MORPHOLOGICAL-CHANGES OF THE TRICEPS SURAE MUSCLE-TENDON UNIT DURING PASSIVE EXTENSION - AN IN-VIVO RABBIT MODEL

Objective. To elucidate the morphological and biomechanical manifestat ion of the triceps surae muscle-tendon unit during passive extension. Design. The instantaneous changes within the load-deformation curve of muscle-tendon unit were analyzed by an in vivo rabbit model. Backgrou nd. Although muscle strains occur more frequently than complete failur es, the failure mechanism of these sub-failure injuries is rarely inve stigated. Monitoring of the instantaneous changes in the load-deformat ion curve allows correlation with the morphological changes that occur during passive extension. Methods. After anesthesia, the triceps sura e muscle of rabbit was dissected and then stretched to failure by a MT S Bionix 858 machine. The morphological changes in failure patterns we re recorded by photographs. Results. The morphological and biomechanic al manifestations of the triceps surae muscle-tendon unit was divided into five different portions: first, the viscoelastic portion with min imal morphologic change; second, the portion of micro-failure with loc al ecchymosis; third and fourth, the portions of macrofailure with seq uential rupture of the muscle fibers; and filth, the portion of ruptur e and separation of muscle parenchyma. Conclusions. A threshold for st retch-induced injury does exist. The threshold of the initiation of mi cro-failure in this model was 16.5% of the strain, which corresponded to 16.6% of the maximal sustainable force. Relevance Elucidation of th e existence of micro-failure in skeletal muscle gives a good reference point to determine the functional capabilities and limitation of musc ular tissue. With this, the deleterious effect of muscle stretching ca n be avoided by limiting muscle stretching to below this limit. (C) 19 98 Elsevier Science Ltd. All rights reserved.