Micro-particle image velocimetry (micro-PIV) is a powerful imaging method for resolving flow fields in microscopic systems, but its application to rapidly deforming gels requires key modifications. Here, we adapt micro-PIV to quantify contraction dynamics of active actomyosin gels without invasive tracer beads by tracking local myosin-generated actin inhomogeneities. We show that accurate displacement measurements depend critically on optimizing the time interval over which displacements are computed and the number of frames used in ensemble correlation, relative to the poroelastic timescale and stage of contraction. To assess reliability, we quantify four complementary metrics: displacement vector fields, displacement-magnitude maps, radial profiles of radial and tangential displacement components, and ensemble-averaged orientation with rotational measures. Incorporating the elastic response of the gel, we extract radial strain profiles under an axisymmetric approximation and demonstrate robustness for irregular geometries and off-center contractions. Across conditions, we observe common radial signatures consisting of inward radial contraction with peak displacement at intermediate radii and effective radial stretching near the boundary. These deformation fields provide a basis for inferring spatial and orientational distributions of motor-generated active stresses using appropriate constitutive models. Our approach advances quantitative analysis of active poroelastic materials and has broad applications in biomaterials design, cytoskeletal dynamics, and morphogenesis.

The collective action of actively contractile units embedded in elastic biopolymer networks plays a crucial role in regulating the network's macroscopic mechanical response. Here, we investigate how the macroscopic boundary stress in model elastic fiber networks depends on the number and nature of embedded contractile units, each exerting an isotropic force dipole, as well as on the bending stiffness of fibers. We find that the macroscopic stress increases nonlinearly with the number of dipoles due to mutual stiffening of initially soft, bending-dominated networks. Using effective medium theory, we relate this enhanced contractility to an increase in the effective average network coordination number due to constraints imposed by the force dipoles. By comparing three distinct force dipole models that differ in their local structures, we demonstrate that the specific manner in which an active unit constrains the network strongly influences the onset and nature of the stiffening transition. Our results highlight that not only the quantity but also the local geometry of force-generating units critically determines the macroscopic mechanical behavior. This framework provides a physical basis for understanding how biological systems-such as molecular motors in the cytoskeleton, or adherent cells in the extracellular matrix-can modulate network-scale nonlinear elastic properties through local tuning of active force-generating units.