title: The effect of arm weight support on upper limb muscle synergies during reaching movements.
abstract: BACKGROUND
  Compensating for the effect of gravity by providing arm-weight support (WS) is a technique often utilized in the rehabilitation of patients with neurological conditions such as stroke to facilitate the performance of arm movements during therapy. Although it has been shown that, in healthy subjects as well as in stroke survivors, the use of arm WS during the performance of reaching movements leads to a general reduction, as expected, in the level of activation of upper limb muscles, the effects of different levels of WS on the characteristics of the kinematics of motion and of the activity of upper limb muscles have not been thoroughly investigated before.
  
  
  METHODS
  In this study, we systematically assessed the characteristics of the kinematics of motion and of the activity of 14 upper limb muscles in a group of 9 healthy subjects who performed 3-D arm reaching movements while provided with different levels of arm WS. We studied the hand trajectory and the trunk, shoulder, and elbow joint angular displacement trajectories for different levels of arm WS. Besides, we analyzed the amplitude of the surface electromyographic (EMG) data collected from upper limb muscles and investigated patterns of coordination via the analysis of muscle synergies.
  
  
  RESULTS
  The characteristics of the kinematics of motion varied across WS conditions but did not show distinct trends with the level of arm WS. The level of activation of upper limb muscles generally decreased, as expected, with the increase in arm WS. The same eight muscle synergies were identified in all WS conditions. Their level of activation depended on the provided level of arm WS.
  
  
  CONCLUSIONS
  The analysis of muscle synergies allowed us to identify a modular organization underlying the generation of arm reaching movements that appears to be invariant to the level of arm WS. The results of this study provide a normative dataset for the assessment of the effects of the level of arm WS on muscle synergies in stroke survivors and other patients who could benefit from upper limb rehabilitation with arm WS.
authors.affiliation: Translational Neural Engineering Laboratory, Center for Neuroprosthetics and Institute of Bioengineering, School of Engineering, École Polytechnique Fédérale de Lausanne (EPFL), BM 3210 Station 17, Lausanne CH-1015, Switzerland. martina.coscia@epfl.ch.
authors.forename: Martina
authors.initials: M
authors.lastname: Coscia
authors.forename: Vincent C K
authors.initials: VC
authors.lastname: Cheung
authors.forename: Peppino
authors.initials: P
authors.lastname: Tropea
authors.forename: Alexander
authors.initials: A
authors.lastname: Koenig
authors.forename: Vito
authors.initials: V
authors.lastname: Monaco
authors.forename: Caoimhe
authors.initials: C
authors.lastname: Bennis
authors.forename: Silvestro
authors.initials: S
authors.lastname: Micera
authors.forename: Paolo
authors.initials: P
authors.lastname: Bonato
mesh_terms: D000328:Adult; D001132:Arm / Q000502:physiology*; D001696:Biomechanical Phenomena; D004576:Electromyography; D005260:Female; D006801:Humans; D008297:Male; D009068:Movement / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology*; D009989:Orthotic Devices; D026741:Physical Therapy Modalities / Q000295:instrumentation; D016059:Range of Motion, Articular / Q000502:physiology