Spin structure functions and intrinsic motion of the constituents

The spin structure functions of the system of quasifree fermions on mass shell are studied in a consistently covariant approach. Comparison with the basic formulas following from the quark-parton model reveals the importance of the fermion motion inside the target for the correct evaluation of the spin structure functions. In particular it is shown, that regarding the moment $\Gamma_{1}$, both the approaches are equivalent for the static fermions, but differ by the factor 1/3 in the limit of massles fermions ($$, in target rest frame). Some other sum rules are discussed as well.