Schoenmakers, PPJM, Crisell, JJ, and Reed, KE. Physiological and perceptual demands of running on a curved nonmotorized treadmill compared with running on a motorized treadmill set at different grades. J Strength Cond Res 34(5): 1197-1200, 2020-The current study compared the physiological and perceptual demands of running on a commercially available curved nonmotorized treadmill (cNMT) with different incline grades on a motorized treadmill (MT). Ten male team-sport athletes completed, after a familiarization session, a 6-minute run at a target velocity of 2.78 m·s on the cNMT (cNMTrun). The mean individual running velocity of cNMTrun was then used as warm-up and experimental running velocity in 3 subsequent visits, in which subjects ran for 6 minutes on the MT set at different grades (4, 6, or 8%). In all experimental trials (cNMTrun, 4MTrun, 6MTrun, and 8MTrun) and in the warm-up of the subjects' third visit (1MTrun), oxygen consumption (V[Combining Dot Above]O2) and heart rate (HR) were monitored, and ratings of perceived exertion (RPE) were obtained. The HR in cNMTrun was significantly higher compared with all MT trials. V[Combining Dot Above]O2 and RPE were significantly higher in cNMTrun compared with 1MTrun and 4MTrun, but not different from 6MTrun and 8MTrun. The relationship between V[Combining Dot Above]O2 and MT grades was highly linear (V[Combining Dot Above]O2 = 34.36 + 1.7 MT grade; r = 0.99), and using linear interpolation, the concave curved design of the cNMT was estimated to mimic a 6.9 ± 3% MT grade. On matched running velocities, V[Combining Dot Above]O2 and RPE responses while running on the cNMT are similar to a 6-8% MT grade. These findings can be used as a reference value by athletes and coaches in the planning of cNMT training sessions and amend running velocities accordingly. Future studies are needed to determine whether this estimate is similar for female runners, or those of a lower body mass.