  By adopting a local QFT framework one can derive in a non-perturbative manner
the constraints imposed by Poincar\'e symmetry on the form factors appearing in
the Lorentz covariant decomposition of the energy-momentum tensor matrix
elements. In particular, this approach enables one to prove that these
constraints are in fact independent of the internal properties of the states
appearing in the matrix elements. Here we outline the rationale behind this
approach, and report on some of the implications of these findings.
