title: Biomechanical properties of muscle-tendon unit under high-speed passive stretch.
abstract: OBJECTIVE
  The purpose of this study was to investigate the strain injury mechanisms of the Achilles muscle-tendon unit during high-speed passive stretch.
  
  
  METHODS
  The high-speed traction device consisted of an impactor which dropped freely to hit one end of a lever, transferring the impact energy to traction energy at the other end. A muscle-tendon unit was attached to the other end of the lever via a force link, and the elongation was recorded with a high-speed camera.
  
  
  BACKGROUND
  The muscle-tendon unit is thought to act viscoelastically. It is generally strain rate dependent, exhibiting higher tensile stress at faster strain rates. However, previous studies of passive stretch in muscle-tendon units usually employed low strain rates.
  
  
  METHODS
  16 fresh Achilles muscle-tendon units were subjected to passive stretch at a test speed of 310 cm s(-1). The history of elongation and the traction force of the muscle-tendon unit during the elongation process were analyzed.
  
  
  RESULTS
  The muscle-tendon units exhibited highly nonlinear mechanical behavior. Most of the elongation occurred in muscle and resulted in structural failure. Failure was not found in the tendon or muscle-tendon junction. Muscle fibers during stretching reached their maximum mechanical strength and then progressively ruptured.
  
  
  CONCLUSIONS
  The strain rate is an important factor in strain injuries of the muscle-tendon unit due to passive stretch. The muscle is a good energy absorber; the rupture process can absorb a great deal of external energy and prevent complete failure of the muscle, while also protecting bone and joints.
  
  
  CONCLUSIONS
  The study of muscle-tendon unit under high-speed stretch could help us to understand the mechanism of strain injuries over passive stretch in real-life situations.
authors.affiliation: Department of Orthopaedics, Institute of Biomedical Engineering, National Cheng Kung University, Medical center, No. 138 Sheng-Li Road, Tainan 701, Taiwan. linrm@mail.ncku.edu.tw
authors.forename: R
authors.initials: R
authors.lastname: Lin
authors.forename: G
authors.initials: G
authors.lastname: Chang
authors.forename: L
authors.initials: L
authors.lastname: Chang
mesh_terms: D000125:Achilles Tendon / Q000293:injuries / Q000502:physiology*; D000818:Animals; D001696:Biomechanical Phenomena; D008297:Male; D011817:Rabbits; D012421:Rupture; D013314:Stress, Mechanical