Although subducting slabs undergo a bending deformation that resists tectonic plate motions, the magnitude of this resistance is not known because of poor constraints on slab strength. However, because slab bending slows the relatively rapid motions of oceanic plates, observed plate motions constrain the importance of bending. We estimated the slab pull force and the bending resistance globally for 207 subduction zone transects using new measurements of the bending curvature determined from slab seismicity. Predicting plate motions Using a global mantle flow model, we constrain the viscosity of the bending slab to be at most similar to 300 times more viscous than the upper mantle: stronger slabs are intolerably slowed by the bending deformation. Weaker slabs, however, cannot transmit a pull force sufficient to explain rapid trenchward plate motions unless slabs stretch faster than seismically observed rates of similar to 10(-15) s(-1). The constrained bending viscosity (similar to 2 x 10(23) Pa s) is larger than previous estimates that yielded similar or larger bending resistance (here similar to 25% of forces). This apparent discrepancy occurs because slabs bend more gently than previously thought, with an average radius of curvature of 390 km that permits subduction Of Strong slabs. This gentle bending may ultimately permit plate tectonics on Earth.