A study of viscoelasticity index for evaluating muscle hypotonicity during static stretching

Static stretching is widely used as a preventative treatment for musculoskeletal disabilities by providing muscle hypotonicity, which results from changes in muscle tissue structure. However, the quantitative evaluation of hypotonicity during stretching has had limited success owing to the confounding factor of mechanical stress relaxation. To resolve this problem, we propose a new evaluation method for hypotonicity based on a viscoelastic muscle model using fractional calculus, which is known to be effective for biomaterials. We made continuous measurements of rectus skin indentation during static stretching as an indicator of reaction force in the rectus femoris muscle. The viscoelastic ratio and modulus were computed from the indentation trace. Both viscoelastic parameters decreased significantly between the early and final phases of stretching. The results suggest that our method is useful for quantitative evaluation of muscle hypotonicity during stretching.