Fig. 5. Results of in vitro measurements of power output from red muscle using the oscillatory work loop technique for the non-heterothermic bonito (Sarda chiliensis) (A,C) and the heterothermic yellowfin tuna (Thunnus albacares) (B,D). Data are from Altringham and Block (1996). (A,B) Power in Wkg^-1 fish for four experimental temperatures, each tested over a range of cycle frequencies. (C,D) The data from A,B normalized by cycle frequency to estimate work per cycle. Normalization by cycle frequency separates the effect of temperature on the force-generating capacity of the muscle from the effect of temperature on the time constants of force development and decay. The results show that yellowfin and, except at the highest temperature bonitos as well, have a modest temperature-dependence in the maximum work per cycle they can generate at the different temperatures. The data also show that, when compared at the same temperatures and frequencies, the non-heterothermic red muscle performs at least as well as the heterothermic red muscle.