Sarcomere dynamic instability and stochastic heterogeneity drive robust cardiomyocyte contraction.

Cardiac contraction is driven by the collective action of cardiomyocytes (CMs) that contain parallel bundles of myofibrils consisting of linear chains of sarcomeres, the basic force-generating units. The dynamics of individual sarcomeres within intact CMs remain incompletely understood. While most models assume uniform, synchronized contractions, recent studies hint at unexpected heterogeneity, whose origins and significance are not yet clear. By combining the culture of fluorescent sarcomere-reporter human induced pluripotent stem cell-derived CMs on micropatterned soft gels of different stiffness (5-85 kPa) with AI-based tracking of sarcomere motion, we found that increasingly stiff substrates inhibited overall CM contraction, but, surprisingly, did not diminish individual sarcomere dynamics. Instead, sarcomeres competed in a tug-of-war, causing increasing heterogeneity, including rapid length oscillations and overextensions (popping). Statistical analysis showed that the heterogeneous dynamics were not caused by static structural differences but were largely stochastic. Stochastic heterogeneity is thus an intrinsic property of cardiac sarcomeres and likely mediates the adaptation of CM contractility to mechanical constraints. A mesoscopic model of coupled sarcomeres shows that these phenomena can be explained by a non-monotonic force-velocity relationship and stochastic fluctuations, where dynamic instability at a critical yielding force creates heterogeneity. Stochastic heterogeneity compensates for structural disorder by randomizing yield events beat-to-beat, preventing damage to specific sarcomeres. Our findings recast cardiac sarcomeres as active, dynamically unstable, and stochastic units engaged in a stochastic tug-of-war, where transient, velocity-dependent forces dominate. We propose that pathological disorder in cardiomyopathy drives a transition from protective stochastic fluctuations to more deterministic, persistently overloaded sarcomeres.