  The physics underlying the complex glassy phenomena, which accompany the
formation of polarons in optimally doped manganites (ODM) is a cumbersome issue
with many unexplained aspects. In this article we present $^{139}$La and
$^{55}$Mn NMR in the temperature range 80K - 900K of ODM
La$_{0.67}$Ca$_{0.33}$MnO$_3$. We show that local lattice distortions,
established in the Paramagnetic (PM) phase for $T<700$K, induce a genuine
spin-glass state, which for $T<T_c$ consolidates with the Ferromagnetic (FM)
state into a single thermodynamic phase. Comparative NMR experiments on
La$_{0.77}$Ca$_{0.23}$MnO$_3$, La$_{0.59}$Ca$_{0.41}$MnO$_3$, and
La$_{0.70}$Sr$_{0.30}$MnO$_3$ demonstrate the dominant role of lattice
distortions, which appear to control (i) the stability of the spin glass phase
component and (ii) the kind (1st or 2nd order) of the PM-FM phase transition.
The experimental results are in agreement with the predictions of the
compressible random bond - random field Ising model, where consideration of a
strain field induced by lattice distortions, is shown to invoke at $T_c$ a
discontinuous (1st order like) change of both the FM and the "glassy"
Edwards-Anderson (EA) order parameters.
