Résumé : A study of the channel e+e- → μ+μ-γISR, where γISR is an initial state radiation photon, is presented using data collected by the DELPHI experiment from 1991 to 1994. The total cross-section at effective annihilation energies (√s′) below the Z0 peak is obtained by using the events with relatively hard initial state radiative photon(s) (Eγ > 1 GeV). The differential cross-section as a function of the muon polar production angle is also determined in order to extract the forward-backward asymmetries for the reaction e+e- → μ+μ- at energies √s′ between 20 and 87 GeV. The ratio of the helicity cross-sections σLL+σRR/σRL+σ LR, where the two subscripts stand for the helicities of the incoming e- and outgoing μ- respectively, is extracted from the differential cross-sections in order to test the Standard Model and to look for new physics near 80 GeV. No deviations from the Standard Model were found. © Springer-Verlag 1997.