Defining the lumbar and trunk-thigh neutral zone from the passive stiffness curve: application to hybrid sit-stand postures and chair design

Minimal data exist on the neutral position for the lumbar spine, trunk, and thighs when adopting a hybrid posture. This study examined sex differences in the neutral zone lumbar stiffness and the lumbar and trunk-thigh angle boundaries of the neutral zone, and determined if the standing lumbar angle fell within the neutral zone. Passive lumbar flexion and extension moment-angle curves were generated for 31 participants (13 M, 18 F), pooled from two datasets, with trunk-thigh angles available for 10 participants. The neutral zone was defined as the low stiffness zone from both the flexion and extension curves. Males demonstrated significantly greater extensor stiffness. Neutral lumbar and trunk-thigh angles ranged on average -22.2 to 0.2 degrees and 124.2 to 159.6 degrees for males and -17.8 to -1.3 degrees and 143.2 to 159.5 degrees for females, respectively. Standing lumbar angles fell outside the neutral zone for 44% of participants. These neutral zone boundaries may inform kinematics for hybrid chair designs. Practitioner summary: Adoption of a neutral spinal posture may be achieved through hybrid chair design, yet minimal data exists on a physiologically defined neutral zone. Using measures of in vivo lumbar stiffness, the lumbar and trunk-thigh angular boundaries of the neutral zone were defined for both males and females.