  Recent analyses of low-mass eclipsing binary stars have unveiled a
significant disagreement between the observations and the predictions of
stellar structure models. Results show that theoretical models underestimate
the radii and overestimate the effective temperatures of low-mass stars but
yield luminosities that accord with observations. A hypothesis based upon the
effects of stellar activity was put forward to explain the discrepancies. In
this paper we study the existence of the same trend in single active stars and
provide a consistent scenario to explain systematic differences between active
and inactive stars in the H-R diagram reported earlier. The analysis is done
using single field stars of spectral types late-K and M and computing their
bolometric magnitudes and temperatures through infrared colours and spectral
indices. The properties of the stars in samples of active and inactive stars
are compared statistically to reveal systematic differences. After accounting
for a number of possible bias effects, active stars are shown to be cooler than
inactive stars of similar luminosity therefore implying a larger radius as
well, in proportions that are in excellent agreement with those found from
eclipsing binaries. The present results generalise the existence of strong
radius and temperature dependences on stellar activity to the entire population
of low-mass stars, regardless of their membership in close binary systems.
