(A) Microtubules form contractile networks in channels with various widths (Scale bar, 500 m, t=10 min). (B) Width of the networks as a function of time in channels with various widths. (C) Fraction contracted as a function of time, (t), calculated from the data in B. The networks all contract to a similar final fraction, while the timescale of contraction differs. (D) The scaling of the characteristic time, , with channel width does not vary as , as would result for a poroelastic timescale, and is not a constant, independent of width, as would result from a viscoelastic timescale. The scaling is well described by an active fluid model (green line analytic scaling, fit to (Equation 6); green dots numerical solution). (E) The characteristic time, , is found to be independent of channel height. The dashed line is the mean value of . (F) is constant for all channel widths and heights, indicating that the network contracts to a constant final density. The dashed line is the mean value of . All panels display mean s.e.m.