The change of tension with repeated activations comparing Neb⁺ and Neb⁻ fibers at 25°C. n = 10 for each curve. (A) In the absence of Pi. (B) In the presence of 8 mM Pi (standard activation). The data were individually normalized to the isometric tension of the first activation, then averaged, and plotted against the number of activations, each lasting ∼1 min.