title: Prolonged Intermittent Trunk Flexion Increases Trunk Muscles Reflex Gains and Trunk Stiffness.
abstract: The goal of the present study was to determine the effects of prolonged, intermittent flexion on trunk neuromuscular control. Furthermore, the potential beneficial effects of passive upper body support during flexion were investigated. Twenty one healthy young volunteers participated during two separate visits in which they performed 1 hour of intermittent 60 seconds flexion and 30 seconds rest cycles. Flexion was set at 80% lumbar flexion and was performed with or without upper body support. Before and after intermittent flexion exposure, lumbar range of motion was measured using inertial measurement units and trunk stability was assessed during perturbations applied in the forward direction with a force controlled actuator. Closed-loop system identification was used to determine the trunk translational admittance and reflexes as frequency response functions. The admittance describes the actuator displacement as a function of contact force and to assess reflexes muscle activation was related to actuator displacement. Trunk admittance gain decreased after unsupported flexion, while reflex gain and lumbar range of motion increased after both conditions. Significant interaction effects confirmed a larger increase in lumbar range of motion and reflex gains at most frequencies analysed following unsupported flexion in comparison to supported flexion, probably compensating for decreased passive tissue stiffness. In contrast with some previous studies we found that prolonged intermittent flexion decreased trunk admittance, which implies an increase of the lumped intrinsic and reflexive stiffness. This would compensate for decreased stiffness at the cost of an increase in cumulative low back load. Taking into account the differences between conditions it would be preferable to offer upper body support during activities that require prolonged trunk flexion.
authors: University of Primorska, Andrej Marušič Institute, Koper, Slovenia.
authors: Matej
authors: http://orcid.org/0000-0002-1496-8675
authors: M
authors: Voglar
authors: MOVE Research Institute Amsterdam, Department of Human Movement Sciences, Vrije Universiteit Amsterdam, Amsterdam, The Netherlands.
authors: Jeffrey
authors: J
authors: Wamerdam
authors: MOVE Research Institute Amsterdam, Department of Human Movement Sciences, Vrije Universiteit Amsterdam, Amsterdam, The Netherlands.
authors: Idsart
authors: I
authors: Kingma
authors: University of Primorska, Andrej Marušič Institute, Koper, Slovenia.|S2P Ltd., Laboratory for Motor Control and Motor Learning, Ljubljana, Slovenia.
authors: Nejc
authors: N
authors: Sarabon
authors: MOVE Research Institute Amsterdam, Department of Human Movement Sciences, Vrije Universiteit Amsterdam, Amsterdam, The Netherlands.
authors: Jaap H
authors: JH
authors: van Dieën
mesh_terms: D000328:Adult; D004576:Electromyography; D015444:Exercise; D005260:Female; D006801:Humans; D008297:Male; D009068:Movement; D018482:Muscle, Skeletal / Q000502:physiology*; D055815:Young Adult