  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 magnetic moments and
semi-inclusive single-spin asymmetries. This imposes additional constraints on
the Sivers function and opens a plausible way to quantifying quark angular
momentum.
