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