Is rat soleus muscle recruited during swimming?

Radiolabeled microspheres were used to measure blood flows (BF) among and within the ankle plantar flexor muscles of rats during swimming and during treadmill running. During swimming, BF was significantly elevated over preexercise quadrupedal standing in plantaris and red and middle portions of gastrocnemius muscles, but was significantly lower in soleus muscles. During treadmill locomotion at 60 m x min(-1), BF in soleus muscle was the same as during preexercise, while BF to plantaris and red and middle gastrocnemius muscles was significantly increased. These data suggested soleus muscle was not active during swimming, which contrasts with terrestrial activities. Measurement of glycogen use in the muscles supported this interpretation. Since hindpaw stroke frequency during swimming is similar to stride frequency during treadmill running at 60 m x min(-1), contraction cycling times do not provide an adequate explanation for the apparent failure to recruit soleus muscle during swimming. We suggest the swimming rat may provide an interesting model for the study of differential slow and fast muscle recruitment patterns.