title: Comparison of knee motion on Earth and in space: an observational study.
abstract: BACKGROUND
  Spaceflight has been shown to cause atrophy, reduced functional capacity, and increased fatigue in lower-limb skeletal muscles. The mechanisms of these losses are not fully understood but are thought to result, in part, from alteration in muscle usage.
  
  
  METHODS
  Knee-joint angles and lower-extremity muscle activity were measured continually, via elecrogoniometry and surface electromyography respectively, from two subjects during entire working days of activity on Earth and onboard the International Space Station (ISS).
  
  
  RESULTS
  On Earth the distribution of angular positions of the knee was typically bimodal, with peaks of >75 degrees of flexion and in almost full extension (<15 degrees of flexion). However, on the ISS, a single peak in the mid-range of the available range of motion was seen. The knee joint was also moved through fewer excursions and the excursions were smaller in amplitude, resulting in a reduced span of angles traversed. The velocities of the excursions in space were lower than those used on Earth.
  
  
  CONCLUSIONS
  These results demonstrate that, in space, overall knee-joint motion is reduced, and there is a transformation in the type of muscle action compared to that seen on Earth, with more isometric action at the expense of concentric and particularly eccentric action.
authors.affiliation: Department of Biomedical Engineering, Lerner Research Institute, Cleveland Clinic, Cleveland, OH, USA. pierrem@ccf.org
authors.forename: Mark C
authors.initials: MC
authors.lastname: Pierre
authors.forename: Kerim O
authors.initials: KO
authors.lastname: Genc
authors.forename: Micah
authors.initials: M
authors.lastname: Litow
authors.forename: Brad
authors.initials: B
authors.lastname: Humphreys
authors.forename: Andrea J
authors.initials: AJ
authors.lastname: Rice
authors.forename: Christian C
authors.initials: CC
authors.lastname: Maender
authors.forename: Peter R
authors.initials: PR
authors.lastname: Cavanagh