The purpose of this study was to investigate whether the action observation changes the learner's leg stiffness, representing mechanical state of global leg spring, during drop jump (DJ) and whether the process to observe the learner's and model's movement is effective method. We used the drop jump as trial movement, since its performance is influenced by the change in leg stiffness. 21 subjects we employed were divided into 3 groups (GPs) according to the observation type-GP1: observation of both the model's and their own DJ; GP2: observation of only the model's DJ; GP3: no observation with rest. The subjects performed DJ pre and post observation. Leg stiffness was estimated using the leg length (240 fps) and vertical ground reaction force (Fz) (2000 Hz) during stance phase and compared between pre- and post-tests in each group. In GP1, kinetic (the leg stiffness and maximum value of Fz) and kinematic (i.e., contact duration and change in leg compression) parameters in posttest were significantly higher and lesser than that in pretest, respectively. The results indicate that the leg stiffness is increased by the action observation and thus the observation procedure is effective to enhance the DJ skill that utilizes the external force. Further, the results suggest that observing learner's own and model's movements is to enhance the effect of action observation.
