Stiffness is defined as the resistance of a body to deformation (Brughelli & Cronin, 2008). It influences how the body interacts with the ground in terms of mechanics and joint kinematics (Farley & Morgenroth, 1999). Optimal stiffness is important in sprinting and jumping as it relates to efficient use of the stretch-shortening cycle. Weighted vests are used to overload the muscles in warm-up and training and have been found to enhance subsequent jumping performance (Faigenbaum et al., 2006). The purpose of this study was to examine the acute effects of wearing a weighted vest on vertical stiffness during hopping in place using a simple spring mass model.