Estimating the Mechanical Behavior of the Knee Joint During Crouch Gait: Implications for Real-Time Motor Control of Robotic Knee Orthoses

Abstract

Individuals with cerebral palsy frequently exhibit crouch gait, a pathological walking pattern characterized by excessive knee flexion. Knowledge of the knee joint moment during crouch gait is necessary for the design and control of assistive devices used for treatment. Our goal was to 1) develop statistical models to estimate knee joint moment extrema and dynamic stiffness during crouch gait, and 2) use the models to estimate the instantaneous joint moment during weight-acceptance. We retrospectively computed knee moments from 10 children with crouch gait and used stepwise linear regression to develop statistical models describing the knee moment features. The models explained at least 90% of the response value variability: peak moment in early (99%) and late (90%) stance, and dynamic stiffness of weight-acceptance flexion (94%) and extension (98%). We estimated knee extensor moment profiles from the predicted dynamic stiffness and instantaneous knee angle. This approach captured the timing and shape of the computed moment (root-mean-squared error: 2.64 Nm); including the predicted early-stance peak moment as a correction factor improved model performance (root-mean-squared error: 1.37 Nm). Our strategy provides a practical, accurate method to estimate the knee moment during crouch gait, and could be used for real-time, adaptive control of robotic orthoses.

A lower-extremity exoskeleton improves knee extension in children with crouch gait from cerebral palsy

“…3 concurrent increase in hip extension would anteriorly shift the center of mass. Increased hamstring activity may also constitute a maladaptive response because of the external torque provided by the motor exceeding the internal flexor moment (38). Additional improvements in posture and muscle patterns may be achieved by refinement of robotic control algorithms to account for internal joint mechanics (38).…”

“…Increased hamstring activity may also constitute a maladaptive response because of the external torque provided by the motor exceeding the internal flexor moment (38). Additional improvements in posture and muscle patterns may be achieved by refinement of robotic control algorithms to account for internal joint mechanics (38). More practice time may also enhance coordination of muscle activity with the external assistance.…”

The Effects of Exoskeleton Assisted Knee Extension on Lower-Extremity Gait Kinematics, Kinetics, and Muscle Activity in Children with Cerebral Palsy

“…Further improvements in posture may be possible by incorporation of more sophisticated control methods that may reduce the total knee moment. For example, real-time estimates of the knee joint moment based on kinematic-kinetic joint coupling during crouch gait could be implemented in robotic control strategies to adjust assistance as a percentage of the instantaneous joint moment 42 . On another note, while improvements in posture were evident up the kinematic chain from the knee (i.e.…”

A Pediatric Knee Exoskeleton With Real-Time Adaptive Control for Overground Walking in Ambulatory Individuals With Cerebral Palsy

“…The goal of this mode is to enhance the volitional effort from the user during overground gait training. We previously developed element-wise knee joint moment estimation models during overground walking in children with CP (Lerner et al, 2016b;Chen et al, 2019). The knee joint of children with CP can be modeled as a dynamic spring during stance phase, with separate stiffness values assigned to the knee joint during flexion and extension.…”

“…For example, an adaptive robust integral of sign error (or adaptive RISE) controller has been developed and implemented for a knee exoskeleton that achieves excellent trajectory tracking while also providing asymptotic stability (Sherwani et al, 2020). A feature-based knee moment estimation algorithm was developed for the NIH pediatric exoskeleton to provide real-time assistance that is proportional to user effort (Lerner et al, 2016b). At the ankle, a human-in-loop assistance strategy was developed by using co-adaptive control based on measured muscle activity and joint kinematics to continuously respond to user needs, resulting in assistance profiles that reduced the metabolic costs of walking in healthy individuals (Jackson and Collins, 2019).…”