From the international space station to the clinic: how prolonged unloading may disrupt lumbar spine stability

Prolonged microgravity exposure is associated with localized low back pain and an elevated risk of post-flight disc herniation. Although the mechanisms by which microgravity impairs the spine are unclear, they should be foundational for developing in-flight countermeasures for maintaining astronaut spine health. Because human spine anatomy has adapted to upright posture on Earth, observations of how spaceflight affects the spine should also provide new and potentially important information on spine biomechanics that benefit the general population. This study compares quantitative measures of lumbar spine anatomy, health, and biomechanics in astronauts before and after 6 months of microgravity exposure on board the International Space Station (ISS). This is a prospective longitudinal study. Six astronaut crewmember volunteers from the National Aeronautics and Space Administration (NASA) with 6-month missions aboard the ISS comprised our study sample. For multifidus and erector spinae at L3–L4, measures include cross-sectional area (CSA), functional cross-sectional area (FCSA), and FCSA/CSA. Other measures include supine lumbar lordosis (L1–S1), active (standing) and passive (lying) flexion-extension range of motion (FE ROM) for each lumbar disc segment, disc water content from T2-weighted intensity, Pfirrmann grade, vertebral end plate pathology, and subject-reported incidence of chronic low back pain or disc injuries at 1-year follow-up. 3T magnetic resonance imaging and dynamic fluoroscopy of the lumbar spine were collected for each subject at two time points: approximately 30 days before launch (pre-flight) and 1 day following 6 months spaceflight on the ISS (post-flight). Outcome measures were compared between time points using paired t tests and regression analyses. Supine lumbar lordosis decreased (flattened) by an average of 11% (p=.019). Active FE ROM decreased for the middle three lumbar discs (L2–L3: −22.1%, p=.049; L3–L4: −17.3%, p=.016; L4–L5: −30.3%, p=.004). By contrast, no significant passive FE ROM changes in these discs were observed (p>.05). Disc water content did not differ systematically from pre- to post-flight. Multifidus and erector spinae changed variably between subjects, with five of six subjects experiencing an average decrease 20% for FCSA and 8%–9% for CSA in both muscles. For all subjects, changes in multifidus FCSA strongly correlated with changes in lordosis (r2=0.86, p=.008) and active FE ROM at L4–L5 (r2=0.94, p=.007). Addi