  The energy conditions provide a very promising model-independent study of the
current acceleration of the universe. However, in order to connect these
conditions with observations, one often needs first to integrate them, and then
find the corresponding constraints on some observational variables, such as the
distance modulus. Those integral forms can be misleading, and great caution is
needed when one interprets them physically. A typical example is that the
transition point of the deceleration parameter $q(z)$ is at about $z \simeq
0.76$ in the $\Lambda$CDM model. However, with the same model when we consider
the dimensionless Hubble parameter $E(z)$, which involves the integration of
$q(z)$, we find that $E(z)$ does not cross the line of $q(z) = 0$ before $z =
2$. Therefore, to get the correct result, we cannot use the latter to determine
the transition point. With these in mind, we carefully study the constraints
from the energy conditions, and find that, among other things, the current
observational data indeed strongly indicate that our universe has ocne
experienced an accelerating expansion phase between the epoch of galaxy
formation and the present.
