FUKUNAGA, T., Y. KAWAKAMI, K. KUBO, and H. KANEHISA. Muscle and tendon interactions during human movements. Exerc. Sport Sci. Rev., Vol. 30, No. 3, pp. 106–110, 2002. Muscle and tendon interaction was estimated in vivo by real-time ultrasonography. Differences between muscles in internal muscle-fiber shortening during isometric actions are due to the elastic properties of tendon. Compliant human tendons allow muscles to contract isometrically during many human movements for efficient force generation.