Mechanical interaction between individual cells and the Extracellular Matrix (ECM) critically regulates cell behaviors including growth, differentiation, and migration. Cell-ECM mechanical interaction has historically been quantified in a simplified context by measuring traction forces of cells cultured on two dimensional linear elastic substrates. However, in vivo, most cells are surrounded by the ECM in a fully three dimensional context, and many tissues are nonlinear owing to their fibrous collagenous microstructure. One such complex ECM environment is that of a tumor and the stromal connective tissue surrounding it. Tumors stiffen in a complex way during progression as cells deposit more collagen than they digest, increasingly express crosslinking enzymes, and exert traction forces to reorganize the ECM. We seek to understand how these complex mechanical changes to the tumor microenvironment guide tumor cell behavior. We first polymerize natively derived collagen I into matrices with varied concentration and crosslinking that mimic the varied ECM of the tumor microenvironment. The resulting matrices have a pore diameters of 1.8-5.1um, tensile small strain stiffness of 12-2100Pa, and stiffen 1.4-10 fold at 15% strain. They span from softer than the ~170Pa stiffness of healthy breast tissue through the ~900 Pa stiffness of the tumor stroma up to nearly the ~4000 Pa stiffness of a breast tumor. After developing a system for tracking single cell generated 3D matrix displacements at high spatial resolution, we apply it to malignant breast cancer cells migrating through each of the collagen matrices. We find that cells exert nearly constant strains across the physiological stiffness range and therefore generate more force in stiffer matrices. Our results also demonstrate the importance of strain-stiffening properties on the collagen matrices in cell-ECM interactions. Cells exert sufficient strains to locally stiffen all matrices studied and local stiffening increases with matrix pore size. Matrix stiffening is accompanied by fiber alignment and greatly extended range of mechanical signal propagation in space. This work highlights the importance of strain-stiffening properties of collagen matrices in cellECM interaction, and in particular, the mechanical signal transmission range. As many biological gels exhibit strain-stiffening behavior, this work can be readily extended to other fields including stem cell engineering where cell-ECM interactions are important.