A study on the visco-elastic properties of the extraocular muscles

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We examined the visco-elastic properties of the slow and fast fibers consisting the superior rectus muscle (SR) and retractor bulbi muscle (RB) of rabbits. Eighteen to nineteen percent of whole muscle tension was produced by slow fibers in SR and one to five percent by those in RB.After long continuous contraction, fatigue-resistant fibers left residual tension in SR,but the tension was almost entirely absent in RB.The frequency response method was used to evaluate dynamic visco-elasticity. The muscle was stretched sinusoidally, with an amplitude of 1 mm at 20 Hz in Lmax. The tension amplitude to sine wave vibrations during tetanus (P) and resting (P_0) were then examined. The dynamic stiffness ratio (P/P_0) was higher in SR (3.1) than in RB (2.0). It was suggested that the visco-elasticity of the activated cross bridges at myosin and actin filaments are greater in slow fibers than in fast fibers.We concluded that these properties support the hypothesis of the slow fibers to stabilize the eye balls.