  We show that in the presence of massive particles such as nucleons, the
standard low energy expansion in powers of meson momenta and light quark masses
in general only converges in part of the low energy region. The expansion of
the scalar form factor $\sigma(t)$, for instance, breaks down in the vicinity
of $t=4M_\pi^2$. In the language of heavy baryon chiral perturbation theory,
the proper behaviour in the threshold region only results if the multiple
internal line insertions generated by relativistic kinematics are summed up to
all orders. We propose a method that yields a coherent representation
throughout the low energy region while keeping Lorentz and chiral invariance
explicit at all stages. The method is illustrated with a calculation of the
nucleon mass and of the scalar form factor to order $p^4$.
