  The dynamics of structural relaxation in a model polymer glass subject to
spatially-homogeneous, time-periodic shear deformation is investigated using
molecular dynamics simulations. We study a coarse-grained bead-spring model of
short polymer chains below the glass transition temperature. It is found that
at small strain amplitudes, the segmental dynamics is nearly reversible over
about $10^4$ cycles, while at strain amplitudes above a few percent, polymer
chains become fully relaxed after a hundred cycles. At the critical strain
amplitude, the transition from slow to fast relaxation dynamics is associated
with the largest number of dynamically correlated monomers as indicated by the
peak value of the dynamical susceptibility. The analysis of individual monomer
trajectories showed that mobile monomers tend to assist their neighbors to
become mobile and aggregate into relatively compact transient clusters.
