Purpose: This study was designed to characterize and compare the vertical (kvert) and leg (kleg) stiffness measured during running in two different footwear conditions on negative, level, and positive slopes, using kinematic data only. Methods: Fourteen male recreational runners (age 23.4 4.4 years, height 177.5 5.2 cm, and body mass 69.5 5.3 kg) were tested on 2 separate days within 1 w…