title: Analysis of the reflexive feedback control loop during posture maintenance.
abstract: In previous work it has been shown in posture experiments of the human arm that reflexive dynamics were substantial for narrow-band stochastic force disturbances. The estimated reflex gains varied substantially with the frequency content of the disturbances. The present study analyses a simplified linear model of the reflexive feedback control loop, to provide an explanation for the observed behaviour. The model describes co-activation and reflexive feedback. The task instruction 'minimize the displacements' is represented mathematically by a cost function that is minimized by adjusting the parameters of the model. Small-amplitude displacements allow the system to be analysed with a quasilinear approach. The optimization results clarify the limited effectiveness of reflexive feedback on the system's closed-loop behaviour, which emanates from the time delay present in the reflex loops. For low-frequency inputs less than 5 Hz, boundary-stable solutions with high reflex gains are predicted to be optimal. Input frequencies near the system's eigenfrequency (about 5 Hz), however, would be amplified and result in oscillatory behaviour. As long as the disturbance does not excite these frequencies, boundary stability will be optimal. The predicted reflex gains show a striking similarity with the estimated reflex gains from the experimental study. The present model analysis also provides a clear explanation for the negative reflex gains, estimated for near-sinusoidal inputs beyond 1.5 Hz.
authors.affiliation: Department of Mechanical Engineering, Delft University of Technology, The Netherlands. e.devlugt@wbmt.tudelft.nl
authors.forename: E
authors.initials: E
authors.lastname: de Vlugt
authors.forename: F C
authors.initials: FC
authors.lastname: van der Helm
authors.forename: A C
authors.initials: AC
authors.lastname: Schouten
authors.forename: G G
authors.initials: GG
authors.lastname: Brouwn
mesh_terms: D005246:Feedback; D011187:Posture; D012018:Reflex