title: Muscle coactivation before and after the impact phase of running following isokinetic fatigue.
abstract: BACKGROUND
  The effects of fatigue on impact loading during running are unclear, with some authors reporting increased impact forces and others reporting decreased forces.
  
  
  OBJECTIVE
  To examine the effects of isokinetic fatigue on muscle cocontraction ratios about the knee and ankle during running.
  
  
  METHODS
  Cross-sectional study.
  
  
  METHODS
  Neuromechanics laboratory.
  
  
  METHODS
  Female middle-distance runners (age  =  21.3 ± 1.93 years) with at least 5 years of training experience.
  
  
  METHODS
  Participants ran on the treadmill at 3.61 m/s before and immediately after the fatigue protocol, which consisted of consecutive, concentric knee extension-flexion at 120°/s until they could no longer produce 30% of the maximum knee-extension moment achieved in the familiarization session for 3 consecutive repetitions.
  
  
  METHODS
  Electromyographic (EMG) amplitude of the vastus medialis (VM), biceps femoris (BF), gastrocnemius (GAS), and tibialis anterior (TA) was recorded using surface electrodes. Agonist∶antagonist EMG ratios for the knee (VM∶BF) and ankle (GAS∶TA) were calculated for the preactivation (PR), initial loading response (LR(1)), and late loading response (LR(2)) phases of running. Hip-, knee-, and ankle-joint angular displacements at initial foot contact were obtained from 3-dimensional kinematic tracings.
  
  
  RESULTS
  Fatigue did not alter the VM∶BF EMG ratio during the PR phase (P > .05), but it increased the ratio during the LR(1) phase (P < .05). The GAS∶TA EMG ratio increased during the LR(1) phase after fatigue (P < .05) but remained unchanged during the PR and LR(2) phrases (P > .05).
  
  
  CONCLUSIONS
  The increased agonist EMG activation, coupled with reduced antagonist EMG activation after impact, indicates that the acute decrease in muscle strength capacity of the knee extensors and flexors results in altered muscle-activation patterns about the knee and ankle before and after foot impact.
authors.affiliation: Laboratory of Neuromechanics, Aristotle University of Thessaloniki, Greece. ekellis@phed-sr.auth.gr
authors.forename: Eleftherios
authors.initials: E
authors.lastname: Kellis
authors.forename: Andreas
authors.initials: A
authors.lastname: Zafeiridis
authors.forename: Ioannis G
authors.initials: IG
authors.lastname: Amiridis
mesh_terms: D000842:Ankle / Q000502:physiology; D000843:Ankle Joint / Q000502:physiology; D001696:Biomechanical Phenomena; D003430:Cross-Sectional Studies; D004576:Electromyography; D005080:Exercise Test; D005260:Female; D006621:Hip Joint / Q000502:physiology; D006801:Humans; D007717:Knee / Q000502:physiology; D007719:Knee Joint / Q000502:physiology; D009068:Movement; D009119:Muscle Contraction; D018763:Muscle Fatigue; D018482:Muscle, Skeletal / Q000293:injuries* / Q000502:physiology*; D052097:Quadriceps Muscle; D012420:Running / Q000502:physiology*; D055815:Young Adult