Power output from a flight muscle of the bumblebee Bombus terrestris .1. Some features of the dorso-ventral flight muscle

1. Isometric contractions from the asynchronous dorso-ventral flight muscle of the bumblebee Bombus terrestris were slow and rather weak. The twitch duration (onset to 50% relaxation) was approximately 300 ms at 30 degrees C and 170 ms at 40 degrees C. The maximum tetanic tension was approximately 40 kN m(-2); the ratio of twitch force to tetanic force was approximately 0.2. 2. The unstimulated muscle was quite resistant to stretch, with a low-frequency stiffness of 730 kN m(-2) at muscle lengths close to that of the muscle in vivo. The length-tension curve for active tetanic tension (that is the increase in tension above the passive level during stimulation) was very narrow, with a half-width equal to only 17% of the optimal length. 3. The muscle strain during tethered flight was approximately 2% peak-to-peak, occasionally reaching 3%. Strain amplitude increased with wing stroke frequency. The thoracic vibration frequency of escape buzzing, during which the wings are not extended but are folded over the abdomen, was approximately twice that of tethered flight but the muscle strain was similar to that of flight.