are almost identical between treadmill and overground running. This included things like step length, ground reaction forces, and muscle activation patterns, which were all largely comparable in both environments​.

There are a few minor differences: runners often land with a slightly flatter foot and more flexed knee on the treadmill, and experience marginally less vertical bounce and shorter strides​. These subtle changes likely occur because the treadmill’s moving belt and uniform surface alter sensory feedback a bit. Treadmill running tends to be a bit "smoother" (lower impact forces), but these are very small adjustments. Importantly, none of these slight biomechanical differences have a big effect on effort.

Most runners adapt to whatever surface they're on. If you feel awkward the first time on a treadmill, it's likely just a short coordination adjustment. The takeaway: the treadmill isn't cheating your legs out of work – you use basically the same muscles in the same way. So if a treadmill run feels harder or slower, it's probably not because your gait is drastically different (assuming you're not holding onto the handrails or doing something unusual).

Perceptual Differences: RPE and the Mental Game

Here's where things get really interesting: your brain has a big say in how hard a run feels. Runners often say treadmill running is mentally tougher – words like "boring" or "tedious" come up a lot. Without changing anything physically, your perception can dramatically color your experience.

Studies have repeatedly found that when runners think they're putting in the same effort on a treadmill as they are outdoors, they often end up running slower on the treadmill. In a 2012 experiment, researchers had participants run on an outdoor track at a self-chosen comfortable pace for 3 minutes, then hop on a treadmill (with the speed display covered) and adjust it until it felt like the same speed​. The result? The self-selected treadmill speed was significantly slower than their actual outdoor running speed​.

When they went back outside afterward, their pace returned to the original level. This showed that our perception of speed is distorted on the treadmill. The runners felt like they were running at the same speed, but objectively, they weren't. The likely reason is the lack of normal visual cues – the movement of scenery and the flow of air – that we're used to when running​. Our brains use those cues (called "optic flow") to gauge how fast we're going. On a treadmill, your environment isn't moving past you, so your brain can be tricked into thinking you're going faster than you really are. That leads you to settle at a slower belt speed unless you have the treadmill’s readout or other cues to recalibrate your sense of pace.

Apart from speed perception, there's the overall perceived effort difference many experience. As mentioned, a moderate treadmill run can feel harder than the same run outside. Why might a treadmill feel tougher even if the physical work is the same? Experts point to a few factors:

• Lack of airflow and cooling: On a treadmill, unless you have a fan set up, there's no wind hitting you. You tend to feel warmer and more uncomfortable, which can boost your RPE. A bit of breeze outside can make a big difference in perceived effort by cooling your body and reducing thermal strain.

  
  

• Reduced sensory variety: Outside, the scenery changes, terrain may vary, and you have a sense of moving somewhere. These stimuli distract you from the discomfort of running. On a treadmill, the environment is static. You're essentially staring at one spot (or maybe a TV screen), and this monotony can make you hyper-aware of every mile ticking by and every little signal of fatigue from your body​ With no change in surroundings, you're more likely to notice every feeling of fatigue, which naturally makes the run feel harder​

  
  

• Mental fatigue and boredom: The repetitive nature of treadmill running can induce mental fatigue. When you're bored or mentally drained, your motivation drops and RPE goes up. (There's even research showing that mental fatigue – say, from a long cognitive task – can decrease physical endurance performance.) If running in place feels like a chore, you might reach your mental limits sooner, even if your muscles and heart could physically go longer.

  
  

• Motivation and enjoyment: Many runners simply find outdoor running more enjoyable. The sights, sounds, and feeling of being in nature or actually going somewhere can boost your mood and motivation. A 2024 review confirmed that people generally enjoy outdoor exercise more than indoor exercise at the same intensity. Greater enjoyment often means lower perceived effort – time flies when you're having fun (or at least it's less miserable!). On a treadmill, you may need to actively find ways to keep your mind engaged (music, podcasts, etc.) to capture some of that outdoor mojo.

  
  

All these perceptual factors help explain why some runners run slower on the treadmill despite similar effort levels – largely because the effort feels harder, so you subconsciously (or consciously) don’t push the pace as much as you would outside.

Interestingly, not everyone finds the treadmill harder. A minority of runners actually prefer the treadmill – often those who appreciate the controlled environment. If you have a great TV show on or music pumping, you might distract yourself enough that the treadmill feels easier. Indeed, studies have shown that self-selected entertainment can improve treadmill enjoyment and even performance (people ran longer with music/TV than with no stimulation)​. But by and large, among recreational runners it's common to hear that the same pace "just feels tougher" on the treadmill.

Why Am I Slower on the Treadmill? Putting It Together

Let's directly address the main conundrum: Why do you run slower on a treadmill despite similar effort? It boils down to the combination of factors we've discussed:

• Perception of speed is skewed: Without the usual visual motion around you, you might think you're running faster than you really are. As a result, you set the treadmill to a slower speed or naturally settle into a slower pace if it's self-paced​. This is a subconscious pacing adjustment – your brain is essentially “tricking” you into easing off the gas.

