Cardiac-like behavior of an insect flight muscle.

The synchronous wing depressor muscles of the hawkmoth Manduca sexta undergo large amplitude motions at lengths that lie entirely on the ascending region of their twitch length-tension curve. Moreover, these muscles bear a striking functional resemblance to mammalian cardiac muscle in both the shape of their length-tension curve and in their working length range. Although operation on the ascending region of the twitch length-tension curve sacrifices maximal force, it does permit the generation of larger forces at greater strains. In the case of cardiac muscle, this mechanical behavior is critical for accommodating the increasing stresses associated with greater ventricular filling. Similar characteristics in moth flight muscle suggest an analogous regulatory mechanism for skeletal muscles performing repetitive oscillatory work; the strong length dependence of force over their working length range should give the wing depressors the capacity to generate larger forces as wing stroke amplitude increases. These results support the notion that the length-tension relationship of muscle can be tuned to function in locomotor muscles.