  We investigate elastic deformations of spin $S=1/2$ antiferromagnetic
$J_1-J_2$ Heisenberg chains, at $M=1/3$ magnetization, coupled to phonons in
the adiabatic approximation. Using a bosonization approach we predict the
existence of non-homogeneous trimerized magnetoelastic phases. A rich ground
state phase diagram is found, including classical and quantum plateau states
for the magnetic sector as well as inequivalent lattice deformations within
each magnetic phase. The analytical results are supported by exact
diagonalization of small clusters.
