  We demonstrate that the shower-to-shower fluctuations of the muon content of
extensive air showers correlate with the fluctuations of a variable of the
first interaction of Ultra High Energy Cosmic Rays, which is computed from the
fraction of energy carried by the hadrons that sustain the hadronic cascade.
The influence of subsequent stages of the shower development is found to play a
sub-dominant role. As a consequence, the shower-to-shower distribution of the
muon content is a direct probe of the hadron energy spectrum of interactions
beyond 100 TeV center of mass energies.
