title: Adaptive effects of spaceflight as revealed by short-term partial weight suspension.
abstract: BACKGROUND
  Human performance and adaptation to altered loading levels is investigated. A previous astronaut jumping study demonstrated significantly altered landing performance following spaceflight, complementing reports of postflight postural and gait instabilities. A dynamic model indicated that leg stiffness changes accounted for the kinematic differences due to adaptation in open-loop modulation of leg impedance. Muscular atrophy or altered stretch reflexes and vestibular sensing could not be discounted.
  
  
  OBJECTIVE
  We hypothesize that partial weight unloading can cause modulation of leg impedance and altered jump landing performance similar to longer-term microgravity exposure, while controlling for muscle atrophy and altered graviceptor inputs. Lower-body impedance changes after partial weight unloading support the hypothesis that postflight differences result primarily from modulation of leg impedance due to reduced postural control demands in microgravity.
  
  
  METHODS
  Six subjects performed six baseline 30-cm downward jumps from a platform, followed by 10 jumps under 60% body weight unloading (the adaptation sequence), and then six additional jumps under full-body loading (termed "adapted jumps"). Joint and mass center kinematics were compared for the baseline and adapted jump landings. A second order model evaluated changes in vertical leg impedance.
  
  
  RESULTS
  Three subjects exhibited significant increased joint angles and rates. Vertical ground reaction forces showed more heavily damped responses after adaptation. Model fits to mass center motion indicated reduced leg stiffness.
  
  
  CONCLUSIONS
  Post-adaptation performance is similar to that of four astronaut subjects who demonstrated reduced postflight leg stiffness. The new study strongly suggests adaptive control of lower limb impedance to accommodate altered gravity levels which can be induced by minimal unloading exposure. Partial weight unloading provides a simple, inexpensive analog to spaceflight for certain postural and movement studies.
authors.affiliation: Lockheed Martin Missiles and Space, Sunnyvale, CA, USA.
authors.forename: D K
authors.initials: DK
authors.lastname: Jackson
authors.forename: D J
authors.initials: DJ
authors.lastname: Newman
mesh_terms: D000222:Adaptation, Physiological; D000328:Adult; D005260:Female; D005684:Gait; D006801:Humans; D018472:Hypogravity; D019957:Motor Skills Disorders / Q000503:physiopathology; D011187:Posture; D013026:Space Flight; D016474:Weight-Bearing