title: The effect of short-term interval training during the competitive season on physical fitness and signs of fatigue: a crossover trial in high-level youth football players.
abstract: OBJECTIVE
  To analyze performance and fatigue effects of small-sided games (SSG) vs high-intensity interval training (HIIT) performed during a 4-wk in-season period in high-level youth football.
  
  
  METHODS
  Nineteen players from 4 youth teams (16.5 [SD 0.8] y, 1.79 [0.06] m, 70.7 [5.6] kg) of the 2 highest German divisions completed the study. Teams were randomly assigned to 1 of 2 training sequences (2 endurance sessions per wk): One training group started with SSG, whereas the other group conducted HIIT during the first half of the competitive season. After the winter break, training programs were changed between groups. Before and after the training periods the following tests were completed: the Recovery-Stress Questionnaire for Athletes, creatine kinase and urea concentrations, vertical-jump height (countermovement jump [CMJ], drop jump), straight sprint, agility, and an incremental field test to determine individual anaerobic threshold (IAT).
  
  
  RESULTS
  Significant time effects were observed for IAT (+1.3%, η(p)(2) = .31), peak heart rate (-1.8%, η(p)(2) = .45), and CMJ (-2.3%, η(p)(2) = .27), with no significant interaction between groups (P > .30). Players with low baseline IAT values (+4.3%) showed greater improvements than those with high initial values (± 0.0%). A significant decrease was found for total recovery (-5.0%, η(p)(2) = .29), and an increase was found for urea concentration (+9.2%, η(p)(2) = .44).
  
  
  CONCLUSIONS
  Four weeks of in-season endurance training can lead to relevant improvements in endurance capacity. The decreases in CMJ height and total-recovery score together with the increase in urea concentration might be interpreted as early signs of fatigue. Thus, the danger of overtaxing players should be considered.
authors.affiliation: Dept of Sport, Exercise and Health, University of Basel, Basel, Switzerland.
authors.forename: Oliver
authors.initials: O
authors.lastname: Faude
authors.forename: Anke
authors.initials: A
authors.lastname: Steffen
authors.forename: Michael
authors.initials: M
authors.lastname: Kellmann
authors.forename: Tim
authors.initials: T
authors.lastname: Meyer
mesh_terms: D000293:Adolescent; D015308:Anaerobic Threshold / Q000502:physiology; D003163:Competitive Behavior / Q000502:physiology; D003402:Creatine Kinase / Q000097:blood; D018592:Cross-Over Studies; D005221:Fatigue / Q000503:physiopathology*; D006339:Heart Rate / Q000502:physiology; D006801:Humans; D008297:Male; D010806:Physical Education and Training / Q000379:methods*; D010807:Physical Endurance / Q000502:physiology; D010809:Physical Fitness / Q000502:physiology*; D012916:Soccer / Q000502:physiology*; D014508:Urea / Q000097:blood
chemical_list: D014508:Urea; D003402:Creatine Kinase