The Tension in a Locust Flight Muscle at Varied Muscle Lengths

1. Tension in a resting muscle increases when the muscle is stretched. If the stretch is maintained, the tension decays (stress relaxation). The time course of stress relaxation in the metathoracic second tergocoxal muscle (Tcx2) of the locust Schistocerca americana was found to be adequately described by a multiexponential function with four or more time constants. These constants were independent of strain, and the slowest had a value of more than 60min. Tension continued to diminish even when stretch was maintained for 2—4h. 2. Tension in a stretched, unstimulated muscle increased with increased length of stretch. Resting tension in the locust Tcx2 at the in vivo length was estimated to be INcm" 2 or less. At 10% strain, resting tension was about 2Ncm~. The stiffness of the unstimulated locust muscle was similar to that of unstimulated frog muscles. 3. The active tetanic tension was maximal (average = 32-4 Ncm~) at slightly less than the in vivo muscle length. Tetanic tension was 50% or more of its maximum value over a range of 80-130% of the in vivo muscle length. 4. The active twitch tension was maximal at slightly greater than the in vivo muscle length. The ratio twitch/tetanic force increased with muscle length. 5. Twitch relaxation time increased with muscle length, but the time to peak for twitch force was nearly independent of muscle length in a stretched muscle.