Northwestern University Feinberg School of Medicine

Northwestern University Prosthetics-Orthotics Center

The Effect of Trunk Flexion on Balance and Energy Expenditure

Co-Principal Investigators: Steven Gard, PhD, and Stephen Ondra, MD**

Student Investigator: Devjani Saha, BS

Co-Investigator: Stefania Fatone, PhD

** Department of Neurosurgery, Northwestern University

Funded by: Medtronic Sofamor-Danek, Inc.; and National Institute on Disability and Rehabilitation Research (NIDRR)


The goal of this project is to better understand the role of trunk posture on the ability to maintain balance. In upright posture, the spine is aligned so that the head and trunk fall directly over the pelvis. Changes in spinal alignment resulting from spinal pathologies may displace the trunk center of mass (TCOM) with respect to the body's base of support. Sufficient displacement of the TCOM may adversely affect the body's ability to maintain upright posture and balance. Compensatory mechanisms, which are metabolically expensive, may then be necessary to restore balance. In order to improve our understanding of how sagittal malalignment of the spine affects stability, this study examined how trunk-flexed postures affect balance, dynamics, and energetics in able bodied individuals during standing and walking.


Motion and force plate data were collected for 14 able-bodied subjects standing and walking with upright and with 25±7° and 50±7° of trunk-flexion from the vertical. Trunk flexion was monitored in real time. Feedback in the form of auditory cues helped subjects maintain their trunk flexion angle within the desired range. Energy expenditure was also monitored at each posture during static standing.


The results of the study suggested that kinematic adaptations, such as an increase in ankle plantarflexion and an increase in hip flexion during standing and sustained stance-phase knee flexion during walking, may be critical for balance control with trunk-flexed postures. Compensatory changes in lower extremity kinematics produced a posterior shift in the TCOM and brought the fore-aft position of the body center of mass closer to the center of the base of support. Sustained stance-phase knee flexion was associated with changes in the vertical ground reaction force (GRFv) profile, including an increase in the average rate of loading, a delay in the timing of the force valley minimum, and a decrease in the magnitude the second peak of the GRFv. Compensatory mechanisms may also have contributed to the observed increase in metabolic energy expenditure during static standing and the decrease in energy conservation during walking. The gait dynamics of able-bodied subjects walking with trunk flexion were similar to that of a patient who was unable to adequately compensate with hip and/or trunk extension for an anterior, sagittal plane spine deformity. However, further research is necessary to explore the impact of trunk alignment on standing and walking in patients with spinal deformities.


Saha D, Gard S, Fatone S. (2008). The Effect of Trunk Flexion on Able-bodied Gait. Gait Posture. 27(4):652-660.

Saha D, Gard S, and Fatone S. (2007). The Effect of Trunk Posture on Static Standing. Spine, 32(15):1605-1611.

Saha, Devjani (2006). The Effect of Trunk Flexion on Standing and Walking, MS Thesis, Biomedical Engineering, Northwestern University.

Saha D, Gard S, Fatone S, Ondra S. (2006). The Effect of Trunk Posture on Global Balance. 56th Annual Congress of Neurological Surgeons, Chicago, Illinois, October 7-12.

Saha D, Gard S, Fatone S. (2006). Vertical Ground Reaction Force During Trunk-flexed Gait. American Society of Biomechanics, Blacksburg, Virginia, September 6-9.