title: Relationships Between Training Load Indicators and Training Outcomes in Professional Soccer.
abstract: BACKGROUND
  In professional senior soccer, training load monitoring is used to ensure an optimal workload to maximize physical fitness and prevent injury or illness. However, to date, different training load indicators are used without a clear link to training outcomes.
  
  
  OBJECTIVE
  The aim of this systematic review was to identify the state of knowledge with respect to the relationship between training load indicators and training outcomes in terms of physical fitness, injury, and illness.
  
  
  METHODS
  A systematic search was conducted in four electronic databases (CINAHL, PubMed, SPORTDiscus, and Web of Science). Training load was defined as the amount of stress over a minimum of two training sessions or matches, quantified in either external (e.g., duration, distance covered) or internal load (e.g., heart rate [HR]), to obtain a training outcome over time.
  
  
  RESULTS
  A total of 6492 records were retrieved, of which 3304 were duplicates. After screening the titles, abstracts and full texts, we identified 12 full-text articles that matched our inclusion criteria. One of these articles was identified through additional sources. All of these articles used correlations to examine the relationship between load indicators and training outcomes. For pre-season, training time spent at high intensity (i.e., >90 % of maximal HR) was linked to positive changes in aerobic fitness. Exposure time in terms of accumulated training, match or combined training, and match time showed both positive and negative relationships with changes in fitness over a season. Muscular perceived exertion may indicate negative changes in physical fitness. Additionally, it appeared that training at high intensity may involve a higher injury risk. Detailed external load indicators, using electronic performance and tracking systems, are relatively unexamined. In addition, most research focused on the relationship between training load indicators and changes in physical fitness, but less on injury and illness.
  
  
  CONCLUSIONS
  HR indicators showed relationships with positive changes in physical fitness during pre-season. In addition, exposure time appeared to be related to positive and negative changes in physical fitness. Despite the availability of more detailed training load indicators nowadays, the evidence about the usefulness in relation to training outcomes is rare. Future research should implement continuous monitoring of training load, combined with the individual characteristics, to further examine their relationship with physical fitness, injury, and illness.
authors.affiliation: Department of Kinesiology, Laboratory of Perception and Performance, Movement Control and Neuroplasticity Research Group, University of Leuven (KU Leuven), Leuven, Belgium. arne.jaspers@kuleuven.be.
authors.forename: Arne
authors.identifier: http://orcid.org/0000-0001-6809-0792
authors.initials: A
authors.lastname: Jaspers
authors.affiliation: Center for Human Movement Sciences, University of Groningen, University Medical Center, Groningen, The Netherlands.
authors.forename: Michel S
authors.initials: MS
authors.lastname: Brink
authors.affiliation: Department of Kinesiology, Laboratory of Perception and Performance, Movement Control and Neuroplasticity Research Group, University of Leuven (KU Leuven), Leuven, Belgium.
authors.forename: Steven G M
authors.initials: SG
authors.lastname: Probst
authors.affiliation: Center for Human Movement Sciences, University of Groningen, University Medical Center, Groningen, The Netherlands.|School of Sports Studies, Hanze University of Applied Sciences, Groningen, The Netherlands.
authors.forename: Wouter G P
authors.initials: WG
authors.lastname: Frencken
authors.affiliation: Department of Kinesiology, Laboratory of Perception and Performance, Movement Control and Neuroplasticity Research Group, University of Leuven (KU Leuven), Leuven, Belgium.
authors.forename: Werner F
authors.initials: WF
authors.lastname: Helsen
mesh_terms: D006339:Heart Rate; D006801:Humans; D010806:Physical Education and Training; D010809:Physical Fitness; D012621:Seasons; D012916:Soccer; D016526:Workload