We show that transport measurements carried out with and without a low appl
ied magnetic field enable one to characterize the vortex dynamics in high-T
-c superconducting films. The application of a low magnetic field induces a
modulation of the critical current that depends on the process involved wh
en the vortices are set in motion. Current-voltage measurements with no app
lied field yield information on the pinning energy, the pinning sites densi
ty, and the way the vortices are moving. Measurements carried out on a set
of YBa2Cu3O7 microbridges show that the vortex dynamics depend primarily on
the pinning sites density. This quantity proves to be strongly dependent o
n the thickness of the superconducting film.