  In the previous paper, based on the $SU(2)_{f}\times SU(2)_{s}$ heavy quark
symmetries of the QCD Lagrangian in the heavy quark limit, the Bethe-Salpeter
equation for the heavy baryon $\Lambda_b$ was established with the picture that
$\Lambda_b$ is composed of a heavy quark and a scalar light diquark. In the
present work, we apply this model to calculate $\mu_\pi^2$ for $\Lambda_b$, the
average kinetic energy of the heavy quark inside $\Lambda_{b}$. This quantity
is particularly interesting since it can be measured in experiments and since
it contributes to the inclusive semileptonic decays of $\Lambda_b$ when
contributions from higher order terms in $1/M_b$ expansions are taken into
account and consequently influences the determination of the
Cabibbo-Kobayashi-Maskawa matrix elements $V_{ub}$ and $V_{cb}$. We find that
$\mu_\pi^2$ for $\Lambda_b$ is $0.25GeV^2$ $\sim$ $0.95GeV^2$, depending on the
parameters in the model including the light diquark mass and the interaction
strength between the heavy quark and the light diquark in the kernel of the BS
equation. We also find that this result is consistent with the value of
$\mu_\pi^2$ for $\Lambda_b$ which is derived from the experimental value of
$\mu_\pi^2$ for the $B$ meson with the aid of the heavy quark effective theory.
