  Significant differences between transverse and longitudinal polarized parton
distributions are found at low energies within a light-front covariant quark
model of the nucleon. These differences are due to relativistic spin effects
introduced by the Melosh rotations and survive evolution to higher $Q^2$
scales. A specific observable related to double-spin asymmetries in lepton pair
production in polarized hadron-hadron collisions is defined. The possibility of
assessing the relevance of these relativistic spin effects in future
experiments at RHIC and HERA--$\vec{N}$ is discussed.
