In recent studies exoskeletons have been proven to augment human mobility and facilitate daily tasks such as walking, running, and hopping. Most exoskeletons are designed to reduce the effort (i.e., metabolic rate) expended by their user while performing aerobic tasks. However, exoskeletons that assist fast, explosive movement, specifically vertical jumping, have yet to be thoroughly examined. Furthermore, a fundamental lack of understanding still prevails regarding the interactions between humans and exoskeletons.

Our main hypothesis was that a passive exoskeleton has the ability to increase vertical jumping height without providing additional external energy. The designed passive knee exoskeleton consists of springs which act in parallel to the quadriceps femoris muscle. These springs store energy in the negative-work phase, during knee flexion, and inject the energy in the consequent positive-work phase, during knee extension. The stored energy can then be utilized to increase the jumping height.

The exoskeleton was tested on ten healthy participants, in two separate experimental sessions, in which they aimed to jump as high as possible. In the first session, participants jumped under five conditions- two without the exoskeleton and three with the exoskeleton and three different spring stiffness levels. The participants jumped without receiving instructions on how to use the exoskeleton. Results showed an increment in jump height as spring stiffness increased, and no difference in height between the jumps with and without the exoskeleton. In the second session, participants jumped under two conditions- without the exoskeleton and with the exoskeleton with the highest spring stiffness level. The participants were trained to better utilize the exoskeleton by exploring different jumping techniques in order to improve their adaption to the exoskeleton.

The second session, including instructions and training with the exoskeleton, resulted in a 6.4%±0.9% (mean ± SE) increase in jumping height compared to jumping without the exoskeleton. To the best of our knowledge, this is the first time a passive exoskeleton is shown to be successful in augmenting vertical jumping. The knowledge accumulated during this study regarding the human-exoskeleton interaction has the potential to assist in the development of fast explosive motion exoskeletons. Subsequently, these results will be used in our laboratory for the development of a model for the human-exoskeleton interaction using an optimal control process, that aims to enable developing different types of exoskeletons in a faster and more economical way.

Sections of this work were presented in the 2019 International and American Society of Biomechanics conference in Calgary, Canada. This work was also presented in 2020 The International Symposium on Wearable Robotics conference in Vigo, Spain (Virtually). Additionally, we are also in preparations for a publication in Science Robotics.