Table 3.

Estimates of stiffness per myosin head (S^∗), force per myosin head (F^∗), and y₀ of individual myosin heads (y₀^∗) calculated from half-sarcomere stiffness (after subtraction of filament compliance) and isometric force generated by the soleus muscle fibers

	Mut R719W∗	Controls for Mut R719W∗	Mut R723G†(H27)	Controls for Mut R723G†
Stiffness per head in rigor (pN/nm)	0.45	0.17	0.84	0.29
Isometric stiffness per head at 10°C (pN/nm)	0.39	0.19	0.52	0.28
Isometric force per head at 10°C (pN)	1.77	0.99	1.99	1.49
Isometric force per head at 20°C (pN)	2.88	1.77	2.56	1.92
y0 per head at 10°C (nm)	4.54	5.14	3.83	5.32

Calculations are based on a filament compliance of 0.025 nm/kPa (Fig. 5). Force and stiffness per head during isometric contraction were calculated based on 111 × 10³ myosin heads/μm² in a half-sarcomere and a fraction of strongly bound myosin heads of 0.66 (52).

For the two controls (that were measured under slightly different conditions) the values for stiffness per head in rigor (0.17 ± 0.05 pN/nm, (n = 7) and 0.29 ± 0.09 pN/nm (n = 6)) and isometric contraction (0.19 ± 0.04 pN/nm, (n = 14) and 0.28 ± 0.07 pN/nm (n = 9)) were not significantly different (all values ± SE).

^∗Calculations based on a fraction of 54% of myosin heads with mutation R719W, 46% wild-type heads (46), and on the values for force and stiffness shown in Table 2 (see also 10). For mutation R719W, changes of cross-bridge turnover kinetics were also taken into account that contribute an increase in F_max and in active fiber stiffness of ∼15%.

^†Calculations based on a fraction of 62% heads with mutation R723G, 38% wild-type heads (14), and on values for force and stiffness shown in Table 1.