Two exceptions to the above general characteristics of muscle activity during steady swimming are eels and tuna
In American eels, there is no longitudinal shift in EMG duty cycle: duty cycle is near constant along the length of this fish (Fig. 3) (Gillis1998).However, there is longitudinal variation in phase, with smaller phase shifts in the anterior of eels. In skipjack tuna, there is no longitudinal shift in phase, although anterior muscle does have a significantly longer duty cycle than posterior muscle (Fig. 3) (Shadwick et al. 1999). Differences in steady swimming mode and in red muscle activation conditions amongst all of these fish lead to questions about the relationship of swimming mode and activation conditions to power production by red muscle.