  We consider the two-chain Heisenberg ladder with antiferromagnetic
interactions. Our approach is based on the description of spin excitations as
triplets above a strong-coupling singlet ground state. It is shown that
interaction with dynamic phonons drastically changes the dispersion of the
elementary triplet excitation (magnon) and the spin structure factor. It is
also shown that a new triplet excitation appears in the spectrum: the bound
state of a magnon and a phonon.
