  Simplicial approaches to quantum gravity such as quantum Regge calculus and
spin foams include configurations where bulk edges can become arbitrarily large
while the boundary edges are kept small. Spikes and spines are prime examples
for such configurations. They pose a significant challenge for a desired
continuum limit, for which the average lengths of edges ought to become very
small. Here we investigate spike and spine configurations in four-dimensional
Lorentzian quantum Regge calculus. We find that the expectation values of
arbitrary powers of the bulk length are finite. To that end, we explore new
types of asymptotic regimes for the Regge amplitudes, in which some of the
edges are much larger than the remaining ones. The amplitudes simplify
considerably in such asymptotic regimes and the geometric interpretation of the
resulting expressions involves a dimensional reduction, which might have
applications to holography.
