title: Active functional stiffness of the knee joint during activities of daily living: a parameter for improved design of prosthetic limbs.
abstract: BACKGROUND
  Exploring knee joint physiological functional stiffness is crucial for improving the design of prosthetic legs that aim to mimic normal gait. This study hypothesizes that knee joint stiffness varies among different activities of daily living, additionally while the knee performs natural movements; the magnitude of the stiffness indicates the degree of energy storage element sufficiency in terms of harvesting/returning energy.
  
  
  METHODS
  This study examined sagittal plane knee moment vs. knee flexion angle curves from 12 able-bodied subjects during activities of daily living. Slopes of these curves were assessed to find the calculated stiffness during the peak energy return and harvest phases so that the activities, which can be performed when the prosthetic knee is supplemented by a spring, were identified.
  
  
  RESULTS
  For the energy return and harvest phases, the stiffness varied between 0.006 and 0.046 Nm/kg deg. and 0 and 0.052 Nm/kg deg. respectively. The optimum energy return phase stiffness was 0.024 (SD 0.013) Nm/kg deg. and energy harvest phase stiffness was 0.011 (SD 0.018) Nm/kg deg.
  
  
  CONCLUSIONS
  Knee joint stiffness varied significantly during activities of daily living, which indicated that a storage unit with a constant stiffness would not be sufficient in providing energy regenerative gait during all activities. However, by controlling the amount and timing of spring compression and release, an energy-regenerative prosthetic knee device could be developed during most of the activities. This study was directed to the development of a complete data set, which determined the torque-angle properties of the healthy knee joint.
authors.affiliation: Department of Chemical and Biomedical Engineering, Cleveland State University, Cleveland, OH, USA; Department of Biomedical Engineering, Lerner Research Institute, Cleveland Clinic, Cleveland, OH, USA; Human Performance and Engineering Laboratory, Kessler Foundation, West Orange, NJ, USA; Department of Research, Children's Specialized Hospital, New Brunswick, NJ, USA. Electronic address: hande.argunsah@gmail.com.
authors.forename: Hande Argunsah
authors.initials: HA
authors.lastname: Bayram
authors.affiliation: Department of Chemical and Biomedical Engineering, Cleveland State University, Cleveland, OH, USA; Department of Biomedical Engineering, Lerner Research Institute, Cleveland Clinic, Cleveland, OH, USA.
authors.forename: Chih-Hao
authors.initials: CH
authors.lastname: Chien
authors.affiliation: Department of Biomedical Engineering, University of Akron, Akron, OH, USA.
authors.forename: Brian L
authors.initials: BL
authors.lastname: Davis
mesh_terms: D000203:Activities of Daily Living; D000328:Adult; D001186:Artificial Limbs; D001696:Biomechanical Phenomena; D005260:Female; D005684:Gait / Q000502:physiology; D006801:Humans; D007719:Knee Joint / Q000502:physiology*; D008297:Male; D009068:Movement; D011474:Prosthesis Design / Q000379:methods*; D019415:Torque; D016138:Walking / Q000502:physiology*
keywords: Healthy human knee; Physiological functional stiffness; Prosthesis design