  Cytoskeletal networks are foundational examples of active matter and central
to self-organized structures in the cell. In vivo, these networks are active
and heavily crosslinked. Relating their large-scale dynamics to properties of
their constituents remains an unsolved problem. Here we study an in vitro
system made from microtubules and XCTK2 kinesin motors, which forms an aligned
and active gel. Using photobleaching we demonstrate that the gel's aligned
microtubules, driven by motors, continually slide past each other at a speed
independent of the local polarity. This phenomenon is also observed, and
remains unexplained, in spindles. We derive a general framework for coarse
graining microtubule gels crosslinked by molecular motors from microscopic
considerations. Using the microtubule-microtubule coupling, and force-velocity
relationship for kinesin, this theory naturally explains the experimental
results: motors generate an active strain-rate in regions of changing polarity,
which allows microtubules of opposite polarities to slide past each other
without stressing the material.
