FIGURE 3.

Representative force records. Records are plotted using linear (top) and logarithmic (bottom) time axes to show the initial phases of force development on an enlarged timescale. (A) Experimental records for a soleus fiber subjected to a rapid shortening/restretch protocol in each of 10 pCa solutions. The fiber was shortened by 20% of its initial length at 0.10 s and restretched to its initial length (step time ∼0.6 ms) at 0.12 s. Records are replotted from Fig. 3 of Campbell (3). (B) Simulated records showing the recovery time course for the 10 levels of Ca²⁺ activation plotted in A and an additional pCa 6.4 activation, as computed using the parameters listed in Table 1. Each record is the average of 1000 individual trials and is normalized to the steady-state force (8.89 ± 0.01 φ₀) in the pCa 4.5 simulation. Note that the simulated records are plotted so that they start at 0.12 s (the same time that the muscle was restretched in the experimental records) and start from zero force rather than from the residual level measured experimentally. (C) Same as for B, but calculated using values of κ_a and κ_m 10⁶ times greater than those listed in Table 1. These calculations describe the model's behavior with effectively rigid thick and thin filaments. The steady-state pCa 4.5 force under these conditions was 8.20 ± 0.01 φ₀ (0.92 ± 0.01 of the corresponding value in the simulations modeling compliant filaments).