QCD Constrains on the Shape of Polarized Quark and Gluon Distributions
  We develop simple analytic representations of the polarized quark and gluon distributions in the nucleon at low $Q^2$ which incorporate general constraints obtained from the requirements of color coherence of gluon couplings at $x \sim 0$ and the helicity retention properties of perturbative QCD couplings at $x \sim 1.$ The unpolarized predictions are similar to the $D_0'$ distributions given by Martin, Roberts, and Stirling. The predictions for the quark helicity distributions are compared with polarized structure functions measured by the E142 experiment at SLAC and the SMC experiment at CERN. 
Stanley J. Brodsky, Matthias Burkardt, and Ivan Schmidt