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. RELEVANCE: Elucidation of the existence of micro-failure in skeletal muscle gives a good reference point to determine the functional capabilities and limitation of muscular tissue. With this, the deleterious effect of muscle stretching can be avoided by limiting muscle stretching to below this limit.