title: Spring-mass behavior during exhaustive run at constant velocity in elite triathletes.
abstract: OBJECTIVE
  The aims of this study were i) to evaluate changes in leg-spring behavior during an exhaustive run in elite triathletes and ii) to determine whether these modifications were related to an increase in the energy cost of running (Cr).
  
  
  METHODS
  Nine elite triathletes ran to exhaustion on an indoor track at a constant velocity corresponding to 95% of the velocity associated with the maximal oxygen uptake (mean ± SD = 5.1 ± 0.3 m·s(-1), time to exhaustion = 10.7 ± 2.6 min). Vertical and horizontal ground reaction forces were measured every lap (200 m) by a 5-m-long force platform system. Cr was measured from pulmonary gas exchange using a breath-by-breath portable gas analyzer.
  
  
  RESULTS
  Leg stiffness (-13.1%, P < 0.05) and peak vertical (-9.2%, P < 0.05) and propulsive (-7.5%, P < 0.001) forces decreased significantly with fatigue, whereas vertical stiffness did not change significantly. Leg and vertical stiffness changes were positively related with modifications of aerial time (R(2) = 0.66, P < 0.01 and R(2) = 0.72, P < 0.01, respectively) and negatively with contact time (R(2) = 0.71, P < 0.01 and R(2) = 0.74, P < 0.01, respectively). Alterations of vertical forces were related with the decrease of the angle of velocity vector at toe off (R(2) = 0.73, P < 0.01). When considering mean values of oxygen uptake, no change was observed from 33% to 100% of the time to exhaustion. However, between one-third and two-thirds of the fatiguing run, negative correlations were observed between oxygen consumption and leg stiffness (R(2) = 0.83, P < 0.001) or vertical stiffness (R(2) = 0.50, P < 0.03).
  
  
  CONCLUSIONS
  During a constant run to exhaustion, the fatigue induces a stiffness adaptation that modifies the stride mechanical parameters and especially decreases the maximal vertical force. This response to fatigue involves greater energy consumption.
authors.affiliation: Research Department, National Institute of Sport, Expertise and Performance, INSEP, Paris, France. giuseppe.rabita@insep.fr
authors.forename: Giuseppe
authors.initials: G
authors.lastname: Rabita
authors.forename: Jean
authors.initials: J
authors.lastname: Slawinski
authors.forename: Olivier
authors.initials: O
authors.lastname: Girard
authors.forename: Frank
authors.initials: F
authors.lastname: Bignet
authors.forename: Christophe
authors.initials: C
authors.lastname: Hausswirth
mesh_terms: D000328:Adult; D000465:Algorithms; D056352:Athletes; D004734:Energy Metabolism / Q000502:physiology; D005260:Female; D006801:Humans; D007866:Leg / Q000502:physiology*; D008297:Male; D009068:Movement; D009119:Muscle Contraction / Q000502:physiology*; D018763:Muscle Fatigue / Q000502:physiology; D010101:Oxygen Consumption; D005082:Physical Exertion / Q000502:physiology*; D012420:Running / Q000502:physiology*; D055815:Young Adult