Lower limb postures resembling sitting and standing alter lumbar angles along the passive stiffness curve

In vivo lumbar passive stiffness is often used to assess time-dependent changes in lumbar tissues and to define the neutral zone. We tested the hypothesis that flexing the hips would alter tension in hip and spine musculature, leading to a more extended passive stiffness curve (i.e., right-shifted), without changes in lumbar stiffness. Twenty participants underwent side-lying passive testing with the lower limbs positioned in Stand, Kneel, and Sit representative postures. Moment-angle curves were constructed from the lumbar angles and the moment at L4/5 and partitioned into three zones. Partially supporting our hypothesis, lumbar stiffness within the low and transition stiffness zones was similar between the Stand and Sit. Contrary to our hypothesis, lumbar angles were significantly larger in the Sit compared to the Stand and Kneel postures at the first and second breakpoints, with average differences of 9.3° or 27.2% of passive range of motion (%PassRoM) in flexion and 5.6° or 16.6 %PassRoM in extension. Increased flexion in the Sit may be linked to increased posterior pelvic tilt and associated lower lumbar vertebrae flexion. Investigators must ensure consistent pelvis and hip positioning when measuring lumbar stiffness. Additionally, the adaptability of the neutral zone to pelvis posture, particularly between standing and sitting, should be considered in ergonomic applications.