  
  

• Higher RPE at a given pace: Because of less cooling, less distraction, and the psychology of being stationary, your perceived exertion is often higher on the treadmill for the same actual pace. When a run feels harder, we usually slow down. So even if you try to match an outdoor speed, it might feel unsustainably hard on the treadmill, causing you to back off to keep the effort manageable.

  
  

• Earlier onset of mental fatigue: You might simply get "tired of" the treadmill faster. One study noted that runners fatigued nearly 9 minutes sooner on a treadmill than overground at the same speed, even though their heart rates and RPE at the point of exhaustion were similar​. The runners didn't physiologically max out; something about the treadmill made them quit earlier. This highlights how boredom or mental strain can cut a run short before your body truly gives out.

  
  

• Less external feedback: Outside, you get lots of feedback – you see distance covered, you feel wind resistance, you adjust naturally to hills or turns. These cues help you regulate your effort without overthinking. On a treadmill, the feedback is mostly numerical (speed, time, distance on a screen). Some runners find that staring at the clock or distance counter makes the run feel longer. If you're watching each second crawl by, you might perceive working harder for each tenth of a mile, whereas outside a few minutes could pass without you fixating on every step.

  
  

Of course, there are individual differences. Some people adapt to treadmills quickly and can translate their outdoor paces pretty closely; others always struggle to get the same feeling indoors. If you're not used to treadmill running, your first few sessions might feel much harder – your body and mind haven't yet learned that this new setup is just another form of running.

Practical Implications for Training

So, what should you do with all this information? Here are some coach-friendly takeaways for training:

• Use RPE as a guide: Treadmill and outdoor paces might not align, so train by effort (RPE or heart rate) rather than forcing an exact pace. If a certain pace feels harder on the treadmill than it does outside, slow down until the effort level is right – you'll still get the intended training benefit because your body responds to the effort, not the specific speed.

  
  

• Adjust for heat: If you're running on a treadmill in a warm room or without a fan, expect a higher heart rate and RPE at a given pace. Use a fan or good ventilation if possible. Otherwise, don’t be surprised if you need to run a bit slower to keep the effort level (or heart rate) where it should be – it's just your body compensating for the warmer conditions.

  
  

• Mind over monotony: If treadmill boredom is your nemesis, have a plan to keep your mind engaged. Cue up a good playlist, watch a favorite show, or try a varied workout (like intervals or fartlek) to break up the time. Research has shown that runners perform better and go longer with self-chosen entertainment than with no distractions​.The more you can occupy your mind, the less you'll fixate on the effort.

  
  

• Don't obsess over pace conversion: There’s no perfect formula to translate outdoor pace to treadmill pace. You might find you’re 10–20 seconds per mile slower on the treadmill at the same effort – and that’s OK. Use the treadmill’s feedback as a guide, but give yourself permission to adjust. Remember that effort is what counts for fitness gains. If you’re hitting the right effort zones, you’re doing it right, whether the pace is slightly different or not.

  
  

• Train both ways: If you have a goal race or regular outdoor runs, make sure to incorporate some outdoor training even if you primarily use a treadmill (and vice versa). Each environment has its own feel. Practicing on the terrain or setting where you’ll be performing (road, track, treadmill, etc.) will help you calibrate your pacing and comfort. For example, if you've been doing all winter training on a treadmill, ease into some outside runs before a spring road race to readjust to pacing with wind, hills, and uneven ground.

  
  

• Leverage treadmill advantages: Despite its quirks, the treadmill can be a valuable training tool. You can precisely control your pace and incline, simulate race courses (by programming hills), and avoid bad weather or unsafe conditions. The softer surface may reduce impact on your joints, which can be helpful if you're prone to injury. Use these advantages to get quality workouts in when outdoor running isn’t practical. Just keep the perceptual differences in mind – what feels "hard" or "easy" on the treadmill might not directly match outdoors, and that's fine.

SOURCES

Kong PW, Koh TM, Tan WC, Wang YS. Unmatched perception of speed when running overground and on a treadmill. Gait Posture. 2012;36(1):46-48. doi:10.1016/j.gaitpost.2012.01.001

Peddie L, Gosselin Boucher V, Buckler EJ, et al. Acute effects of outdoor versus indoor exercise: a systematic review and meta-analysis. Health Psychol Rev. 2024;18(4):853-883. doi:10.1080/17437199.2024.2383758

Van Hooren B, Fuller JT, Buckley JD, et al. Is Motorized Treadmill Running Biomechanically Comparable to Overground Running? A Systematic Review and Meta-Analysis of Cross-Over Studies. Sports Med. 2020;50(4):785-813. doi:10.1007/s40279-019-01237-z

van Middelaar, Robbert P.; Wouda, Frank J.; and Reenalda, Jasper (2024) "RUNNERS FATIGUE EARLIER ON A TREADMILL VERSUS OVERGROUND: KINEMATIC EVALUATION OF A RUN-TO-EXHAUSTION," ISBS Proceedings Archive: Vol. 42: Iss. 1, Article 181.

Available at: https://commons.nmu.edu/isbs/vol42/iss1/181