  Despite the great success of the Huxley sliding filament model proposed half
a century ago for actin-myosin linkages (cross-bridges), it fails to explain
the force-velocity behavior of stretching skeletal muscles. Huxley's two-state
kinetic equation for cross-bridge proportions is therefore reconsidered and a
new solution to the problem of steady muscle eccentric and concentric
contractions is reported. When the second law of statistical thermodynamics is
applied to cross-bridge proportions, the weakly bound states appear to be
correlated to the strongly bound states via structural and kinetic intrinsic
muscle characteristics. The explicit force-velocity curve fits the empirical
tension-velocity data on frog muscle shortening using only one adjustable
parameter, while the Huxley model employed four parameters.
