[Studies on kinetic viscoelasticity of slow muscle fibers--1. Tension, fatigue resistance and stiffness in rabbit extraocular muscles].

To evaluate the mechanical properties of the slow fibers and fast fibers which make up the extraocular muscle, I studied the contractibility and viscoelasticity properties of the superior rectus muscle (SR) and the retractor bulbi muscle (RB) of rabbits. Eighteen to nineteen percent of whole muscle tension was produced by slow fibers in SR and 1 to 5% 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 examine kinetic viscoelasticity. 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 (Po) were then examined. The dynamic stiffness ratio (P/Po) was higher in SR (3.1) than in RB (2.0). I suggest that the viscoelasticity of the activated cross bridge is greater in slow fibers than in fast fibers.