  We show that the sum rule recently proved by Uraltsev in the heavy quark
limit of QCD holds in relativistic quark models \`a la Bakamjian and Thomas,
that were already shown to satisfy Isgur-Wise scaling and Bjorken sum rule.
This new sum rule provides a {\it rationale} for the lower bound of the slope
of the elastic IW function $\rho^2 \geq {3 \over 4}$ obtained within the BT
formalism some years ago. Uraltsev sum rule suggests an inequality
$|\tau_{3/2}(1)| > |\tau_{1/2}(1)|$. This difference is interpreted in the BT
formalism as due to the Wigner rotation of the light quark spin, independently
of a possible LS force. In BT models, the sum rule convergence is very fast,
the $n = 0$ state giving the essential contribution in most of the
phenomenological potential models. We underline that there is a serious
problem, in the heavy quark limit of QCD, between theory and experiment for the
decays $B \to D^*_{0,1}(broad) \ell \nu$, independently of any model
calculation.
