  The passive transient response of tetanized muscles is usually simulated
using the classical Huxley-Simmons (HS) model. It predicts negative effective
elastic stiffness in the state of isometric contractions (stall conditions),
which can potentially trigger spatial inhomogeneity at the scale of the whole
muscle fiber. Such instability has not been observed. Here we argue that the
passive stabilization of the homogeneous state in real muscles may be due to
the steric short-range interaction between individual myosin heads, which
competes with the long-range elastic interaction induced by semi-rigid myosin
backbones. We construct a phase diagram for the HS-type model accounting for
such competing interactions and show that the resulting mechanical response in
stall conditions is strongly influenced by a tricritical point. In addition to
the coherent pre- and post-power stroke configurations anticipated by the
original HS theory, the augmented model predicts the stability of
configurations with pre- and post-power stroke cross-bridges finely mixed. In
this new "phase," the overall stiffness of a half-sarcomere is positive, which
suggests that it may be adequately representing the physiological state of
isometric contractions.
