  Electron-induced one-nucleon knock-out observables are computed for moderate
to high momentum transfer making use of semi-relativistic expressions for the
one-body and two-body meson-exchange current matrix elements. Emphasis is
placed on the semi-relativistic form of the $\Delta$-isobar exchange current
and several prescriptions for the dynamical-equivalent form of the
$\Delta$-propagator are analyzed. To this end, the inclusive transverse
response function, evaluated within the context of the semi-relativistic
approach and using different prescriptions for the $\Delta$-propagator, is
compared with the fully relativistic calculation performed within the scheme of
the relativistic Fermi gas model. It is found that the best approximation
corresponds to using the traditional static $\Delta$-propagator. These
semi-relativistic approaches, which contain important aspects of relativity,
are implemented in a distorted wave analysis of quasielastic $(e,e'p)$
reactions. Final state interactions are incorporated through a phenomenological
optical potential model and relativistic kinematics is assumed when calculating
the energy of the ejected nucleon. The results indicate that meson exchange
currents may modify substantially the $TL$ asymmetry for high missing momentum.
