Kirchhoff's kinetic analogy relates the deformation of an incompressible elastic rod to the classical dynamics of rigid body rotation. We extend the analogy to compressible filaments and find that the extension is similar to the introduction of relativistic effects into the dynamical system. The extended analogy reveals a surprising symmetry in the deformations of compressible elastica. In addition, we use known results for the buckling of compressible elastica to derive the explicit solution for the motion of a relativistic nonlinear pendulum. We discuss cases where the extended Kirchhoff analogy may be useful for the study of other soft matter systems.Analogies to dynamical problems have been used to simplify the physics of various condensed-matter systems, ranging from the deformation of elastic bodies to the order-parameter profile across an interface between coexisting phases. A particularly well known example is Kirchhoff's kinetic analogy [1]. In this theory the threedimensional (3D) deformation of a slender elastic rod is reduced to the bending deformation of an incompressible curve, representing the mid-axis of the rod. This problem, in turn, is analogous to the dynamics of a rigid body rotating about a fixed point, where the distance along the curve and its local curvature are analogous, respectively, to time and angular velocity. When the filament is confined to a two-dimensional (2D) plane (the celebrated Euler elastica [2]), the equation of equilibrium coincides with the equation of motion of a physical pendulum [1,19].In the examples above the elastic system was reduced to an indefinitely thin, incompressible body, whose equilibrium shape follows the trajectory of a classical dynamical system. In the present work we show that relaxing the incompressibility constraint introduces terms akin to relativistic corrections to classical dynamics. Within this analogy, the compression modulus, Y , plays the role of the relativistic particle's rest mass, and the bendability parameter (Y /B) 1/2 ≡ h −1 , where B is the bending modulus, is analogous to the speed of light. The limit of an incompressible rod (h → 0) corresponds to the nonrelativistic limit.Despite the relevance to real systems, including compressible fluid membranes [4], there have not been many studies of compressible elastica (see [5] and references therein). Following these works, we consider the 2D deformation of a compressible filament, represented by a planar curve of relaxed length L. The same model applies to thin elastic sheets, as well as fluid membranes [6], provided that they are deformed along a single direction. The deformation away from the flat, stress-free * ozzoshri@tau.ac.il