Closing the Wearable Gap: Mobile Systems for Kinematic Signal Monitoring of the Foot and Ankle

Abstract

Interviews from strength and conditioning coaches across all levels of athletic competition identified their two biggest concerns with the current state of wearable technology: (a) the lack of solutions that accurately capture data “from the ground up” and (b) the lack of trust due to inconsistent measurements. The purpose of this research is to investigate the use of liquid metal sensors, specifically Liquid Wire sensors, as a potential solution for accurately capturing ankle complex movements such as plantar flexion, dorsiflexion, inversion, and eversion. Sensor stretch linearity was validated using a Micro-Ohm Meter and a Wheatstone bridge circuit. Sensors made from different substrates were also tested and discovered to be linear at multiple temperatures. An ankle complex model and computing unit for measuring resistance values were developed to determine sensor output based on simulated plantar flexion movement. The sensors were found to have a significant relationship between the positional change and the resistance values for plantar flexion movement. The results of the study ultimately confirm the researchers’ hypothesis that liquid metal sensors, and Liquid Wire sensors specifically, can serve as a mitigating substitute for inertial measurement unit (IMU) based solutions that attempt to capture specific joint angles and movements.

Closing the Wearable Gap—Part IV: 3D Motion Capture Cameras Versus Soft Robotic Sensors Comparison of Gait Movement Assessment

“…Regardless of the challenges, this research team remains confident that an accurate gait assessment that rivals the accuracy of 3D motion capture systems can still be achieved using only four off-the-shelf SRS sensors. As is consistent with the Conclusions Section in Part II of the "closing the wearable gap" paper series: "the solution verified on human participants within this study essentially creates a real-time, continuous, and consistent electric goniometer that a user can wear to assess their movements accurately" [8].…”

“…Therefore, utilizing the mean MAE for fit considers that not all errors may be preventable and should not overly penalize the fit score. However, given that the goal of this entire project and "closing the wearable gap" paper series is to design a device that meets the needs of the practitioners by accurately capturing foot-ankle data "from the ground up" [8], then perhaps the more penalized score provided by mean RMSE is a more realistic appropriation of fit. Variance by five degrees in the calculation of a joint angle may not be viewed by a practitioner as problematic for flexion-based movements given that the foot-ankle joint can move in much wider arcs in the sagittal plane.…”

“…Due to non-homogenous magnetic fields, mainly from construction building materials and magnetic interference, distortion and drift affect a sensor's vertical and horizontal data [5]. A sensor system that could simulate stretch and strain around the joints may offer an alternative method to gait analysis rather than using stiff, circuit board based IMUs [8]. Consistency and calibration issues regularly result from sensors on the market, a potential solution may lie in the form of a different type of sensor, such as stretchable soft robotic sensors (SRS) [8].…”

“…Due to the reoccurring issues with wearable technology, the purpose of this paper is to continue the research and development narrative for building a new SRS-based wearable solution that closes the WST gap identified by the practitioners [8]. This paper suggests alternative methods for capturing the human gait cycle in a real-life setting for more precise rehabilitation techniques and improvement of athletic performance.…”

“…This paper suggests alternative methods for capturing the human gait cycle in a real-life setting for more precise rehabilitation techniques and improvement of athletic performance. Luczak et al [8] and Saucier et al [9] recommend a more accurate technology-stretchable SRS accompanied by liquid conductive material, placed in garments, that can be directly placed on joints to analyze joint kinematics [8,9]. There are several benefits for using SRS which include (a) the capability to capture biomechanical strain without the occlusion of errors that normally occur in 3D motion capture systems and eliminate drift that can arise in IMU sensors [8]; (b) the realization of small changes in electromechanical specifications during loading and unloading; and (c) limited interference observed by the consumer.…”