title: Human muscular and postural responses in unstable load lifting.
abstract: METHODS
  Most studies of manual material handling have focused on stable (fixed) loads. The muscular response during unstable load handling deserves more investigative attention.
  
  
  OBJECTIVE
  To investigate muscular and postural responses during unstable load lifting, and to quantify the threat posed by such loads in the workplace.
  
  
  BACKGROUND
  Accurate assessment of load weight is important for lifters. Unfortunately, this information is not always available to individuals handling containers of liquids or shifting loads, or those encountering impact loading or a sudden unexpected load.
  
  
  METHODS
  In this study, 12 subjects were subjected to time-variant loads, in which a box weighing 18 kg was used to generate a sudden 12 (kg. m)/second load impact internally during the lifting process. Responses were investigated as a function of different load-shift conditions (anterior-to-posterior and posterior-to-anterior rolling loads and stable load), two foot placements (wide and narrow stance), and two lifting techniques (stoop and squat). Normalized electromyography signals, joint angles, and total linear length for center of pressure (COP-length) were investigated.
  
  
  RESULTS
  Lifting an unstable load significantly increases total lifting time, COP-length, and muscular contraction levels. When responding to impact momentum, subjects attempted to lift with greater elbow extension and shoulder and trunk flexion. A more flexed knee joint (approximately an additional 5 degrees ) was observed during preparation for load impact in the anterior-to-posterior rolling task, allowing the formation of a more stable platform to compensate for the load variation. The largest erector spinae contraction for unstable lifting was approximately 12% to 25% greater maximal voluntary contraction than for the stable analog.
  
  
  CONCLUSIONS
  The normalized electromyography data demonstrate that the central nervous system detects and responds to the need to stabilize the joints closest to the location of load-shift perturbation.
authors.affiliation: Department of Industrial Management, National Taiwan University of Science and Technology, Taipei, Taiwan, Republic of China. yhlee@im.ntust.edu.tw
authors.forename: Yung-Hui
authors.initials: YH
authors.lastname: Lee
authors.forename: Tzu-Hsien
authors.initials: TH
authors.lastname: Lee
mesh_terms: D000054:Acceleration; D000328:Adult; D001696:Biomechanical Phenomena; D004576:Electromyography; D006801:Humans; D017770:Lifting; D008297:Male; D018482:Muscle, Skeletal / Q000502:physiology*; D004856:Postural Balance / Q000502:physiology*; D011187:Posture / Q000502:physiology*; D011930:Reaction Time / Q000502:physiology; D013314:Stress, Mechanical; D013997:Time Factors