title: Aging effect on muscle synergies in stepping forth during a forward perturbation.
abstract: OBJECTIVE
  We explored changes in muscle interactions during healthy aging as a window into neural control strategies of postural preparation to action/perturbation. In particular, we quantified the strength of multi-muscle synergies stabilizing the center of pressure (COP) displacement during the preparation for making a step associated with support surface translations.
  
  
  METHODS
  Young and elderly subjects were required to make a step in response to support surface perturbations. Surface muscle activity of 11 leg and trunk muscles was analyzed to identify sets of 4 muscle modes (M-modes). Linear combinations of M-modes and their relationship to changes in the COP displacement in the anterior-posterior direction were then determined. Uncontrolled manifold analysis was performed to determine variance components in the M-mode space and indices of M-mode synergy stabilizing COP displacement.
  
  
  RESULTS
  Prior to the step initiation, the older subjects showed strong synergies that stabilized COP displacement to forward perturbation of the support surface. However, the synergy indices were significantly lower than those of the young subjects during preparation for making a step. The timings of early postural adjustment (EPA) and anticipatory postural adjustment (APA) were consistently earlier in the young subjects as compared to the older subjects. For both groups, the timing of EPA did not change across tasks, while APA showed delayed timing in response to the support surface translations.
  
  
  CONCLUSIONS
  We infer that changes in the indices of synergies with age may present challenges for the control of postural preparation to external perturbation in older adults. They may lead to excessive muscle co-contractions and low stability of COP displacement. The results reported here could have clinical relevance when identifying the risk of making a step, which has been linked to an increased risk of falls among the elderly.
authors.affiliation: Tianjin Key Lab of Exercise Physiology and Sports Medicine, Department of Health and Exercise Science, Tianjin University of Sport, 51 Weijin South Street, Hexi District, Tianjin, 300381, China. yunwang70@hotmail.com.
authors.forename: Yun
authors.initials: Y
authors.lastname: Wang
authors.affiliation: Institute of Sports and Health Science, 3-10-31, Kagamiyama, Higashi-hiroshima, Hiroshima, 739-0046, Japan.
authors.forename: Kazuhiko
authors.initials: K
authors.lastname: Watanabe
authors.affiliation: Department of Rehabilitation Science, Faculty of Health Sciences, Hokkaido University, N12-W5, Kita-ku, Sapporo, 060-0826, Japan.
authors.forename: Tadayoshi
authors.initials: T
authors.lastname: Asaka
mesh_terms: D000328:Adult; D000368:Aged; D000375:Aging / Q000502:physiology*; D016022:Case-Control Studies; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009119:Muscle Contraction; D018482:Muscle, Skeletal / Q000254:growth & development / Q000502:physiology*; D011187:Posture; D016138:Walking
keywords: Anticipatory adjustments; EMG; Perturbation; Stepping; Synergy