IMU-Based Kinematics Estimation Accuracy Affects Gait Retraining Using Vibrotactile Cues.

Wearable sensing using inertial measurement units (IMUs) is enabling portable and customized gait retraining for knee osteoarthritis. However, the vibrotactile feedback that users receive directly depends on the accuracy of IMU-based kinematics. This study investigated how kinematic errors impact an individual's ability to learn a therapeutic gait using vibrotactile cues. Sensor accuracy was computed by comparing the IMU-based foot progression angle to marker-based motion capture, which was used as ground truth. Thirty subjects were randomized into three groups to learn a toe-in gait: one group received vibrotactile feedback during gait retraining in the laboratory, another received feedback outdoors, and the control group received only verbal instruction and proceeded directly to the evaluation condition. All subjects were evaluated on their ability to maintain the learned gait in a new outdoor environment. We found that subjects with high tracking errors exhibited more incorrect responses to vibrotactile cues and slower learning rates than subjects with low tracking errors. Subjects with low tracking errors outperformed the control group in the evaluation condition, whereas those with higher error did not. Errors were correlated with foot size and angle magnitude, which may indicate a non-random bias. The accuracy of IMU-based kinematics has a cascading effect on feedback; ignoring this effect could lead researchers or clinicians to erroneously classify a patient as a non-responder if they did not improve after retraining. To use patient and clinician time effectively, future implementation of portable gait retraining will require assessment across a diverse range of patients.

Predicting knee adduction moment response to gait retraining with minimal clinical data.

Knee osteoarthritis is a progressive disease mediated by high joint loads. Foot progression angle modifications that reduce the knee adduction moment (KAM), a surrogate of knee loading, have demonstrated efficacy in alleviating pain and improving function. Although changes to the foot progression angle are overall beneficial, KAM reductions are not consistent across patients. Moreover, customized interventions are time-consuming and require instrumentation not commonly available in the clinic. We present a regression model that uses minimal clinical data-a set of six features easily obtained in the clinic-to predict the extent of first peak KAM reduction after toe-in gait retraining. For such a model to generalize, the training data must be large and variable. Given the lack of large public datasets that contain different gaits for the same patient, we generated this dataset synthetically. Insights learned from a ground-truth dataset with both baseline and toe-in gait trials (N = 12) enabled the creation of a large (N = 138) synthetic dataset for training the predictive model. On a test set of data collected by a separate research group (N = 15), the first peak KAM reduction was predicted with a mean absolute error of 0.134% body weight * height (%BW*HT). This error is smaller than the standard deviation of the first peak KAM during baseline walking averaged across test subjects (0.306%BW*HT). This work demonstrates the feasibility of training predictive models with synthetic data and provides clinicians with a new tool to predict the outcome of patient-specific gait retraining without requiring gait lab instrumentation.

Vibrotactile Feedback Improves Foot Placement Perception on Stairs for Lower-Limb Prosthesis Users.

Lower-limb amputees demonstrate decreased performance in stair ambulation compared to their intact-limb counterparts. An estimated 21% of amputees can navigate stairs without a handrail; almost 33% do not use stairs at all. The absence of tactile sensation on the bottom of the foot, creating uncertainty in foot placement, may be overcome by integrating sensory feedback into prosthesis design. Here we describe the design and evaluation of a haptic feedback system worn on the thigh to provide vibrotactile cues of foot placement with respect to stair steps. Tactor discrimination and foot placement awareness tests were performed to analyze system efficacy. Control participants wearing ski boots (N=10) and below-knee amputees (N=2) could discriminate individual tactor vibrations with 95.4% and 90.1% accuracy, respectively. The use of vibrotactile feedback increased accuracy in reporting foot placement by 15% and 17.5%, respectively. These results suggest that using vibrotactile arrays for sensory feedback may improve stair descent performance in lower-limb amputees.

Corneal Potential Maps Measured With Multi-Electrode Electroretinography in Rat Eyes With Experimental Lesions.

Purpose: Conventional full-field flash electroretinography (ERG) yields a single response waveform that can be useful in the early detection and diagnosis of many diseases affecting the retina. It is an objective measurement that probes the entire retina. However, localized areas of dysfunction have relatively small influence on ERG amplitudes compared to normal ranges. Here we evaluate the use of corneal potential maps obtained in response to full-field flash stimuli for sensitivity to local areas of retinal damage. Methods: A contact lens electrode array was used to record 25 ERG waveforms simultaneously following saturating full-field flash stimuli (multi-electrode electroretinography, meERG) in rats. Waveforms were evaluated for a-wave and b-wave amplitudes; these values were normalized and further evaluated for spatial differences across the corneal surface. Cluster analysis and a support vector machine approach were used to classify meERG responses from healthy eyes and eyes with central (photocoagulation) or peripheral (cryocoagulation) experimental lesions. Results: A normative normalized corneal potential map was obtained from healthy eyes (n = 26). Corneal potential maps from eyes with experimental lesions (n = 13) could be classified with sensitivity and specificity of approximately 80% based solely on the normalized spatial distribution of corneal potentials, that is, with no knowledge of absolute amplitudes. Conclusions: Corneal potential maps obtained in response to full-field flash stimuli are altered in eyes with scotomas in the central and far-peripheral retina. The meERG approach yields useful spatial information following a single brief flash, analogous to body-surface potential maps used to evaluate heart and brain.