Effect of added mass on human unipedal hopping at three frequencies

Although more commonly done by children, hopping appears to be a rich source of neuromuscular and biomechanical information on adults. Given prior research on the independent effects of hopping frequency and added mass, this study assessed whether these would interact to affect vertical stiffness, contact duration, and lower extremity kinematics during unipedal hopping. Vertical force and two-dimensional kinematics were measured in 10 healthy males hopping at three frequencies: their preferred hopping frequency and frequencies 20% higher and 20% lower, in two conditions with added mass (body mass+10% and body mass+20%). Vertical stiffness was directly related to hopping frequency, while hip flexion, knee flexion, and ankle dorsiflexion were inversely related to hopping frequency. Additional mass significantly increased ankle dorsiflexion and contact duration but did not significantly affect hip flexion, knee flexion, or vertical stiffness. The differential response of vertical stiffness to hopping frequency and added mass was consistent with predictions based on a mass-spring model. The interactive effect of frequency and added mass on the kinematics of the lower extremity and contact period were consistent with earlier studies of the independent effects of hopping frequency and added mass.