title: Locomotion in simulated zero gravity: ground reaction forces.
abstract: BACKGROUND
  Exercise is likely to be an important countermeasure to bone demineralization, which remains a concern for astronauts during long-duration spaceflight. However, loads on the feet during exercise with 1 G equivalent gravity replacement are not known. The purpose of this study was to compare ground reaction forces (GRFs) during over-ground and simulated zero gravity (0 G) locomotion.
  
  
  OBJECTIVE
  It was hypothesized that sufficient gravity replacement loading could be applied to the subjects such that GRF profiles similar to those seen in 1 G would occur during locomotion in a zero-gravity locomotion simulator (ZLS).
  
  
  METHODS
  GRFs were measured during overground walking and running, and during locomotion in two restraint harness designs in the ZLS with an initial loading of 1 body weight. Load cells measured the gravity replacement load (GRL) in the ZLS. Joint angles at the hip and knee were also measured by goniometers.
  
  
  RESULTS
  Peak forces were greater in overground locomotion than in the ZLS; however, loading rates were greater in the ZLS running conditions than in overground running. The knee joint was more flexed at key times in the support phase during running in the ZLS compared with overground.
  
  
  CONCLUSIONS
  Large loads and loading rates can be generated at the feet during simulated 0 G exercise although peak forces during running in the ZLS are less than overground running at the same speed. The refinement of the gravity replacement system to provide a constant 1 G load should be considered.
authors.affiliation: Center for Locomotion Studies, Department of Kinesiology, The Pennsylvania State University, University Park, PA, USA. mccroryjl@msx.upmc.edu
authors.forename: Jean L
authors.initials: JL
authors.lastname: McCrory
authors.forename: Janice
authors.initials: J
authors.lastname: Derr
authors.forename: Peter R
authors.initials: PR
authors.lastname: Cavanagh
mesh_terms: D000328:Adult; D000704:Analysis of Variance; D001696:Biomechanical Phenomena; D001862:Bone Resorption / Q000209:etiology / Q000517:prevention & control; D004867:Equipment Design; D015444:Exercise / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D012420:Running / Q000502:physiology*; D013026:Space Flight; D016138:Walking / Q000502:physiology*; D016474:Weight-Bearing / Q000502:physiology; D014893:Weightlessness / Q000009:adverse effects; D018544:Weightlessness Countermeasures; D018474:Weightlessness Simulation