title: Tendon Contraction After Cyclic Elongation Is an Age-Dependent Phenomenon: In Vitro and In Vivo Comparisons.
abstract: BACKGROUND
  Tendons are viscoelastic tissues that deform (elongate) in response to cyclic loading. However, the ability of a tendon to recover this elongation is unknown.
  
  
  OBJECTIVE
  Tendon length significantly increases after in vivo or in vitro cyclic loading, and the ability to return to its original length through a cell-mediated contraction mechanism is an age-dependent phenomenon.
  
  
  METHODS
  Controlled laboratory study.
  
  
  METHODS
  In vitro, rat tail tendon fascicles (RTTfs) from Sprague-Dawley rats of 3 age groups (1, 3, and 12 months) underwent 2% cyclic strain at 0.17 Hz for 2 hours, and the percentages of elongation were determined. After loading, the RTTfs were suspended for 3 days under tissue culture conditions and photographed daily to determine the amount of length contraction. In vivo, healthy male participants (n = 29; age, 19-49 years) had lateral, single-legged weightbearing radiographs taken of the knee at 60° of flexion immediately before, immediately after, and 24 hours after completing eccentric quadriceps loading exercises on the dominant leg to fatigue. Measurements of patellar tendon length were taken from the radiographs, and the percentages of tendon elongation and subsequent contraction were calculated.
  
  
  RESULTS
  In vitro, cyclic loading increased the length of all RTTfs, with specimens from younger (1 and 3 months) rats demonstrating significantly greater elongation than those from older (12 months) rats (P = .009). The RTTfs contracted to their original length significantly faster (P < .001) and in an age-dependent fashion, with younger animals contracting faster. In vivo, repetitive eccentric loading exercises significantly increased patellar tendon length (P < .001). Patellar tendon length decreased 24 hours after exercises (P < .001) but did not recover completely (P < .001). There was a weak but significant (R (2) = 0.203, P = .014) linear correlation between the amount of tendon contraction and age, with younger participants (<30 years) demonstrating significantly more contraction (P = .014) at 24 hours than older participants (>30 years).
  
  
  CONCLUSIONS
  Cyclic tendon loading results in a significant increase in tendon elongation under both in vitro and in vivo conditions. Tendons in both conditions demonstrated an incomplete return to their original length after 24 hours, and the extent of this return was age dependent.
  
  
  CONCLUSIONS
  The age- and time-dependent contraction of tendons, elongated after repetitive loading, could result in transient alterations in the mechanobiological environment of tendon cells. This, in turn, could induce the onset of catabolic changes associated with the pathogenesis of tendinopathy. These results suggest the importance of allowing time for contraction between bouts of repetitive exercise and may explain why age is a predisposing factor in tendinopathy.
authors.affiliation: Laboratory for Comparative Orthopaedic Research, College of Veterinary Medicine, Michigan State University, East Lansing, Michigan, USA.
authors.forename: Michael
authors.initials: M
authors.lastname: Lavagnino
authors.affiliation: MedSport, University of Michigan Health System, Ann Arbor, Michigan, USA.
authors.forename: Asheesh
authors.initials: A
authors.lastname: Bedi
authors.affiliation: MedSport, University of Michigan Health System, Ann Arbor, Michigan, USA.
authors.forename: Christopher P
authors.initials: CP
authors.lastname: Walsh
authors.affiliation: MedSport, University of Michigan Health System, Ann Arbor, Michigan, USA.
authors.forename: Elizabeth R
authors.initials: ER
authors.lastname: Sibilsky Enselman
authors.affiliation: Laboratory for Comparative Orthopaedic Research, College of Veterinary Medicine, Michigan State University, East Lansing, Michigan, USA.
authors.forename: Shahin
authors.initials: S
authors.lastname: Sheibani-Rad
authors.affiliation: Laboratory for Comparative Orthopaedic Research, College of Veterinary Medicine, Michigan State University, East Lansing, Michigan, USA arnoczky@cvm.msu.edu.
authors.forename: Steven P
authors.initials: SP
authors.lastname: Arnoczky
mesh_terms: D000328:Adult; D000818:Animals; D001696:Biomechanical Phenomena / Q000502:physiology; D006801:Humans; D007537:Isometric Contraction / Q000502:physiology*; D008297:Male; D052580:Muscle Stretching Exercises; D017847:Patellar Ligament / Q000502:physiology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D013314:Stress, Mechanical; D013710:Tendons / Q000502:physiology*; D016474:Weight-Bearing / Q000502:physiology*
keywords: aging; contraction; elongation; patellar; rat tail; tendon