  Crossbridge binding and force in active muscle is dependent on the radial
spacing between the thick and the thin filaments. This radial lattice spacing
has been shown through spatially explicit modeling and experimental efforts to
greatly affect isometric, force production in muscle. It has recently been
suggested that this radial spacing might also be able to drive differences in
mechanical function, or net work, under dynamic oscillations like those which
occur in muscles in vivo. However, previous spatially explicit models either
had no radial spacing dependence, meaning the lattice spacing could not be
investigated, or did include radial spacing dependence but could not reproduce
in vivo net work during dynamic oscillations and only investigated isometric
contractions. Here we show the first spatially explicit model to include radial
crossbridge dependence which can produce mechanical function similar to real
muscle. Using this spatially explicit model of a half sarcomere, we show that
when oscillated at strain amplitudes and frequencies like those in the hawk
moth Manduca sexta, net work does depend on the lattice spacing. In addition,
we can prescribe a trajectory of lattice spacing changes in the spatially
explicit half sarcomere model and investigate the extent to which the time
course of lattice spacing changes can affect mechanical function. We simulated
a half sarcomere undergoing dynamic oscillations and prescribed the Poisson's
ratio of the lattice to be either 0 (constant lattice spacing) or 0.5
(isovolumetric lattice spacing changes). We also simulated net work using
lattice spacing data taken from Manduca sexta which has a variable Poisson's
ratio. Our simulation indicates that the lattice spacing can change the
mechanical function of muscle, and that in some cases a 1 nm difference can
switch the net work of the half sarcomere model from motor like to brake like.
