title: A new method for sudden mechanical perturbation with axial load, to assess postural control in sitting and standing.
abstract: Sudden application of load along a sagittal or coronal axis has been used to study trunk stiffness, but not axial (vertical) load. This study introduces a new method for sudden-release axial load perturbation. Prima facie validity was supported by comparison with standard mechanical systems. We report the response of the human body to axial perturbation in sitting and standing and within-day repeatability of measures. Load of 20% of body weight was released from light contact onto the shoulders of 22 healthy participants (10 males). Force input was measured via force transducers at shoulders, output via a force plate below the participant, and kinematics via 3-D motion capture. System identification was used to fit data from the time of load release to time of peak load-displacement, fitting with a 2nd-order mass-spring-damper system with a delay term. At peak load-displacement, the mean (SD) effective stiffness measured with this device for participants in sitting was 12.0(3.4)N/mm, and in standing was 13.3(4.2)N/mm. Peak force output exceeded input by 44.8 (10.0)% in sitting and by 30.4(7.9)% in standing. Intra-class correlation coefficients for within-day repeatability of axial stiffness were 0.58 (CI: -0.03 to 0.83) in sitting and 0.82(0.57-0.93) in standing. Despite greater degrees of freedom in standing than sitting, standing involved lesser time, downward displacement, peak output force and was more repeatable in defending upright postural control against the same axial loads. This method provides a foundation for future studies of neuromuscular control with axial perturbation.
authors.affiliation: The University of Queensland, Centre of Clinical Research Excellence in Spinal Pain, Injury and Health, School of Health and Rehabilitation Sciences, Brisbane 4072, Australia. Electronic address: a.claus1@uq.edu.au.
authors.forename: Andrew P
authors.initials: AP
authors.lastname: Claus
authors.affiliation: Center for Lifespan Psychology, Max Planck Institute for Human Development, Berlin, Germany.
authors.forename: Julius
authors.initials: J
authors.lastname: Verrel
authors.affiliation: The University of Queensland, School of Information Technology and Electrical Engineering, Brisbane 4072, Australia.
authors.forename: Paul E I
authors.initials: PEI
authors.lastname: Pounds
authors.affiliation: The University of Queensland, Centre of Clinical Research Excellence in Spinal Pain, Injury and Health, School of Health and Rehabilitation Sciences, Brisbane 4072, Australia.
authors.forename: Renee C
authors.initials: RC
authors.lastname: Shaw
authors.affiliation: The University of Queensland, Centre of Clinical Research Excellence in Spinal Pain, Injury and Health, School of Health and Rehabilitation Sciences, Brisbane 4072, Australia.
authors.forename: Niamh
authors.initials: N
authors.lastname: Brady
authors.affiliation: The University of Queensland, Centre of Clinical Research Excellence in Spinal Pain, Injury and Health, School of Health and Rehabilitation Sciences, Brisbane 4072, Australia.
authors.forename: Min T
authors.initials: MT
authors.lastname: Chew
authors.affiliation: The University of Queensland, Centre of Clinical Research Excellence in Spinal Pain, Injury and Health, School of Health and Rehabilitation Sciences, Brisbane 4072, Australia.
authors.forename: Thomas A
authors.initials: TA
authors.lastname: Dekkers
authors.affiliation: The University of Queensland, Centre of Clinical Research Excellence in Spinal Pain, Injury and Health, School of Health and Rehabilitation Sciences, Brisbane 4072, Australia.
authors.forename: Paul W
authors.initials: PW
authors.lastname: Hodges
mesh_terms: D000293:Adolescent; D000328:Adult; D001696:Biomechanical Phenomena; D005260:Female; D006801:Humans; D008297:Male; D009038:Motion; D011187:Posture / Q000502:physiology*; D012782:Shoulder; D060726:Torso / Q000502:physiology; D055815:Young Adult
keywords: Axial; Perturbation; Postural control; Spine; Stiffness