  Using flavor twisted boundary conditions, we study nucleon matrix elements of
the vector current. We twist only the active quarks that couple to the current.
Finite volume corrections due to twisted boundary conditions are determined
using partially twisted, partially quenched, heavy baryon chiral perturbation
theory, which we develop for the graded group SU(7|5). Asymptotically these
corrections are exponentially small in the volume, but can become pronounced
for small twist angles. Utilizing the Breit frame does not mitigate volume
corrections to nucleon vector current matrix elements. The derived expressions
will allow for better controlled extractions of the isovector magnetic moment
and the electromagnetic radii from simulations at zero lattice momentum. Our
formalism, moreover, can be applied to any nucleon matrix elements.
