Morphological changes of the triceps surae muscle-tendon unit during passive extension: an in vivo rabbit model

Objective. To elucidate the morphological and biomechanical manifestation 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. Background. Although muscle strains occur more frequently than complete failures, the failure mechanism of these sub-failure injuries is rarely investigated. Monitoring of the instantaneous changes in the load-deformation curve allows correlation with the morphological changes that occur during passive extension. Methods. After anesthesia, the triceps surae muscle of rabbit was dissected and then stretched to failure by a MTS Bionix 858 machine. The morphological changes in failure patterns were recorded by photographs. Results. The morphological and biomechanical manifestations of the triceps surae muscle-tendon unit was divided into five different portions: first, the viscoelastic portion with minimal morphologic change; second, the portion of micro-failure with local ecchymosis; third and fourth, the portions of macrofailure with sequential rupture of the muscle fibers; and fifth, the portion of rupture and separation of muscle parenchyma. Conclusions. A threshold for stretch-induced injury does exist. The threshold of the initiation of micro-failure in this model was 16.5% of the strain, which corresponded to 16.6% of the maximal sustainable force.