  The dynamics of many glassy systems are known to exhibit string-like hopping
motions each consisting of a line of particles displacing one and other. By
using molecular dynamics simulations of glassy polymers, we show that these
motions become highly repetitive back-and-forth motions as temperature
decreases and do not necessarily contribute to net displacements. Particle hops
which constitute string-like motions are reversed with a high probability,
reaching 73% and beyond at low temperature. Structural relaxation rate is then
dictated not by a simple particle hopping rate but instead by the rate at which
particles break away from hopping repetitions. We propose that disruption of
string repetitions and hence also structural relaxations are brought about by
pair-interactions between strings.
