Calculation of the t-force from the principal bend image of a bull sperm at the switch-point of the beat. The method we used to analyze the magnitude of the t-force, with and without ADP, is illustrated for one of the cells included in the data set. The sperm cell is reactivated with 0.1 mM ATP and is stuck to the slide by its head with the flagellum beating freely. The selected frame is the one that exhibits the greatest curvature in the region 5–10 *μ*m from the head-tail junction (i.e., the next captured image showed less curvature and a switch in direction of beating in that same segment). The curvature is measured and the radius of curvature, *r*, is recorded. The radius of curvature, stiffness, and effective diameter are used in the formula shown to find the tension on the outermost axonemal component that is inducing the bend (see text for details). The tension on the axoneme multiplied by the curvature of the bend yields the global component of the t-force acting across the axoneme. The actual calculation for the specific cell shown is given as an example. This cell was treated with 4 mM ADP and yielded a t-force of 2.5 nN/*μ*m.

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