  Physical interaction between individuals plays an important role in human
motor learning and performance during shared tasks. Using robotic devices,
researchers have studied the effects of dyadic haptic interaction mostly
focusing on the upper-limb. Developing infrastructure that enables physical
interactions between multiple individuals' lower limbs can extend the previous
work and facilitate investigation of new dyadic lower-limb rehabilitation
schemes.
  We designed a system to render haptic interactions between two users while
they walk in multi-joint lower-limb exoskeletons. Specifically, we developed an
infrastructure where desired interaction torques are commanded to the
individual lower-limb exoskeletons based on the users' kinematics and the
properties of the virtual coupling. In this pilot study, we demonstrated the
capacity of the platform to render different haptic properties (e.g., soft and
hard), different haptic connection types (e.g., bidirectional and
unidirectional), and connections expressed in joint space and in task space.
With haptic connection, dyads generated synchronized movement, and the
difference between joint angles decreased as the virtual stiffness increased.
This is the first study where multi-joint dyadic haptic interactions are
created between lower-limb exoskeletons. This platform will be used to
investigate effects of haptic interaction on motor learning and task
performance during walking, a complex and meaningful task for gait
rehabilitation.
