  The relationship between velocities, tractions, and intercellular stresses in
the migrating epithelial monolayer are currently unknown. Ten years ago, a
method known as Monolayer Stress Microscopy (MSM) was suggested from which the
intercellular stresses could be computed given a traction field. The core
assumption of MSM is that the intercellular stresses within the monolayer
behave similarly to passive systems like a Hookean solid (an elastic sheet) or
a Newtonian fluid (thin fluid film), implying a relation between the
displacements/velocities and tractions. Due to the lack of independently
measured intercellular stresses, validation of MSM is difficult. An alternative
approach, which we give here, is based on simultaneous measurements of the
monolayer velocity field and the cell/substrate tractions. With limited
assumptions, the velocity field suffices to compute tractions, which we can
then compare directly with those measured by traction force microscopy. We find
that the calculated tractions and measured tractions are uncorrelated. Since
both classical MSM and a purely viscous description of the relation between
displacements or velocities and tractions depends on a linear constitutive law,
it follows that some modification of these approaches is needed. One possible
resolution is the inclusion of an active force. To this end, we give a new
relationship between the active force density and the measured velocity(or
displacement) field, and tractions, which by Newton's laws, must be obeyed.
