title: Estimation of ground reaction forces and ankle moment with multiple, low-cost sensors.
abstract: BACKGROUND
  Wearable sensor systems can provide data for at-home gait analyses and input to controllers for rehabilitation devices but they often have reduced estimation accuracy compared to laboratory systems. The goal of this study is to evaluate a portable, low-cost system for measuring ground reaction forces and ankle joint torques in treadmill walking and calf raises.
  
  
  METHODS
  To estimate the ground reaction forces and ankle joint torques, we developed a custom instrumented insole and a tissue force sensor. Six healthy subjects completed a collection of movements (calf raises, 1.0 m/s walking, and 1.5 m/s walking) on two separate days. We trained artificial neural networks on the study data and compared the estimates to a multi-camera motion system and an instrumented treadmill. We evaluated the relative strength of each sensor by testing each sensor's ability to predict the ankle joint torque calculated from a reference inverse kinematics algorithm. We assessed model accuracy through root mean squared error and normalized root mean square error. We hypothesized that the estimation of the models would have normalized root mean square error measures less than 10 %.
  
  
  RESULTS
  For walking at 1.0 and walking at 1.5 m/s, the single-task, intra-day and multi-task, intra-day predictions had normalized root mean square error less than 10 % for all three force components and both center of pressure components. For the calf raise task, the single-task, intra-day and multi-task, intra-day predictions had normalized root mean square error less than 10 % for only the anterior-posterior center of pressure. The multi-task, intra-day model had similar predictions to the single-task, intra-day model. The normalized root mean square error of predictions from the insole sensor alone were less than 10 % for walking at 1.0 m/s and 1.5 m/s. No sensor was sufficient for the calf raise task. The combination of the insole sensor and the tendon sensor had lower normalized root mean square error than the individual sensors for all three tasks.
  
  
  CONCLUSIONS
  The proposed sensor system provided accurate estimates for five of the six components of the ground reaction kinetics during walking at 1.0 and 1.5 m/s and one of the six components during the calf raise task. The normalized root mean square error of the predictions of the ground reaction forces were similar to published studies using commercial devices. The proposed system of low-cost sensors can provide useful estimations of ankle joint torque for both walking and calf raises for future studies in mobile gait analysis.
authors.affiliation: School of Kinesiology, University of Michigan, 401 Washtenaw Ave CCRB, Ann Arbor, MI, USA.
authors.forename: Daniel A
authors.initials: DA
authors.lastname: Jacobs
authors.affiliation: School of Kinesiology, University of Michigan, 401 Washtenaw Ave CCRB, Ann Arbor, MI, USA.
authors.forename: Daniel P
authors.initials: DP
authors.lastname: Ferris
mesh_terms: D061725:Accelerometry / Q000295:instrumentation* / Q000379:methods; D000328:Adult; D000818:Animals; D000843:Ankle Joint / Q000502:physiology*; D001696:Biomechanical Phenomena / Q000502:physiology; D002417:Cattle; D005260:Female; D005684:Gait / Q000502:physiology*; D006801:Humans; D008297:Male; D008875:Middle Aged; D016571:Neural Networks, Computer; D016138:Walking / Q000502:physiology