Characteristics of tension recruitment and mechanical activation in mammalian skeletal muscle

Characteristics of summation of isometric mechanical responses were studied in in situ flexor digitorum longus (FDL) (fast-twitch) and soleus (slow-twitch) muscles of the cat at 37°C. The pattern of tension recruitment in short tetanic contractions was examined at different stimulation frequencies. At frequencies leading to the development of maximum tetanic tension, the second stimulus activation made the largest individual contribution to tension development. The tension developed with two stimuli was dependent on the interval separating them. When the first two stimuli were fixed at a short interval such that the muscle developed the highest two-response tension, the tension developed following application of the third stimulus was independent over a wider range of two-to-three stimulus intervals. Application of a stimulus shortly after another, in general, lengthened the contraction. The results from the two muscles were basically similar when allowance was made for the difference in their speed of contraction. The marked tension potentiation seen in the twitch of FDL, with staircase effect and with cooling, was largely absent in the well-fused double response. We suggest that the mechanical activation occurring during the isometric twitch is incomplete in both muscle fiber types and that a second stimulus applied shortly after the first brings about nearly complete mechanical activation.