Uniaxial elongation and compression data of linear and branched polymer melts as well as of crosslinked rubbers are analyzed with respect to their nonlinear strain measure. This is found to be the result of two contributions: (1) affine orientation of network strands, and (2) isotropic strand extension. Network strand extension is caused by an increasing restriction of lateral movement of polymer chains due to deformation, and is modeled by a molecular stress functionf, which in the tube concept of Doi and Edwards is the inverse of the relative tube diameter. Up to moderate strains,f2is found to be linear in the average stretch for melts as well as for rubbers, which corresponds to a constant volume of the tube. At large strains, melts show maximum molecular tension depending on the degree of long‐chain branching, while rubbers show maximum extensibility. The influence of crosslink density on the strain measure is considered.