The force-velocity curve in passive whole muscle is asymmetric about zero velocity

The force-velocity property of passive muscle was investigated to determine if a discontinuity of slope occurred at zero velocity. Isolated, unstimulated whole frog sartorius muscles were subjected to constant-velocity stretches and releases using a servo-controlled lever. The force due to damping (ΔT) was calculated by subtracting the tension measured at a very low speed (1.0 mm s−1) from the tension measured at the same length while the muscle was shortening or lengthening at a particular test speed. The experiments were performed over a range of speeds at each of several lengths and at two temperatures. For comparison, the same experiments were performed using a strip of pure latex rubber and a steel spring. Curves showing the magnitude of ΔT vs velocity were nearly symmetric about the zero-velocity axis for the steel spring and the rubber strip, but were markedly asymmetric for passive muscle, showing a positive ΔT for lengthening at all speeds that was between four and 11 times the negative ΔT for shortening at the same speed, depending on the temperature and initial stretch length. The force due to damping at a given speed increased with extension above the rest length in passive muscle but decreased with increasing length in experiments using the latex strip. Predictions obtained from a mathematical model based on a damping element in series with a lightly damped spring were fitted to the experimental measurements of ΔT vs velocity. The damping parameter provisionally representing interfilamentary sliding was between six and 12 times larger for lengthening than for shortening. We conclude that the force-velocity curve in passive whole muscle, as in activated muscle, is asymmetric about zero velocity. Furthermore, the slope of the force-velocity curve is discontinuous at zero velocity.