In this paper we consider the extension of the classical Hamilton - Cartan variation theory to fibrous varieties. It is known that in field theory, we can assign various Hamiltonian equations corresponding to the Lepage equivalents of Lagrangian to the variational problem represented by Lagrangian. The case of affine Lagrangians in the second derivatives is studied using differential geometry tools. A new condition of regularity and strong regularity and Legendre's transformation are found.