  Nonperturbative self-energy of a bound quark is computed gauge-invariantly in
the framework of background perturbation theory. The resulting Delta m^2_q is
negative and is a universal function of string tension and current quark mass.
The shift of hadron mass squared, M^2, due to Delta m^2_q is negative, large
for light quarks, and solves the long-standing problem of Regge intercepts in
relativistic quark models.
