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Standing up shifts blood to dependent parts of the body, and blood vessels in the leg become filled. The orthostatic blood volume accumulation in the small vessels is relatively unknown, although these may contribute significantly. We hypothesized that in healthy humans exposed to the upright posture, volume accumulation in small blood vessels contributes significantly to the total fluid volume accumulated in the legs. Considering that near-infrared spectroscopy (NIRS) tracks postural blood volume changes within the small blood vessels of the lower leg, we evaluated the NIRS-determined changes in oxygenated (Δ[O(2)Hb]), deoxygenated (Δ[HHb]) and total haemoglobin tissue concentration (Δ[tHb]) and in total leg volume by strain-gauge plethysmography during 70 deg head-up tilt (HUT; n = 7). In a second experiment, spatial and temporal reproducibility were evaluated with three NIRS probes applied on two separate days (n = 8). In response to HUT, an initially fast increase in [O(2)Hb] was followed by a gradual decline, while [HHb] increased continuously. The increase in [tHb] during HUT was closely related to the increase in total leg volume (r(2) = 0.95 ± 0.03). After tilt back, [O(2)Hb] declined below and [HHb] remained above baseline, whereas all NIRS signals gradually returned to baseline. Spatial heterogeneity was observed, and for two probes [tHb] was highly correlated between days (r(2) = 0.92 ± 0.09 and 0.91 ± 0.12), but less for the third probe (r(2) = 0.44 ± 0.36). The results suggest a non-linear accumulation of blood volume in the small vessels of the leg, with an initial fast phase followed by a more gradual increase at least partly contributing to the relocation of fluid during orthostatic stress.

Background: The unipodal stance task is a clinical task that quantifies postural stability and alignment of the lower limb joints, while weight bearing on one leg. As persons with knee osteoarthritis (KOA) have poor postural and knee joint stability, objective assessment of this task might be useful. Objective: To investigate the discriminant validity of three-dimensional joint kinematics and centre of mass displacement (COM) between healthy controls and persons with knee KOA, during unipodal stance using inertial sensors. Additionally, the reliability, agreement and construct validity are assessed to determine the reproducibility and accuracy of the discriminating parameters. Methods: Twenty healthy controls and 19 persons with unilateral severe KOA were included. Five repetitions of the unipodal stance task were simultaneously recorded by an inertial sensor system and a camera-based system (gold standard). Statistical significant differences in kinematic waveforms between healthy controls and persons with severe knee KOA were determined using one-dimensional statistical parametric mapping (SPM1D). Results: Persons with severe knee KOA had more lateral trunk lean towards the contralateral leg, more hip flexion throughout the performance of the unipodal stance task, more pelvic obliquity and COM displacement towards the contralateral side. However, for the latter two parameters the minimum detectable change was greater than the difference between healthy controls and persons with severe knee KOA. The construct validity was good (coefficient of multiple correlation 0.75, 0.83 respectively) and the root mean squared error (RMSE) was low (RMSE <1.5°) for the discriminant parameters. Conclusion: Inertial sensor based movement analysis can discriminate between healthy controls and persons with severe knee KOA for lateral trunk lean and hip flexion, but unfortunately not for the knee angles. Further research is required to improve the reproducibility and accuracy of the inertial sensor measurements before they can be used to assess differences in tasks with a small range of motion. [ABSTRACT FROM AUTHOR]