Biomechanical properties of muscle-tendon unit under high-speed passive stretch

Objective. The purpose of this study was to investigate the strain injury m echanisms of the Achilles muscle-tendon unit during high-speed passive stre tch. Design. The high-speed traction device consisted of an impactor which dropp ed freely to hit one end of a lever, transferring the impact energy to trac tion energy at the other end. A muscle-tendon unit was attached to the othe r 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 faste r strain rates. However, previous studies of passive stretch in muscle-tend on units usually employed low strain rates. Methods. 16 fresh Achilles muscle-tendon units were subjected to passive st retch at a test speed of 310 cm s(-1). The history of elongation and the tr action force of the muscle-tendon unit during the elongation process were a nalyzed. Results. The muscle-tendon units exhibited highly nonlinear mechan ical behavior. Most of the elongation occurred in muscle and resulted in st ructural failure. Failure was not found in the tendon or muscle-tendon junc tion. Muscle fibers during stretching reached their maximum mechanical stre ngth and then progressively ruptured. Conclusion. The strain rate is an important factor in strain injuries of th e muscle-tendon unit due to passive stretch. The muscle is a good energy ab sorber; the rupture process can absorb a great deal of external energy and prevent complete failure of the muscle, while also protecting bone and join ts. Relevance The study of muscle-tendon unit under high-speed stretch could he lp us to understand the mechanism of strain injuries over passive stretch i n real-life situations. (C) 1999 Elsevier Science Ltd. All rights reserved.