Comparative Biomechanics of Thick Filaments and Thin Filaments with Functional Consequences for Muscle Contraction

Table 1

Mechanical properties of thick filaments and thin filaments.

Filament	Measurement	Method	Muscle Source	Stiffness (pN/nm)	Young’s Modulus (GPa)	Persistence Length (𝜇m)
Thick^(a)	Longitudinal stiffness	X-ray diffraction	Frog skeletal	252	1.9 [30]	642
Thick^(a)	Flexural rigidity	AFM	Mouse cardiac	165	0.8	639 [8]
Thick^(a)	Flexural rigidity	AFM	Fruit fly IFM	442	3.0	1742 [9]
Actin^(b)	Flexural rigidity	thermal fluctuations	Rabbit skeletal	19	0.8	9 [10]
Actin^∗(b)	Flexural rigidity	thermal fluctuations	Rabbit skeletal	37	1.5	18 [10, 12]
Actin^∗(b)	Longitudinal stiffness	microneedle	Rabbit skeletal	44 [13]	1.8	21
Actin^∗ + Tm^(c)	Longitudinal stiffness	microneedle	Rabbit skeletal	65 [13]	1.3	63
Actin + Tm + Cad^(c)	Flexural rigidity	thermal fluctuations	Rabbit skeletal + turkey gizzard	21	0.4	20 [10]
Thin^(c)	Fiber longitudinal stiffness	X-ray diffraction	Frog skeletal	125	2.5 [30]	121
Thin^(c)	Fiber longitudinal stiffness	mechanics	Rabbit skeletal	46–68 [32]	0.9–1.4	44–66

Measured values are listed in bold, along with the referenced study. Stiffness values represent the spring constant for a filament length (L) of 1000 𝜇m. To compare between measurements, the calculations for stiffness (k), Young’s modulus (E), and persistence length (L_p) required an estimate of filament geometry listed in the footnotes. Otherwise, E = kL/A_c, where A_c is cross-sectional area [13] and L_p = EI/k_BT, where I is the second moment of inertia, k_B is the Boltzman constant, and temperature (T) was 300 K [129].
^(a)Calculations assumed a solid, cylindrical filament backbone of radius 6.5 nm for skeletal thick filaments [30] and 8 nm [130] for cardiac thick filaments. For insect flight muscle filaments, calculations assumed a hollow cylinder with an outer radius of 7.5 nm and an inner radius of 3 nm [131].
^(b)Calculations for undecorated actin filaments assumed a solid, cylindrical filament backbone of radius 2.8 nm [13]. *In the presence of phalloidin.
^(c)Calculations for thin filaments and decorated actin filaments assumed a solid, cylindrical filament backbone of radius 4 nm [30].
Abbreviations: Tm: Tropomyosin; Cad: Caldesmon; Thin: Native thin filament; Thick: Native thick filament; AFM: Atomic force microscope; IFM: Indirect flight muscles.