title: A Correction Equation for Jump Height Measured Using the Just Jump System.
abstract: OBJECTIVE
  To determine the concurrent validity and reliability of the popular Just Jump system (JJS) for determining jump height and, if necessary, provide a correction equation for future reference.
  
  
  METHODS
  Eighteen male college athletes performed 3 bilateral countermovement jumps (CMJs) on 2 JJSs (alternative method) that were placed on top of a force platform (criterion method). Two JJSs were used to establish consistency between systems. Jump height was calculated from flight time obtained from the JJS and force platform.
  
  
  RESULTS
  Intraclass correlation coefficients (ICCs) demonstrated excellent within-session reliability of the CMJ height measurement derived from both the JJS (ICC = .96, P < .001) and the force platform (ICC = .96, P < .001). Dependent t tests revealed that the JJS yielded a significantly greater CMJ jump height (0.46 ± 0.09 m vs 0.33 ± 0.08 m) than the force platform (P < .001, Cohen d = 1.39, power = 1.00). There was, however, an excellent relationship between CMJ heights derived from the JJS and force platform (r = .998, P < .001, power = 1.00), with a coefficient of determination (R2) of .995. Therefore, the following correction equation was produced: Criterion jump height = (0.8747 × alternative jump height) - 0.0666.
  
  
  CONCLUSIONS
  The JJS provides a reliable but overestimated measure of jump height. It is suggested, therefore, that practitioners who use the JJS as part of future work apply the correction equation presented in this study to resultant jump-height values.
authors.affiliation: Directorate of Sport, Exercise and Physiotherapy, University of Salford, Salford, UK.
authors.forename: John J
authors.initials: JJ
authors.lastname: McMahon
authors.forename: Paul A
authors.initials: PA
authors.lastname: Jones
authors.forename: Paul
authors.initials: P
authors.lastname: Comfort
mesh_terms: D006801:Humans; D008297:Male; D059385:Plyometric Exercise / Q000706:statistics & numerical data*; D015203:Reproducibility of Results; D055815:Young Adult