title: Trunk muscle co-activation using functional electrical stimulation modifies center of pressure fluctuations during quiet sitting by increasing trunk stiffness.
abstract: BACKGROUND
  The purpose of this study was to examine the impact of functional electrical stimulation (FES) induced co-activation of trunk muscles during quiet sitting. We hypothesized that FES applied to the trunk muscles will increase trunk stiffness. The objectives of this study were to: 1) compare the center of pressure (COP) fluctuations during unsupported and FES-assisted quiet sitting - an experimental study and; 2) investigate how FES influences sitting balance - an analytical (simulation) study.
  
  
  METHODS
  The experimental study involved 15 able-bodied individuals who were seated on an instrumented chair. During the experiment, COP of the body projected on the seating surface was calculated to compare sitting stability of participants during unsupported and FES-assisted quiet sitting. The analytical (simulation) study examined dynamics of quiet sitting using an inverted pendulum model, representing the body, and a proportional-derivative (PD) controller, representing the central nervous system control. This model was used to analyze the relationship between increased trunk stiffness and COP fluctuations.
  
  
  RESULTS
  In the experimental study, the COP fluctuations showed that: i) the mean velocity, mean frequency and the power frequency were higher during FES-assisted sitting; ii) the frequency dispersion for anterior-posterior fluctuations was smaller during FES-assisted sitting; and iii) the mean distance, range and centroidal frequency did not change during FES-assisted sitting. The analytical (simulation) study showed that increased mechanical stiffness of the trunk had the same effect on COP fluctuations as the FES.
  
  
  CONCLUSIONS
  The results of this study suggest that FES applied to the key trunk muscles increases the speed of the COP fluctuations by increasing the trunk stiffness during quiet sitting.
authors.affiliation: Institute of Biomaterials and Biomedical Engineering, University of Toronto, 164 College Street, Toronto, ON, M5S 3G9, Canada. matija.milosevic@utoronto.ca.|Rehabilitation Engineering Laboratory, Lyndhurst Centre, Toronto Rehabilitation Institute - University Health Network, 520 Sutherland Drive, Toronto, ON, M4G 3V9, Canada. matija.milosevic@utoronto.ca.
authors.forename: Matija
authors.initials: M
authors.lastname: Milosevic
authors.affiliation: Institute of Biomaterials and Biomedical Engineering, University of Toronto, 164 College Street, Toronto, ON, M5S 3G9, Canada. k.masani@utoronto.ca.|Rehabilitation Engineering Laboratory, Lyndhurst Centre, Toronto Rehabilitation Institute - University Health Network, 520 Sutherland Drive, Toronto, ON, M4G 3V9, Canada. k.masani@utoronto.ca.
authors.forename: Kei
authors.initials: K
authors.lastname: Masani
authors.affiliation: Institute of Biomaterials and Biomedical Engineering, University of Toronto, 164 College Street, Toronto, ON, M5S 3G9, Canada. noelwu20@gmail.com.|Rehabilitation Engineering Laboratory, Lyndhurst Centre, Toronto Rehabilitation Institute - University Health Network, 520 Sutherland Drive, Toronto, ON, M4G 3V9, Canada. noelwu20@gmail.com.
authors.forename: Noel
authors.initials: N
authors.lastname: Wu
authors.affiliation: Institute of Biomaterials and Biomedical Engineering, University of Toronto, 164 College Street, Toronto, ON, M5S 3G9, Canada. kmcconvi@ee.ryerson.ca.|Rehabilitation Engineering Laboratory, Lyndhurst Centre, Toronto Rehabilitation Institute - University Health Network, 520 Sutherland Drive, Toronto, ON, M4G 3V9, Canada. kmcconvi@ee.ryerson.ca.|Department of Electrical and Computer Engineering, Ryerson University, 350 Victoria Street, Toronto, ON, M5B 2K3, Canada. kmcconvi@ee.ryerson.ca.
authors.forename: Kristiina M V
authors.initials: KM
authors.lastname: McConville
authors.affiliation: Institute of Biomaterials and Biomedical Engineering, University of Toronto, 164 College Street, Toronto, ON, M5S 3G9, Canada. milos.popovic@utoronto.ca.|Rehabilitation Engineering Laboratory, Lyndhurst Centre, Toronto Rehabilitation Institute - University Health Network, 520 Sutherland Drive, Toronto, ON, M4G 3V9, Canada. milos.popovic@utoronto.ca.
authors.forename: Milos R
authors.initials: MR
authors.lastname: Popovic
mesh_terms: D004558:Electric Stimulation; D005260:Female; D006801:Humans; D008297:Male; D018482:Muscle, Skeletal / Q000502:physiology*; D004856:Postural Balance / Q000502:physiology; D011187:Posture / Q000502:physiology; D060726:Torso / Q000502:physiology