FORCE RESPONSES TO FAST RAMP STRETCHES IN STIMULATED FROG SKELETAL-MUSCLE FIBERS

Force responses to fast ramp stretches at various velocities were reco rded from single muscle fibres isolated from either lumbricalis digiti TV or tibialis anterior muscle of the frog (Rana esculenta) at sarcom ere length between 2.15 and 3.25 mu m at 15 degrees C. Stretches were applied at rest, at tetanus plateau and during the tetanus rise. Stret ches with the same velocity but different accelerations were imposed t o the fibre to evaluate the effect of fibre inertia on the force respo nses. Length changes were measured at sarcomere level with either a la ser diffractometer or a striation follower apparatus. The force respon se to a fast ramp stretch could be divided into two phases. The initia l fast one (phase 1) lasts for the acceleration period during which th e stretching velocity rises up to the steady state. The second slower phase (phase 2) lasts for the remainder of the stretch and corresponds to the well-known elastic response of the fibre. Most of this paper i s concerned with phase 1. The amplitude of the initial fast phase was proportional to the stretching velocity as expected from a viscous res ponse. This viscosity was associated with a very short (about 10 mu s) relaxation time. The amplitude of the fast phase increased progressiv ely with tension during the tetanus rise and scaled down with sarcomer e length approximately in the same way as tetanic tension and fibre st iffness. These data suggest that activated fibres have a significant i nternal viscosity which may arise from crossbridge interaction.