  The spin-lattice coupling plays an important role in strongly frustrated
magnets. In ZnCr$_2$O$_4$, an excellent realization of the Heisenberg
antiferromagnet on the ``pyrochlore'' network, a lattice distortion relieves
the geometrical frustration through a spin-Peierls-like phase transition at
$T_c = 12.5$ K. Conversely, spin correlations strongly influence the elastic
properties of a frustrated magnet. By using infrared spectroscopy and published
data on magnetic specific heat, we demonstrate that the frequency of an optical
phonon triplet in ZnCr$_2$O$_4$ tracks the nearest-neighbor spin correlations
above $T_c$. The splitting of the phonon triplet below $T_c$ provides a way to
measure of the spin-Peierls order parameter.
