Data on muon pair production obtained by the OPAL collaboration at and around the Z peak are analysed. Small angular mismatches between the directions of the two muons are used to estimate the effects of initial state photon radiation and initial-final state radiation interference on the forward-backward asymmetry of muon pair production and its dependence on the invariant mass of the pair. Effective vector and axial-vector couplings of the Z bozon are determined in a model-independent way and compared to the Standard Model expectations.