Video microscopy demonstrated the uncoupled power and recovery strokes of HSP40⁻ twitching flagella. (A) Images recorded at 500 Hz revealed the asymmetrical waveforms of power (p) and recovery strokes (r) in each beat cycle of a forward-swimming wild-type cell. Note that the power stroke bends near the flagellar base before the recovery stroke is completed, rendering a curved flagellar tip (first and last panels). (B) HSP40⁻ jiggling cells were taken at a frame rate of ∼12 Hz. The four cells that were not stuck to the slide (left) were further analyzed. (1) Two flagella crossing each other at the anterior end of the cell body after sequentially undergoing a sluggish full recovery stroke without the subsequent power stroke. The next powerful stroke without subsequent recovery stroke sent one flagellum underneath the cell body (lines point to the flagellar tips). (2) One flagellum stalled at the beginning of an active stroke. Upon completion, both flagella recovered to the normal position. (3) Both flagella beat alternately first but then stalled at the end of recovery stroke, rendering crossed flagella as well. (4) A traced diagram showed that one flagellum, after stalling, bent near the base but stalled again in the middle of the power stroke. After wavering, the flagellum underwent recovery stroke followed by a swift power stroke and stalled. (C) Recording at 500 Hz showed that swift power strokes also bend near the base but ∼50% slower than wild type. The recording was stamped in seconds.