POWER OUTPUT FROM A FLIGHT-MUSCLE OF THE BUMBLEBEE BOMBUS-TERRESTRIS .2. CHARACTERIZATION OF THE PARAMETERS AFFECTING POWER OUTPUT

1. Length-tension relationships and work output were investigated in t he intact, dorso-ventral flight muscle of the bumblebee Bombus terrest ris. The muscle is an asynchronous muscle. Like other asynchronous fli ght muscles, it has high resting stiffness and produces relatively low active force in response to tetanic stimulation. 2. The muscle shows shortening deactivation and stretch activation, properties that result in delayed force changes in response to step changes in length, a pha se lag between force and length during imposed sinusoidal strain and, under appropriate conditions, positive work output during oscillatory length change. 3. Work loops were used to quantify work output by the muscle during imposed sinusoidal oscillation. The curves relating net work per cycle with muscle length, oscillatory strain and oscillatory frequency were all roughly bell-shaped. The work-length curve was narr ow. The optimum strain for net work per cycle was approximately 3 %, w hich is probably somewhat greater than the strain experienced by the m uscle in an intact, flying bumblebee. The optimum frequency for net wo rk output per cycle was 63 Hz (30 degrees C). The optimum frequency fo r power output was 73 Hz, which agrees well with the normal wing strok e frequency if allowance is made for the elevated temperature (approxi mately 40 degrees C) in the thorax of a flying bumblebee. The optimal strain for work output was not strongly dependent on oscillation frequ ency. 4. Resilience (that is the work output during shortening/work in put during lengthening) for unstimulated muscle and dynamic stiffness (=Delta stress/Delta strain) for both stimulated and unstimulated musc les were determined using the strain (3 %) and oscillation frequency ( 64 Hz) which maximize work output in stimulated muscles. Unstimulated muscle is a good energy storage device. Its resilience increased with increasing muscle length (and increasing resting force) to reach value s of over 90 %. The dynamic stiffness of both stimulated and unstimula ted muscles increased with muscle length, but the increase was relativ ely grater in unstimulated muscle, and at long muscle lengths the stif fness of unstimulated muscle exceeded that of stimulated muscle. Effec tively, dynamic stiffness is reduced by stimulation! This is taken as indicting that part of the stiffness in an unstimulated muscle reflect s structures, possibly attached cross bridges, whose properties change upon stimulation.