Although almost thirty years have paased since the discovery that muscle contraction results from relative sliding of the myofilaments, the molecular mechanism of the crossbridge motion responsible for contraction still remains as a matter of speculation. In this article, the problems of the sliding filament models of Huxley (1957)and Huxley and Simmons(1971), which are central in the field of muscle research, are discussed in relation to the followillg six topics, i.e. (1) constancy of the myofilament lengths during contraction, (2) experimental evidence for and against the Huxley-Simmons contraction model, (3) experimental evidence against the myosin head rotation mechanism, (4) problems on the sarcomere length-force relation, (5) possible mechanism of the force enhancement after stretch and. (6) relation between the velocity of shortening and the number of attached cross-bridges.
It is concluded that the above models are now far from being complete, and from the standpoint of muscle physiology, much more experimental work with physical and chemical techniques should be performed on the preparations in which the three-dimensional myofilament lattice structures are preserved.

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For the last ten years, a variety of NMR imaging techniques have been proposed. These techniques are currently finding useful application in medicine, physiology, biology, and materials science. This article reviews the basis and the applications of the NMR imaging techniques.

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