50% body weight loading reduces stature increases and lumbar disc expansion from 4 h hyper-buoyancy floatation versus 15 min sitting upright.

Microgravity is associated with stature increases, back pain and post-flight intervertebral disc (IVD) herniation. This study aims to determine whether 30 s seated 50% body weight (BW) axial loading is comparable to 15 min sitting upright in 1 g upon changes in stature, anterior lumbar IVD height (via ultrasound), passive vertebral stiffness (VS), and back pain induced by 4 h hyper-buoyancy floatation (HBF) unloading. Sixteen (seven male) healthy volunteers had stature, lumbar IVD height (L2-S1), passive VS (C1-L5) and back pain assessed before and following 4 h HBF, and immediately after participants performed a 30 s seated squat with 50% of their BW or 15 min sitting upright. Four hours of HBF unloading induced significant increments in stature (+1.6 ± 0.5 cm; P < 0.001), IVD height (L2-L3: P = 0.002; L3-L4: P < 0.001; L4-L5: P = 0.013; L5-S1: P < 0.001) and back pain (2.90 ± 1.26; P < 0.001) with no differences between 1 and 1.5 BW. Stature, IVD height increments and back pain were similarly attenuated in both reloading groups. Passive VS was unchanged by 4 h HBF or reloading. HBF-induced back pain positively correlated with stature (P = 0.01) and lumbar IVD height changes (L2-L3: P = 0.03; L3-L4: P = 0.01; L5-S1: P = 0.02). Four hours of HBF increased stature, lumbar IVD height and induced moderate back pain that were similarly (albeit not entirely) ameliorated by both 15 min upright sitting and 30 s of 50% BW axial loading, with no changes in passive VS observed. IVD geometric changes appear key to space adaptation back pain and stature increments that can be rapidly modulated by brief periods of axial loading.