Quark spin and momentum distributions of the nucleon

We analyze the recent European Muon Collaboration (EMC) data on the proton spin asymmetry in a relativistic constituent-quark model of the nucleon. In a convolution approach we calculate the x dependence of the spin-weighted and spin-averaged quark distributions. We estimate the size of orbital angular momentum carried by the valence constituents, 〈Lz〉, to be sizable (about 20--25 % of the proton spin). This depletes the valence-quark contribution to the proton spin, but not enough to account for the spin deficit claimed by the EMC.