TE. MORRIS-PATERSON1, EJ.JONES1, C-C TSAI2,3, H HASEGAWA2,3, O CARMICHAEL4; KA. VAN SOMEREN5, ZA .PUTHUCHEARY7.8, DA. GREEN1,6, SB. ZANELLO1, I. ROSENZWEIG2,3; SDR HARRIDGE1 Exposure to a micro-gravity (μG) environment, particularly in the absence of counter measures, is known to induce a loss of skeletal muscle mass and function, with the postural muscles of the lower limb most affected (Narici & De Boaer 2011). We have recently used supine whole-body unloading on a hyper-saline filled water bed (hyper buoyancy floatation, HBF) as an analogue of μG and demonstrated an average ~1kg loss of total muscle mass after 7 days of HBF unloading (Morris-Patterson et al. 2018). The aim of the present study was to identify changes in muscle mass in specific regions of the body following this 7-day unloading period using magnetic resonance imaging (MRI). Twelve male subjects aged (27.3±4.2 yrs) completed the study. Six weeks prior to unloading each subject underwent a one-week control period during which time their diet was controlled. Pre and post the control period and at standardised time of day subjects undertook an MRI (Siemens MAGNETOM Verio 3T, Siemens Healthcare, Erlangen, Germany). For the intervention period the subjects were asked to lie supine on the HBF for 7 days, during which time they were also fed a controlled diet. Subjects were allowed a maximum of 15 mins per day when they were not on the HBF (for personal hygiene etc). One day prior to and 1.5-3hrs post-unloading, further scans were performed. Up to 400 images were taken along the length of the body, with every 10th image used in the derivation of skeletal muscle mass. All 12 subjects who were entered in the intervention arm of the study, successfully completed the 7-day period of unloading. No significant changes were observed over the control period. However, the results showed that the unloading period induced muscle loss which was proportionally greatest in the lower leg (Pre 3.81 ± 0.68 vs Post 3.65 ± 0.67kg, 4.2%, mean+bSD, ANOVA with repeated measures, p<0.05), followed by the upper leg (10.37 ± 1.83 vs 10.02kg, 3.4%, p<0.05) and then the trunk (12.56 ± 2.01 vs 12.22 ± 1.99 kg, 2.8%, p<0.05). No changes were observed in the lower arm (1.54 ± 0.3 vs 1.49 ± 0.32), the upper arm (3.46 ± 0.79 3.41 ± 0.79 kg) or the neck (kg) 0.45 ± 0.13 vs 0.46 ± 0.11 kg) as a result of the unloading period. The results showed that HBF unloading was able to induce regional changes in skeletal muscle mass in a manner comparable to that observed in space flight, supporting its utility as an analogue of μG.