Spontaneous oscillatory contraction (SPOC) in striated muscle: focusing on mechanistic models.

The contractile apparatus of striated muscle, including both skeletal and cardiac muscle, intrinsically exhibits self-sustained oscillatory behavior under intermediate activation states between contraction and relaxation. This phenomenon, termed SPOC (spontaneous oscillatory contraction), provides unique insights into the dynamic regulation of force generation. In this review, we first highlight experimental findings that characterize SPOC and then focus on theoretical frameworks developed to capture its essential features. A key feature of these models is the assumption that the lattice spacing, that is, the distance between the thick and thin filaments, varies dynamically during muscle contraction and that actomyosin activity depends on this spacing. This assumption naturally explains the emergence of SPOC. Particular attention is given to models constructed for single sarcomeres, isolated myofibrils, and small bundles of myofibrils, through which the underlying mechanisms of SPOC can be systematically understood. Finally, by extending the mathematical structure of these models, we propose that SPOC may exhibit chaotic properties, thereby providing a comprehensive understanding of its mechanisms and physiological significance.