  Passive mechanical response of skeletal muscles at fast time scales is
dominated by long range interactions inducing cooperative behavior without
breaking the detailed balance. This leads to such unusual "material properties"
as negative equilibrium stiffness and different behavior in force and
displacement controlled loading conditions. Our fitting of experimental data
suggests that "muscle material" is finely tuned to perform close to a critical
point which explains large fluctuations observed in muscles close to the stall
force.
