title: Inward torque and high-friction handles can reduce required muscle efforts for torque generation.
abstract: OBJECTIVE
  The effects of handle friction and torque direction on muscle activity and torque are empirically investigated using cylindrical handles.
  
  
  BACKGROUND
  A torque biomechanical model that considers contact force, friction, and torque direction was evaluated using different friction handles.
  
  
  METHODS
  Twelve adults exerted hand torque in opposite directions about the long axis of a cylinder covered with aluminum or rubber while grip force, torque, and finger flexor electromyography (EMG) were recorded. In addition, participants performed grip exertions without torque, in which they matched the EMG level obtained during previous maximum torque exertions, to allow us to determine how grip force was affected by the absence of torque.
  
  
  RESULTS
  (a) Maximum torque was 52% greater for the high-friction rubber handle than for the low-friction aluminum handle. (b) Total normal force increased 33% with inward torque (torque applied in the direction fingertips point) and decreased 14% with outward torque (torque in the direction the thumb points), compared with that with no torque. Consequently, maximum inward torque was 45% greater than maximum outward torque. (c) The effect of torque direction was greater for the high-friction rubber handle than for the low-friction aluminum handle.
  
  
  CONCLUSIONS
  The results support the proposed model, which predicts a large effect of torque direction when high-friction handles are gripped.
  
  
  CONCLUSIONS
  Designing tasks with high friction and inward rotations can increase the torque capability of workers of a given strength, or reduce required muscle activities for given torque exertions, thus reducing the risk of fatigue and musculoskeletal disorders.
authors: Center for Ergonomics, University of Michigan, Ann Arbor, MI 48109, USA. najins@umich.edu
authors: Na Jin
authors: NJ
authors: Seo
authors: Thomas J
authors: TJ
authors: Armstrong
authors: Don B
authors: DB
authors: Chaffin
authors: James A
authors: JA
authors: Ashton-Miller
mesh_terms: D000328:Adult; D004576:Electromyography; D004867:Equipment Design; D006804:Ergonomics; D005260:Female; D018737:Hand Strength / Q000502:physiology*; D006801:Humans; D008297:Male; D009119:Muscle Contraction / Q000502:physiology*; D019415:Torque; D014481:United States