The Quark Spin Distributions of the Nucleon

The quark helicity measured in polarized deep inelastic scattering is different from the quark spin in the rest frame of the nucleon. We point out that the quark spin distributions $\Delta q_{RF}(x)$ are connected with the quark helicity distributions $\Delta q(x)$ and the quark transversity distributions $\delta q(x)$ by an approximate relation: $\Delta q_{RF}(x) + \Delta q(x)=2 \delta q(x)$. This relation will be useful in order to measure the rest frame (or quark model) spin distributions of the nucleon once the quark helicity distributions and quark transversity distributions are themselves measured. We also calculate the $x$-dependent quark transversity distributions $\delta q(x)$ and quark spin distributions $\Delta q_{RF}(x)$ in a light-cone SU(6) quark-spectator model, and present discussions on possible effect from the sea quark-antiquark pairs.