To understand better how scup can swim twice as fast as carp with its red muscle, we measured the mechanical properties of red muscle bundles in scup. The values of the mean maximum velocity of shortening (Vmax) at 10 degrees C (3.32 muscle lengths s-1) and at 20 degrees C (5.55 muscle lengths s-1; Q10 = 1.69) were nearly the same as those in carp. Isometric force, however, was approximately 50% greater (183 kN m-2; Q10 = 1.08). The maximal power generation was correspondingly about 50% greater in scup than in carp (71 W kg-1 at 10 degrees C and 134 W kg-1 at 20 degrees C; Q10 = 1.88). The larger power output of its muscle may be important in the faster swimming of the scup. In addition, the fact that scup use a less undulatory style of swimming means that, when they are swimming twice as fast, their red muscle shortens at the same velocity (V) and with the same V/Vmax (0.37, i.e. where maximum power is generated) as that of carp. The importance of V/Vmax is further shown by the comparison of scup swimming at different temperatures. The 1.69-fold higher Vmax at 20 degrees C than at 10 degrees C enables scup to swim with a 1.67-fold faster V at 20 degrees C. Thus, at both 10 degrees C and 20 degrees C, red muscle is used only over the same narrow range of V/Vmax (0.17-0.37), where experiments on isolated muscle suggest that power and efficiency are maximal. Therefore, V/Vmax appears to be an important design constraint that limits the range of velocities over which muscle is used in vivo, both at different temperatures and in fast- and slow-locomoting species.