title: Preserved and impaired aspects of feed-forward grip force control after chronic somatosensory deafferentation.
abstract: BACKGROUND
  Although feed-forward mechanisms of grip force control are a prerequisite for skilled object manipulation, somatosensory feedback is essential to acquire, maintain, and adapt these mechanisms.
  
  
  OBJECTIVE
  Individuals with complete peripheral deafferentation provide the unique opportunity to study the function of the motor system deprived of somatosensory feedback.
  
  
  METHODS
  Two individuals (GL and IW) with complete chronic deafferentation of the trunk and limbs were tested during cyclic vertical movements of a hand-held object. Such movements induce oscillating loads that are typically anticipated by parallel modulations of the grip force. Load magnitude was altered by varying either the movement frequency or object weight.
  
  
  RESULTS
  GL and IW employed excessive grip forces probably reflecting a compensatory mechanism. Despite this overall force increase, both deafferented participants adjusted their grip force level according to the load magnitude, indicating preserved scaling of the background grip force to physical demands. The dynamic modulation of the grip force with the load force was largely absent in GL, whereas in IW only slower movements were clearly affected.
  
  
  CONCLUSIONS
  The authors hypothesize that the deafferented patients may have utilized visual and vestibular cues and/or an efferent copy of the motor command of the arm movement to scale the grip force level. Severely impaired grip force-load coupling in GL suggests that sensory information is important for maintaining a precise internal model of dynamic grip force control. However, comparably better performance in IW argues for the possibility that alternative cues can be used to trigger a residual internal model.
authors.affiliation: Clinical Neuropsychology Research Group, Academic Hospital München-Bogenhausen, Munich, Germany. Joachim.Hermsdoerfer@extern.lrz-muenchen.de
authors.forename: J
authors.initials: J
authors.lastname: Hermsdörfer
authors.forename: Z
authors.initials: Z
authors.lastname: Elias
authors.forename: J D
authors.initials: JD
authors.lastname: Cole
authors.forename: B M
authors.initials: BM
authors.lastname: Quaney
authors.forename: D A
authors.initials: DA
authors.lastname: Nowak
mesh_terms: D000222:Adaptation, Physiological / Q000502:physiology; D001696:Biomechanical Phenomena; D002908:Chronic Disease; D003463:Cues; D005246:Feedback / Q000502:physiology; D005260:Female; D006225:Hand / Q000294:innervation / Q000503:physiopathology*; D018737:Hand Strength / Q000502:physiology*; D006801:Humans; D008297:Male; D008875:Middle Aged; D009069:Movement Disorders / Q000175:diagnosis / Q000209:etiology / Q000503:physiopathology*; D009119:Muscle Contraction / Q000502:physiology; D053580:Muscle Strength / Q000502:physiology; D018482:Muscle, Skeletal / Q000294:innervation / Q000503:physiopathology*; D009473:Neuronal Plasticity / Q000502:physiology; D011115:Polyneuropathies / Q000175:diagnosis / Q000503:physiopathology*; D011434:Proprioception / Q000502:physiology; D011597:Psychomotor Performance / Q000502:physiology; D012678:Sensation Disorders / Q000175:diagnosis / Q000209:etiology / Q000503:physiopathology*; D014110:Touch / Q000502:physiology; D016474:Weight-Bearing / Q000502:physiology