  Knowledge on joint impedance during walking in various conditions is relevant
for clinical decision-making and the development of robotic gait trainers, leg
prostheses, leg orthotics and wearable exoskeletons. Whereas ankle impedance
during walking has been experimentally assessed, knee and hip joint impedance
during walking have not been identified yet. Here we developed and evaluated a
lower limb perturbator to identify hip, knee and ankle joint impedance during
treadmill walking. The lower limb perturbator (LOPER) consists of an actuator
connected to the thigh via rods. The LOPER allows to apply force perturbations
to a free-hanging leg, while standing on the contralateral leg, with a
bandwidth of up to 39 Hz. While walking in minimal impedance mode, the
interaction forces between LOPER and the thigh were low (<5 N) and the effect
on the walking pattern was smaller than the within-subject variability during
normal walking. Using a non-linear multibody dynamical model of swing leg
dynamics, the hip, knee and ankle joint impedance were estimated at three time
points during the swing phase for nine subjects walking at a speed of 0.5 m/s.
The identified model was well able to predict the experimental responses for
the hip and knee, since the mean variance accounted (VAF) for was 99% and 96%,
respectively. The ankle lacked a consistent response and the mean VAF of the
model fit was only 77%, and therefore the estimated ankle impedance was not
reliable. The averaged across-subjects stiffness varied between the three time
points within 34-66 and 0-3.5 Nm/rad for the hip and knee joint respectively.
The damping varied between 1.9-4.6 and 0.02-0.14 Nms/rad for hip and knee
respectively. The developed LOPER has a negligible effect on the unperturbed
walking pattern and allows to identify hip and knee impedance during the swing
phase.
