Towards bridging the gap from molecular forces to the movement of organisms.

Muscles are responsible for generating the forces required for the movement of multicellular organisms. Microscopically, these forces arise as a consequence of motor proteins (myosin) pulling and sliding along actin filaments. Current knowledge states that the molecular forces between actin and myosin are linear in nature [Huxley and Simmons (1971) Nature (London) 233, 533-538] and that the physiologically observed non-linearities (e.g. Hill's force-velocity relationship) are a consequence of non-linearities in the attachment/detachment ratios. However, this view has been disputed recently [Nielsen (2002) J. Theor. Biol. 219, 99-119], inspired by results from protein pulling experiments showing that proteins often have non-linear entropic force-extension profiles. Irrespective of the case, the present study aims at integrating such basic force-producing properties into large-scale simulations of muscle, which may accommodate macroscopic properties of muscles, e.g. the catch-like effect, the Henneman principle and accurate twitch force and motor unit size distributions. As a test of the underlying principles, a model of the biceps caput breve muscle is presented and compared with experimental data.