title: The force-velocity curve in passive whole muscle is asymmetric about zero velocity.
abstract: 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 (delta T) 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 delta T 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 delta T for lengthening at all speeds that was between four and 11 times the negative delta T 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 delta T vs velocity. The damping parameter provisionally representing interfilamentary sliding was between six and 12 times larger for lengthening than for shortening.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Division of Applied Sciences, Harvard University, Cambridge, MA 02138, USA.
authors.forename: P C
authors.initials: PC
authors.lastname: Krause
authors.forename: J S
authors.initials: JS
authors.lastname: Choi
authors.forename: T A
authors.initials: TA
authors.lastname: McMahon
mesh_terms: D000818:Animals; D001696:Biomechanical Phenomena; D008954:Models, Biological; D009132:Muscles / Q000502:physiology*; D011894:Rana pipiens