  We discuss a kinetically constrained model in which real-valued local
densities fluctuate in time, as introduced recently by Bertin, Bouchaud and
Lequeux. We show how the phenomenology of this model can be reproduced by an
effective theory of mobility excitations propagating in a disordered
environment. Both excitations and probe particles have subdiffusive motion,
characterised by different exponents and operating on different time scales. We
derive these exponents, showing that they depend continuously on one of the
parameters of the model.
