The influence of thermal acclimation on power production during swimming I. In vivo stimulation and length change pattern of scup red muscle

Ectothermal animals are able to locomote in a kinematically similar manner over a wide range of temperatures. It has long been recognized that there c an be a significant reduction in the power output of muscle during swimming at low temperatures because of the reduced steady-state (i.e. constant act ivation and shortening velocity) power-generating capabilities of However, an additional reduction in power involves the interplay between the non-ste ady-state contractile properties of the muscles (i.e. the rates of activati on and relaxation) and the in vivo stimulation and length change pattern th e muscle undergoes during locomotion. In particular, it has been found that isolated scup (Stenotomus chrysops) red muscle working under in vivo stimu lus and length change conditions (measured in warm-acclimated scup swimming at low temperatures) generates very little power for swimming. Even though the relaxation of the muscle has slowed greatly, warm-acclimated fish swim with the same tail-beat frequencies and the same stimulus duty cycles at c old temperatures, thereby not affording the slow-relaxing muscle any extra time to relax. We hypothesize that considerable improvement in the power output of the red muscle at low temperatures could be achieved if cold acclimation resulted in either a faster muscle relaxation rate or in the muscle being given more time to relax (e.g. by shortening the stimulus duration or reducing the ta il-beat frequency), We test these hypotheses in this paper and the accompan ying paper. Scup were acclimated to 10 degreesC (cold-acclimated) and 20 degreesC (warm -acclimated) for at least 6 weeks, Electromyograms (EMGs) and high-speed ci ne films were taken of fish swimming steadily at 10 degreesC and 20 degrees C, At 10 degreesC, we found that, although there were no differences in tai l-beat frequency, muscle strain or stimulation phase between acclimation gr oups, cold-acclimated scup had EMG duty cycles approximately 20 % shorter t han warm-acclimated scup, In contrast at 20 degreesC, there was no differen ce between acclimation groups in EMG duty cycle, nor in any other muscle le ngth change or stimulation parameter. Thus, in response to cold acclimation, there appears tp be a reduction in E MG duty cycle at low swimming temperatures that is probably due to an alter ation in the operation of the pattern generator. This novel acclimation pro bably improves muscle power output at low temperatures compared with that o f warm-acclimated fish, an expectation we test in the accompanying paper us ing the work-loop technique.