Preserving sarcomere number after tenotomy requires stretch and contraction

INTRODUCTION: Passive stretch therapy is utilized to improve the range of motion of chronically shortened muscles. However, human studies show conflicting results as whether passive stretch is clinically effective. METHODS: The soleus muscles of adult rats were tenotomized to induce muscle shortening adaptation. Muscles included were non-treated normal, subjected to daily static stretch, or lengthened and isometrically contracted for 20 min/day. Muscle fiber structure was analyzed histochemically. Sarcomeres per millimeter length were counted to assess the effect of treatment. RESULTS: Passive stretch significantly reduced central core lesion formation, but sarcomere loss was not prevented. The addition of isometric contraction during static stretch significantly (P < 0.001) reduced sarcomere loss. CONCLUSIONS: Passive stretch alone does not prevent shortening adaptation. Contraction is required in combination with stretch to preserve the number of sarcomeres in series. The combination of stretch and contraction is necessary to maintain proper muscle fiber length.