Table 1.

Estimates of XB cycling kinetics

	Fibres from normal mice (α-MHC)		Fibres from PTU-treated mice (β-MHC)
	α-MHC(cTnI)	α-MHC(ssTnI)	β-MHC(cTnI)	β-MHC(ssTnI)
ktr	17.0 ± 0.67	17.0 ± 0.41	8.04 ± 0.17a	8.87 ± 0.24b,c
gapp	5.02 ± 0.54	5.12 ± 0.35	1.73 ± 0.12a	2.06 ± 0.08b,c
fapp	11.9 ± 0.75	11.9 ± 0.59	6.32 ± 0.23a	6.72 ± 0.26c
Tension cost	7.58 ± 0.32	7.16 ± 0.19	2.99 ± 0.12a	3.54 ± 0.10b,c
c	44.7 ± 2.98	43.1 ± 2.40	19.8 ± 0.77a	22.0 ± 1.18c

The rate constant of tension redevelopment, k_tr (s⁻¹), was determined based on a rapid slack/restretch protocol, and is reported here at maximal Ca²⁺ activation. The intercept of the exponential trend of k_tr vs. activation level (Fig. 6) was used to determine g_app (s⁻¹). f_app (s⁻¹) was determined by taking the difference of k_tr and g_app, since at max activation k_tr≈f_app+g_app (Brenner & Eisenberg, 1986). Tension cost (pmol mN⁻¹ mm⁻¹ s⁻¹) is reported as the slope of the regression from the relationship between tension and ATPase activity. c (s⁻¹) is the model-predicted rate constant of XB detachment, and correlates with tension cost. Values are means ± SEM. Number of determinants is at least 10 for each group. ^aP < 0.05 vs. α-MHC(cTnI); ^bP < 0.05 vs. β-MHC(cTnI); ^cP < 0.05 vs. α-MHC(ssTnI).