Distribution of myosin attachment times predicted from viscoelastic mechanics of striated muscle.

We demonstrate that viscoelastic mechanics of striated muscle, measured as elastic and viscous moduli, emerge directly from the myosin crossbridge attachment time, t(att), also called time-on. The distribution of t(att) was modeled using a gamma distribution with shape parameter, p, and scale parameter, β. At 5 mM MgATP, β was similar between mouse α-MyHC (16.0 ± 3.7 ms) and β-MyHC (17.9 ± 2.0 ms), and p was higher (P < 0.05) for β-MyHC (5.6 ± 0.4 no units) compared to α-MyHC (3.2 ± 0.9). At 1 mM MgATP, p approached a value of 10 in both isoforms, but β rose only in the β-MyHC (34.8 ± 5.8 ms). The estimated mean t(att) (i.e., pβ product) was longer in the β-MyHC compared to α-MyHC, and became prolonged in both isoforms as MgATP was reduced as expected. The application of our viscoelastic model to these isoforms and varying MgATP conditions suggest that t(att) is better modeled as a gamma distribution due to its representing multiple temporal events occurring within t(att) compared to a single exponential distribution which assumes only one temporal event within t(att).