Effect of Tendon Mechanical Properties on Strain of Triceps Surae Muscle-Tendon Unit under Eccentric Contraction

Abstract

It has been recognized that there are intramuscular site-specific differences in muscle injury. Based on the accumulating evidence of inter-individual variation in the mechanical properties of human tendinous tissues(TT), we investigated how the differences in Young's modulus of TT affect the strain experienced by muscle and TT during forced lengthening of muscle-tendon unit, with computer simulation. PURPOSE: We tested the effect of different Young's modulus of TT on the magnitude and distribution of strain in human triceps surae muscle-tendon complex under eccentric contraction. Our hypothesis was that the different Young's modulus influences the strain not only in TT but also in muscle tissues. METHODS: Finite element analysis was used for computation of strain in tissues. A hexahedral constitutive model based on MR images was constructed. Young's modulus and Poisson's ratio of muscle tissues during maximal eccentric contraction were estimated from our experimental data and previous reports. The variation of TT stiffness was directly measured by human experiments with ultrasonography (n=30), and the calculated maximal (STIF: 2439.0MPa) and minimal (COMP: 272.4MPa) Young's modulus were used for analysis. Anatomical origins of muscle were fully fixed. A 20mm lengthening was applied to simulate eccentric contraction by displacing the end point of Achilles tendon. RESULTS: The change in Young's modulus of TT altered the magnitude and distribution of strain in both muscle and TT. In both conditions, strain of muscle tissues was concentrated on the origin of soleus muscle. The stain was larger in STIF than in COMP (COMP: 0.068+/-0.068 and STIF: 0.173+/-0.149). The strain at the muscle-tendon junction did not show significant difference. In COMP, larger strain was determined at the portion with smallest cross-sectional area in Achilles tendon (COMP: 0.144+/-0.042 and STIF: 0.047+/-0.014). CONCLUSIONS: The elongation of compliant TT decreased strain at the origin of soleus muscle by 61%, suggesting the role of compliant tendon in the reduction of muscle damage by decreasing the amplitude and distribution of sizable strains in muscle tissues.