  Large-scale force generation is essential for biological functions such as
cell motility, embryonic development, and muscle contraction. In these
processes, forces generated at the molecular level by motor proteins are
transmitted by disordered fiber networks, resulting in large-scale active
stresses. While these fiber networks are well characterized macroscopically,
this stress generation by microscopic active units is not well understood. Here
we theoretically study force transmission in these networks, and find that
local active forces are rectified towards isotropic contraction and strongly
amplified as fibers collectively buckle in the vicinity of the active units.
This stress amplification is reinforced by the networks' disordered nature, but
saturates for high densities of active units. Our predictions are
quantitatively consistent with experiments on reconstituted tissues and
actomyosin networks, and shed light on the role of the network microstructure
in shaping active stresses in cells and tissue.
