Proof-of-concept of a Robotic Knee Exoskeleton in Healthy Subjects and Hemiparetic Patients During Gait.

• Brief name: Proof-of-concept of a unilateral robotic knee exoskeleton and evaluation of its effects over gait symmetry.
• Materials: The investigators have developed a Knee-Ankle-Foot orthosis (KAFO) composed of two joints aligned to the knee and ankle of the user. The length of its bars and the positions of its braces can be tailored to the anthropometry of different users. The knee joint is actuated by a DC brushless motor EC-60 flat 408057 (Maxon ag, Switzerland) coupled with a harmonic drive CSD-20-160-2AGR (Harmonic Drive LLC, EE.UU.). The transmission ratio of 1:60 of this system enables the application of a mean torque of 35Nm. The ankle joint of the prototype remains non-actuated and unlimited, enabling its free movement in the sagittal plane. The total weight of the KAFO is about 4kg.

The prototype is equipped with sensors that provide information on system variables that are used for its control in real-time, such as the flexion angle of the robot joint or the interaction torque between user and robot. In addition, the gait kinematic of the user is measured by Inertial Measurement Units (IMUs) and the contact of both feet with the floor by Force Sensing Resistors (FSRs).

The system uses an Adaptive Frequency Oscillator to estimate the continuous gait phase of the contralateral limb and synchronically assists the paretic leg following two different control strategies: (1) Replicating the movement of the sound leg (Echo Strategy) or (2) Inducing a healthy gait pattern on the paretic leg (Pattern strategy). The action of the robot depends on the gait phase of the assisted leg: during stance the robot reinforces the limb so the system composed of the leg and the exoskeleton can load the user's weight and not collapse, while during the swing phase the robot guides the limb's movement according to the Assisted-As-Needed (AAN) paradigm creating a force tunnel around the prescribed trajectory.

• Procedures: Subjects were instructed to walk on a treadmill at a constant speed during trials of 5 minutes. All subjects carried out four different kinds of trials: (1) NoExo: subjects only wore the inertial sensors and the insole pressure sensors to acquire their basal motion; (2) Free: subjects wore the exoskeleton although the actuator was mechanically decoupled, so it enabled the free movement of the knee; (3) Echo: the device provided gait assistance following the Echo-control strategy; and (4) Pattern: the device provided gait assistance following the Pattern strategy. During trials, patients wore a safety harness, although it did not support any weight. Previously to the execution of the trials, the gait velocity was self-selected by the subjects to a comfortable level. Subjects rested between trials for five minutes at least, in order to avoid adaptation and learning effects from trial to trial.

Healthy subjects also performed a previous trial under variable gait speed (VariableSpeed). They walked over the treadmill as in the NoExo condition, although the gait speed randomly varies from 1km/h to 3km/h in 0.2km/h steps for 15 seconds at least.

• Intervention providers A physiotherapist and an engineer are present during the trials. The first is responsible of assessing the basal gait of the subjects and evaluating his/her state while the assistance is provided. The engineer is responsible of managing the device and recording the data.

Both researchers monitor fidelity to the intervention by direct supervision.

• Modes of delivery The protocol is provided to one participant at a time. Each patient completes the protocol once.
• Location Hospital Beata María Ana (Madrid, Spain)
• Tailoring and modifications The protocol remains unaltered across applications. The only adaptation to patients is the choice of the comfortable gait speed for the trials.