Fluid to solid transition in muscles

Roboticists have long sought to develop actuators that emulate the performance of muscle. These have mostly centered on the power and force capacity of muscle and more recently on muscle's stiffness and damping characteristics. However, a critical aspect that remains missing in muscle-inspired actuators, and also poorly studied in muscle, is an activation-dependent fluid-to-solid transition in muscle's mechanical response. For example, a highly activated muscle resembles a solid-like material that maintains posture and provides stiff resistance, whereas muscle under low activation yields like a fluid without much resistance against rapid motions. Understanding how this transition may arise in muscle could guide the development of new muscle-like actuators that use similar principles as muscle to achieve a fluid-to-solid transition. Current understanding of the sarcomere is based on mean-field models of Huxley-based crossbridge cycles. We find that models of varying complexity, from two- to five-state models of actomyosin, all fail to capture the fluid-to-solid transition. In analogy with jamming transitions in disordered solids, we postulate potential nonequilibrium dynamics that may underlie the fluid-to-solid transition in muscle.