Failure sites and peak tensile forces of the composite triceps surae muscle by passive extension in the rabbit

The triceps surae muscle-tendon unit of rabbit was tested by a passive extension technique. The effects of different stretch rates were evaluated. Functioning as a composite muscle-tendon unit, the triceps surae responded viscoelastically to the tensile loads. The sites of failure were consistent at the muscular portion near the musculotendinous junction. Failure occurred within 0.1-1.0 mm of the distal musculotendinous junction for the soleus muscle and within 5-10 mm proximal to the distal musculotendinous junction in the lateral head of gastrocnemius muscle. In the medial head of gastrocnemius muscle failure occurred within 15-30 mm proximal to the distal musculotendinous junction. There was a greater peak tensile force requirement and a greater energy absorption when tested with faster stretch rates. These results furnish evidence that the biomechanical property of composite musculotendinous unit is strain-rate dependent and behaves viscoelastically.