Hyper-buoyancy flotation increases cervical disc height and reduces vertebral stiffness, with only partial reversal after acute 1 g axial loading.

Exposure to microgravity is associated with stature increases, moderate-to-severe back/neck pain and elevated lumbar and cervical intervertebral disc (IVD) herniation risk post-flight. Whilst lumbar pathophysiology has been investigated, little attention has been placed on the cervical spine. Thus, the aim of this study was to determine the effect of 4 h of hyper-buoyancy flotation (HBF) and subsequent 15 min of upright 1 g exposure on stature, cervical IVD height, cervical muscle cross-sectional area (CSA) and thickness, passive cervical vertebral stiffness, and their association with stature changes and/or neck pain. To investigate this, 12 healthy volunteers (five male; 30.2 ± 7.2 years, 66.1 ± 11.7 kg, 173.4 ± 8.5 cm) participated. Stature was assessed at Pre HBF, after 4 h of supine HBF (Post HBF), and following 15 min of upright 1 g exposure (Post 1G). Cervical IVD height (C3-T1), longus colli and semispinalis cervicis CSA and thickness, passive vertebral stiffness (C2-L5), and neck pain were assessed at Pre HBF, HBF0, HBF2, HBF4 and Post 1G. Results indicated that 4 h HBF significantly increased stature (+1.6 ± 0.5 cm), which was partially reduced after acute 1 g exposure (-0.4 ± 0.3 cm). Cervical IVD height increased at all levels following HBF, with partial reversal at Post 1G, particularly in the lower cervical region. Longus colli and semispinalis cervicis CSA and thickness were unchanged. Passive vertebral stiffness significantly decreased (across the entire column and most regions) following HBF and was only partially restored after 1 g exposure. Neck pain increased during HBF and was reduced after 1 g exposure. No associations were observed between neck pain and stature or IVD changes. However, neck pain at HBF4 was positively correlated with mid-cervical vertebral stiffness (R = 0.587, P = 0.045). Overall, 4 h HBF induced mild neck pain, increased stature and cervical IVD height, all of which were partially reduced after 15 min of upright 1 g exposure. No changes in longus colli or semispinalis cervicis muscle thickness or CSA were observed. Passive cervical vertebral stiffness decreased following HBF and remained below baseline after 1 g exposure. A positive correlation between neck pain at HBF4 and mid-cervical vertebral stiffness suggests modulation of vertebral column stabilisation may contribute to symptoms. The lack of association between neck pain and IVD expansion suggests that vertebral flattening may play a role and therefore warrants direct evaluation with HBF and following spaceflight.