The purpose of this study was to determine if lower body negative pressure (LBNP) treadmill exercise maintains lumbar spinal compressive properties, curvature and back muscle strength after 28-days of 6° head down tilt (HDT) bed rest. We hypothesize that LBNP treadmill exercise will maintain lumbar spine compressibility, lumbar lordosis and back muscle strength after 28-days of 6° HDT bed rest. Fifteen healthy identical twin pairs (14 women and 16 men) participated in this study. One identical twin was randomly assigned to the non-exercise control (CON) group and their sibling was assigned to the exercise (EX) group. The lumbar spine was significantly more compressible PostBR when compared to PreBR in the CON (p=0.01). Lumbar spine compressibility PostBR was not significantly different compared to PreBR in the EX group (p=0.89). In both the CON and EX groups there were no significant changes PostBR in lumbar lordosis compared to PreBR. Back muscle strength significantly decreased in the CON group PostBR (p=0.002), whereas in the EX group back muscle strength was not significantly different to PreBR values. A significant increase in lumbar spine compressibility in the CON group suggests that spinal deconditioning to gravity occurs during 28-day bed rest. Changes in the mechanical properties of the lumbar spine may be an early indicator of lumbar intervertebral disc degeneration. Supine LBNP treadmill exercise provides axial loads to the lumbar spine and may prevent lumbar spine deconditioning associated with HDT bed rest.