The high-frequency wing beat of higher-order insects is driven by self-sustained oscillations of constantly activated flight muscles. However, whether its underlying mechanism is based on flight muscle-specific features or on preexisting contractile functions is unknown. Here, we recorded X-ray diffraction movies, at a rate of 5000 frames/second, simultaneously from the two antagonistic flight muscles of bumblebees during wing beat. Signals that occurred at the right timing for triggering each wing-beat stroke were resolved in both muscles. The signals likely reflect stretch-induced myosin deformation, which would also enhance force in vertebrate muscles. The results suggest that insects use a refined preexisting force-enhancing mechanism for high-frequency wing beat, rather than developing a novel mechanism.