The force-velocity properties of a crustacean muscle during lengthening

Muscle force during active lengthening was characterized for scaphognathite levator muscle L2B from the crab Carcinus maenas. The muscle was tetanically stimulated and, during the peak of the contraction, stretched at constant velocity. The total strain was approximately 4 %, the strain rates ranged from 0.03 to 1.6 muscle lengths s-1 (L s-1), and the temperature was 15 degreesC. Force increased throughout stretch. During low-velocity stretch, up to approximately 0.3 L s-1, force rose during isovelocity stretch along an approximately exponential trajectory. The asymptotic force approached during the stretch increased and the time constant of the response decreased with increasing strain rate. With stretch at 0.6 L s-1 and greater, the force increased to a distinct yield point, reached after a strain of approximately 1 %, after which force continued to increase but with a slope approximately one-quarter as great as that before yield. Because force changes continuously during constant-velocity lengthening, the adequate descriptor for the force-velocity relationship in a lengthening crab muscle is not a two-dimensional force-velocity curve, but rather a three-dimensional force-velocity-time or force-velocity-strain surface. Stimulating muscle L2B at 20 Hz or 50 Hz gives a smoothly fused tetanic contraction in which muscle activation is only partial and the plateau force reached is less than that at the optimum stimulus frequency of approximately 100 Hz. The force-velocity relationships of a partially activated muscle are not simply those of a fully activated one scaled down in proportion to the reduction in the maximum isometric force. At low stretch velocities, the asymptotic force approached is larger in proportion to the pre-stretch isometric tension, and the time constant of the force increase is greater, in partially activated than in fully activated muscles. At high stretch velocities, the force at yield relative to the pre-stretch force, and the relative values of the slopes of the force increase before and after yield, are all greater in partially activated than in fully activated muscles, while the strain at yield is smaller.