title: Hamstring injuries have increased by 4% annually in men's professional football, since 2001: a 13-year longitudinal analysis of the UEFA Elite Club injury study.
abstract: BACKGROUND
  There are limited data on hamstring injury rates over time in football.
  
  
  OBJECTIVE
  To analyse time trends in hamstring injury rates in male professional footballers over 13 consecutive seasons and to distinguish the relative contribution of training and match injuries.
  
  
  METHODS
  36 clubs from 12 European countries were followed between 2001 and 2014. Team medical staff recorded individual player exposure and time-loss injuries. Injuries per 1000 h were compared as a rate ratio (RR) with 95% CI. Injury burden was the number of lay off days per 1000 h. Seasonal trend for injury was analysed using linear regression.
  
  
  RESULTS
  A total of 1614 hamstring injuries were recorded; 22% of players sustained at least one hamstring injury during a season. The overall hamstring injury rate over the 13-year period was 1.20 injuries per 1000 h; the match injury rate (4.77) being 9 times higher than the training injury rate (0.51; RR 9.4; 95% CI 8.5 to 10.4). The time-trend analysis showed an annual average 2.3% year on year increase in the total hamstring injury rate over the 13-year period (R(2)=0.431, b=0.023, 95% CI 0.006 to 0.041, p=0.015). This increase over time was most pronounced for training injuries-these increased by 4.0% per year (R(2)=0.450, b=0.040, 95% CI 0.011 to 0.070, p=0.012). The average hamstring injury burden was 19.7 days per 1000 h (annual average increase 4.1%) (R(2)=0.437, b=0.041, 95% CI 0.010 to 0.072, p=0.014).
  
  
  CONCLUSIONS
  Training-related hamstring injury rates have increased substantially since 2001 but match-related injury rates have remained stable. The challenge is for clubs to reduce training-related hamstring injury rates without impairing match performance.
authors.affiliation: Division of Community Medicine, Department of Medical and Health Sciences, Linköping University, Linköping, Sweden Football Research Group, Linköping, Sweden UEFA Medical Committee, Nyon, Switzerland.
authors.forename: Jan
authors.initials: J
authors.lastname: Ekstrand
authors.affiliation: Division of Community Medicine, Department of Medical and Health Sciences, Linköping University, Linköping, Sweden Football Research Group, Linköping, Sweden.
authors.forename: Markus
authors.initials: M
authors.lastname: Waldén
authors.affiliation: Football Research Group, Linköping, Sweden Division of Physiotherapy, Department of Medical and Health Sciences, Linköping University, Linköping, Sweden.
authors.forename: Martin
authors.identifier: http://orcid.org/0000-0002-6883-1471
authors.initials: M
authors.lastname: Hägglund
mesh_terms: D000041:Absenteeism; D001265:Athletic Injuries / Q000453:epidemiology; D017281:Cost of Illness; D004812:Epidemiologic Methods; D005060:Europe / Q000453:epidemiology; D000070633:Hamstring Muscles / Q000293:injuries*; D006801:Humans; D008297:Male; D012008:Recurrence; D012916:Soccer / Q000293:injuries* / Q000639:trends
keywords: Muscle injury; Recurrent; Soccer; Strain; Thigh