  Star formation depends strongly on both the local environment of galaxies and
the internal dynamics of the interstellar medium. To disentangle the two
effects, we obtained, in the framework of the AMIGA project, Halpha and Gunn r
photometric data for more than 200 spiral galaxies lying in very low-density
regions of the local Universe. We characterise the Halpha emission, tracing
current star formation, of the 45 largest and least inclined galaxies observed
for which we estimate the torques between the gas and the bulk of the optical
matter. We subsequently study the Halpha morphological aspect of these isolated
spiral galaxies. Using Fourier analysis, we focus on the modes of the spiral
arms and also on the strength of the bars, computing the torques between the
gas and newly formed stars (Halpha), and the bulk of the optical matter (Gunn
r).
