Nanomechanical analysis reveals decreased tendon collagen fibril modulus after spaceflight

Microgravity presents a unique physiological stressor, causing rapid musculoskeletal (MSK) deterioration across species. Although microgravity-induced muscle and bone loss is well documented, its effects on tendon remain poorly understood, particularly at the nanoscopic-level. Tendon is composed of collagen fibrils and other extracellular matrix (ECM) components, whose nano-morphology and mechanics influences MSK health and performance. Atomic force microscopy (AFM) provides the necessary resolution to directly probe these nanoscopic-level mechanical features. Understanding the rate and magnitude of collagen fibril maladaptation is essential for developing targeted countermeasures to preserve MSK function during long-duration missions. Therefore, the objective of this study was to determine the effects of 37 d of spaceflight (SF) on the morphology (width, height) and compressive nanomechanics (Young’s Modulus, YM) of individual collagen fibrils. We hypothesized that SF collagen fibrils would be smaller and exhibit lower YM compared to ground controls (GC). Methods: Female C57BL/6J mice (n = 14) were exposed to 37 d of SF aboard the International Space Station (ISS) and compared with GC. Following storage at -80°C, gastrocnemius (GAS) and quadriceps (QUAD) tendons were mechanically isolated from the muscle and underwent a chemical and mechanical process to isolate individual collagen fibrils while preserving native structure. Hydrated samples were imaged using AFM (JPK NanoWizard 4a) to compare morphology (width, height) and compressive nanomechanics (YM). All samples were imaged via quantitative-imaging (QI) mode with a 2 nN setpoint, 40 µm/sec z-speed, and 1×1 µm scan size using a qp-Bio-AC-CB1 cantilever (tip radius = 10 nm, k = 0.3 N/m). Resulting micrographs were processed in the JPK Data Processing Suite and analyzed in Gwyddion to extract fibril-specific width, height, and YM. SF and GC measurements were compared via ANOVA and ANCOVA in SPSS (α = 0.05).Results: A total of 167 individual fibrils were analyzed (SF = 85, GC = 82). Fibril width was greater in SF (205 ± 71 nm) compared to GC (178 ± 57 nm; p < 0.01). SF fibrils exhibited reduced YM, revealing a significant 30% decline after 37 d aboard the ISS (SF = 2.09 ± 0.60 MPa; GC = 2.97 ± 1.08 MPa; p < 0.001). Width inversely correlated with YM (r = -0.44), but did not account for the observed mechanical differences after statistical correction. Following SF, GAS fibrils exhibited a greater reduction in YM (37%) compared to QUAD fibrils (21%). Conclusions: Sustained microgravity exposure markedly reduces tendon collagen fibril YM. This decline likely reflects disrupted mechanotransduction; in the absence of normal physiological loading, collagen turnover, crosslink formation, and ECM remodeling are diminished. The larger reduction observed in GAS fibrils suggests that tendons normally exposed to higher baseline loading experience proportionally greater deterioration during unloading. Future work should consider the role of fibril hydration, alignment, and packing using alternative imaging techniques (e.g., small-angle X-ray scattering, SAXS) to gain deeper structural insights. Together, these findings identify disrupted tendon nanomechanics as a key component of microgravity-induced deterioration, underscoring the need to address collagen fibril vulnerability when developing countermeasures for long-duration spaceflight. Funding: CSUBIOTECH grant to JRB. This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.