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.