Non-Axial Muscle Stress & Stiffness

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9626736 Zahalak Striated muscle, whether found in the limb musculature or muscular organs like the diaphragm and the heart, is composed of thin elongated living cells called muscle fibers. It has been assumed, almost invariably, that such muscle functions as a uniaxial force generator, producing forces and experiences length changes in the direction of the fibers. With rare exceptions, experimental investigations of muscle mechanics have focused on contractions and forces in the fiber directions, and all current theories assume that when embedded in a mass of contractile tissue muscle fibers behave exactly as measured in uniaxial experiments. But at least one class of experiments--those involving osmotically-induced expansion and compression transverse to the fibers--suggest that non-axial deformations of contractile tissue can greatly reduce muscle force. At present there exists no method for estimating quantitatively the effects of non-axial deformations, which occur often in normal physiological function, on the active stresses generated by striated muscle. The goal of this research is to develop a quantitative theory of non-axial deformation effects in muscle. The proposed theory is a generalization of the well established mathematical theory of muscle contraction at the molecular level, first formulated by the English biophysicist, A.F. Huxley. Successful completion of this work will have important implications at both the levels of applied biomechanics and fundamental biophysics. At the applied level, an ability to predict quantitatively the effects of non- axial deformations on active muscle stress will clearly identify the circumstances when these effects are important and when they am not, and lead to improved descriptions of muscle function in studies of locomotion, motor control, cardiac mechanics, respiratory mechanics, and related problems. At the level of basic biophysics, such a quantitative theory may suggest some non- traditional exper iments--for example, those involving axial shear, transverse shear, or torsion--that will provide now information about the fundamental molecular mechanisms of muscle contraction. ***