Statistical analysis

All values are shown as mean ± SEM. Constrained non-linear least squares fitting of Equations (1, 2) to moduli was performed using sequential quadratic programming methods in Matlab (v 7.9.0, The Mathworks, Natick MA). All statistical tests were performed using SPSS (IBM Statistics, Chicago, IL). A two-way ANOVA was used to assess effects of genotype and sarcomere length for parameter estimates from (i) the 3-parameter Hill fits to the tension-pCa relationships and (ii) the parameter estimates from fits to Equation (2) for the nucleotide handling rates. All other relationships were analyzed using linear mixed models with pCa, frequency, or MgATP as a repeated measure, followed by a least significant difference post-hoc comparison of the means between genotype or sarcomere length. Statistical significance is reported at p < 0.05.