title: Mechanical properties of the human patellar tendon, in vivo.
abstract: BACKGROUND
  Mechanical properties of the human patellar tendon are largely based on cadaver studies, which may not necessarily be applicable to the human patellar tendon, in vivo. While ultrasonography is an attractive non-invasive method for studying human, in vivo, muscle-tendon behaviour, it has mostly been used to examine Achilles tendon properties. We examine the accuracy of human patellar tendon elongation, in vivo, and its within-day and between-day reproducibility.
  
  
  METHODS
  Subjects performed four maximal 10-s ramp isometric knee extension trials to examine within-day reproducibility, and the procedure was repeated on a separate day to assess between-day reproducibility. Continuous and simultaneous ultrasonography based measurement of both tibial and patellar movement together with patellar tendon force yielded mechanical estimates.
  
  
  RESULTS
  Longitudinal tibial displacement corresponded to 45 (SEM 8) % of the overall tibia-patella displacement. For within-day reproducibility there was no difference between trials for stiffness [trial a, 4334 (SEM 562) N/mm; trial b, 4273 (SEM 533) N/mm], strain [trial a, 6.9 (SEM 0.6) %; trial b, 6.8 (SEM 0.7) %] or elastic modulus [trial a, 1.09 (SEM 0.12) GPa; trial b, 1.09 (SEM 0.10) GPa]. The within day correlation coefficient and typical error were 0.95 and 9.9% for stiffness, 0.97 and 5.5% for strain, and 0.94 and 9.4% for elastic modulus. The corresponding values for between days were 0.94 and 8.7% for stiffness, 0.98 and 3.7% for strain, and 0.86 and 9.6% for elastic modulus.
  
  
  CONCLUSIONS
  The present methodology of assessing human patellar tendon elongation, strain and stiffness, in vivo, is accurate and reproducible which permits its use in future investigations of patellar tendon biomechanics and the potential effects of various interventions.
authors.affiliation: Institute of Sports Medicine Copenhagen, Bispebjerg Hospital, Bispebjerg Bakke 23, Denmark. philip.hansen@dadlnet.dk
authors.forename: P
authors.initials: P
authors.lastname: Hansen
authors.forename: J
authors.initials: J
authors.lastname: Bojsen-Moller
authors.forename: P
authors.initials: P
authors.lastname: Aagaard
authors.forename: M
authors.initials: M
authors.lastname: Kjaer
authors.forename: S P
authors.initials: SP
authors.lastname: Magnusson
mesh_terms: D000328:Adult; D001696:Biomechanical Phenomena / Q000379:methods; D004548:Elasticity; D006801:Humans; D007537:Isometric Contraction / Q000502:physiology*; D007719:Knee Joint / Q000502:physiology*; D008297:Male; D017847:Patellar Ligament / Q000000981:diagnostic imaging* / Q000502:physiology*; D016059:Range of Motion, Articular / Q000502:physiology; D013314:Stress, Mechanical; D013710:Tendons / Q000000981:diagnostic imaging* / Q000502:physiology*; D014463:Ultrasonography