  It is shown that the proton "spin crisis'' or "spin puzzle" can be understood
by the relativistic effect of quark transversal motions due to the
Melosh-Wigner rotation. The quark helicity $\Delta q$ measured in polarized
deep inelastic scattering is actually the quark spin in the infinite momentum
frame or in the light-cone formalism, and it is different from the quark spin
in the nucleon rest frame or in the quark model. The flavor asymmetry of the
Melosh-Wigner effect for the valence $u$ and $d$ quarks and the intrinsic sea
$q \bar{q}$ pairs are also the important ingredients in a SU(6)
quark-spectator-diquark model framework to understand the "spin puzzle". Such a
picture of the spin structure can be tested by use of several simple relations
to measure the quark spin distributions in the quark model.
