Force–velocity curves depict the effect of afterload on velocity of contraction of cardiac muscle. A, When the force of contraction is higher (i.e., when the muscle contracts against higher afterload), velocity of shortening is reduced. Velocity is highest (V_m) at zero afterload. Increased preload shifts the curve upward, but V_m, a measure of contractility, is unchanged. B, Positive inotropic influences such as sympathetic stimulation change V_m. The force–velocity curve is shifted upward.