  The determination of quark angular momentum requires the knowledge of the
generalized parton distribution $E$ in the forward limit. We assume a
connection between this function and the Sivers transverse-momentum
distribution, based on model calculations and theoretical considerations. Using
this assumption, we show that it is possible to fit at the same time nucleon
anomalous magnetic moments and semi-inclusive single-spin asymmetries. This
imposes additional constraints on the Sivers function and opens a plausible way
to quantifying the quark angular momentum.
