title: "Pumped-up propulsion" during front crawl swimming.
abstract: OBJECTIVE
  It is currently held that propulsion in human front crawl swimming is achieved by lift and drag forces predominantly generated by the hands. Calculation of these propulsive forces relies on the quasi-steady assumption that the fluid dynamic behavior of a hand model in a flow channel (constant velocity and orientation) is similar to that of a hand of a real swimmer. However, both experimental and theoretical analyses suggest that this assumption is questionable and that unsteady and rotational propulsion mechanisms play a significant role. Theoretical considerations suggest that arm rotation could lead to a proximodistal pressure gradient, which would induce significant axial flow along the arm toward the hand.
  
  
  METHODS
  To gain insight into such mechanisms, we used tufts to study the flow directions around the arm and hand during the front crawl, which consists of a glide, an insweep, and an outsweep phase. In a second experiment, we measured pressure during the stroke at various points along the arm and hand.
  
  
  RESULTS
  It was observed that 1) the flow during insweep and part of the outsweep was highly unsteady; 2) the arm movements were largely rotational; 3) a clear axial flow component, not in the direction of the arm movement, was observed during insweep and outsweep; and 4) both the V-shaped "contracting" arrangement of the tufts during the outsweep and pressure recordings point to a pressure gradient along the direction of the arm during the outsweep, as predicted on theoretical grounds.
  
  
  CONCLUSIONS
  Our results demonstrate the reality of the predicted rotational and unsteady effects during front crawl swimming. We hypothesize that the axial flow observed during the outsweep has a propulsion-enhancing effect by increasing the pressure difference over the hand. Further investigation is required to establish more accurately the role of axial flow on propulsion.
authors.affiliation: Institute for Fundamental and Clinical Human Movement Science, Faculty of Human Movement Science, Vrije Universiteit, Amsterdam, The Netherlands. h_m_toussaint@fbw.vu.nl
authors.forename: Huub M
authors.initials: HM
authors.lastname: Toussaint
authors.forename: Coen
authors.initials: C
authors.lastname: Van den Berg
authors.forename: Wiero J
authors.initials: WJ
authors.lastname: Beek
mesh_terms: D001132:Arm / Q000502:physiology*; D001696:Biomechanical Phenomena; D006225:Hand / Q000502:physiology; D006801:Humans; D008954:Models, Biological; D009068:Movement / Q000502:physiology*; D011312:Pressure; D012399:Rotation; D013550:Swimming / Q000502:physiology*