Filaments repel while muscles propel: conservation of energy explains length-dependent lattice spacing in sarcomeres

The radial lattice spacing (LS) of actin and myosin filaments within a sarcomere changes substantially during muscle contraction. While these changes have been phenomenologically attributed to the constant-volume characteristic of lattice unit cells, the underlying mechanism remained unresolved. Here, I present a novel model that, for the first time, explains these observations by invoking the principle of constant internal energy. Based on electrostatic repulsion between charged filaments in an ionic medium, the model predicts length-dependent LS adaptations that maintain an energetic equilibrium as filament overlap varies. The resulting LS behavior closely follows experimental data across a wide range of sarcomere lengths. Rooted in fundamental physics and applicable to different muscle types, this approach provides new insight into the structural dynamics of the sarcomere and its role in muscle force generation.