Ma H, Zhu Y, Xiao Y, Kan G, Jin X, Yang KH, King AI. Dynamic changes of macaque cancellous bone following head-down bed rest. Aviat Space Environ Med 2014; 85:130–4. Introduction: Skeletal unloading during a spaceflight could result in bone loss and osteopenia, ultimately leading to poor bone strength. The purpose of the present study was to investigate the influence of bone loss on the dynamic behavior of cancellous bone. Methods: Microgravity-induced bone loss and osteopenia were simulated in a macaque head-down bed rest (HDBR) model, in which 20 macaques were laid on a bed tilted by -6° from the horizontal. These macaques were randomly divided into control (Con) and head down bed rest (HDBR) groups. After 28 d, 5 macaques chosen at random from each group were tested for bone density and mechanical properties, and the obtained data was used to develop a density-based constitutive equation; the remaining animals were tested only for bone density in order to attain statistical power. A split Hopkinson bar was used to monitor the dynamic response of cancellous bone. Cancellous bone deformation under high strain rate conditions was recorded by high-speed videos. Results: Compared with the Con group, the Young’s modulus of cancellous bone from HDBR macaque lumbar vertebrae were decreased by 6.03%. Based on the static and dynamic experimental results, parameters in the Maxwell nonlinear viscoelasticity material model were estimated. Discussion: This model of cancellous bone under high strain rate was useful to establish the medical tolerance and evolution criteria of impact-related trauma by finite element method calculations.