Tension is built during contraction of tethered bundles. (

*a*) Schematic illustrating how the displacement of a bead bound to the top surface of an elastic hydrogel can be used to determine the forces applied on the bead. A force,

*F*, applied to the surface-bound bead is balanced by the elastic restoring force exerted by the underlying gel, resulting in a bead displacement Δ

*x* such that

*F* =

*k*_{eff} Δ

*x*, where

*k*_{eff} is an effective spring constant determined by the gel elastic properties. Further details of this measurement are discussed in . (

*b*) Images of OG-myosin (inverted contrast) in a bundle with

= 1.4. The underlying gel has a shear elastic modulus of 54 Pa. Time = 0 s delineates addition of 1 mM ATP. Scale bar is 5

*μ*m; see . (

*c*) Tensile force (

*left axis, red squares*) and contour length (

*right axis, open triangles*) as a function of time for the bundle shown in (

*b*). (

*d*) Contraction speed versus tensile force for data shown in (

*b*). The dashed line approximates the force-velocity relationship observed at long times and high loads. Arrows indicate times = 0 s, 5 s, and 90 s. (

*e*) Histogram of maximum tensile force, or stall force, of contractile bundles with

= 1.4 and contracted with buffer containing 1 mM ATP. (

*f*) Stall forces calculated in (

*e*) plotted as a function of initial bundle contour length.

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