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P.J. Reiser, D.G. Gentry, S. Bicer, K.M. Gentry; Passive Viscoelastic Properties of Global and Orbital Layers of Dog Rectus Muscles . Invest. Ophthalmol. Vis. Sci. 2005;46(13):5719.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To measure and compare passive viscoelastic properties of the global and orbital layers of dog rectus muscles. Methods: Small strips (∼200 um thick and 2 mm long) of the global and orbital layers of dog rectus muscles were mounted between an isometric tension transducer and a servo–controlled motor under conditions which ensured complete relaxation. Strips from the predominantly fast tibialis cranialis and predominantly slow gastrocnemius muscles were utilized to compare extraocular muscle strips with limb muscle strips. Sinusoidal length oscillations (2, 5 and 10% of strip length, each at 0.2, 1, 10 and 25 Hz) were applied to one end of the strips. The resultant force oscillations were recorded. The elastic modulus (calculated from the peak–to–peak stiffness), elasticity (determined as the inverse of the curvature of the stress/strain relationship during the lengthening phase), and viscosity (the area within the stress–strain loops during the 5% oscillations at 1 Hz) were calculated. Results: The mean elastic moduli (i.e., stiffness) of both layers of rectus muscles were significantly greater than that of fast and slow limb muscles. The mean elastic modulus of global strips was ∼35% greater (P<.05) than that of orbital strips. The elasticity in the global strips, compared to the orbital strips, was lower (greater curvature, therefore greater stiffness; P<.05), especially with the 10% length oscillations. The viscosity of the global strips was 50–60% greater (P<.05) than that of orbital strips and of fast and slow limb muscle strips. Orbital viscosity did not differ from fast or slow limb muscle viscosity. Conclusions: The global and orbital layers of dog rectus muscles have different passive viscoelastic properties, with the global layer having lower elasticity (greater stiffness) and a greater viscous component. Furthermore, the viscoelastic properties of both layers of dog rectus muscle differ significantly from those of fast and slow limb skeletal muscles. The different viscoelastic properties of the global and orbital layers of dog rectus muscles may complement differences in active contractile properties of the two layers during oculomotor functions.
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