Tension recovery in permeabilized rat soleus muscle fibers after rapid shortening and restretch

Permeabilized rat soleus muscle fibers were subjected to rapid shortening/restretch protocols (20% muscle length, 20 ms duration) in solutions with pCa values ranging from 6.5 to 4.5. Force redeveloped after each restretch but temporarily exceeded the steady-state isometric tension reaching a maximum value ∼2.5 s after relengthening. The relative size of the overshoot was <5% in pCa 6.5 and pCa 4.5 solutions but equaled 17% ± 4% at pCa 6.0 (approximately half-maximal Ca²⁺ activation). Muscle stiffness was estimated during pCa 6.0 activations by imposing length steps at different time intervals after repeated shortening/restretch perturbations. Relative stiffness and relative tension were correlated (p < 0.001) during recovery, suggesting that tension overshoots reflect a temporary increase in the number of attached cross-bridges. Rates of tension recovery (k_tr) correlated (p < 0.001) with the relative residual force prevailing immediately after restretch. Force also recovered to the isometric value more quickly at 5.7 ≤ pCa ≤ 5.9 than at pCa 4.5 (ANOVA, p < 0.05). These results show that k_tr measurements underestimate the rate of isometric force development during submaximal Ca²⁺ activations and suggest that the rate of tension recovery is limited primarily by the availability of actin binding sites.