We analyse the muon longitudinal polarization asymmetry ∆ long in the decay K + → π + µ + µ − . It is stressed that, since the muon helicities are not Lorentz-invariant quantities, the magnitude of ∆ long depends in general on the reference frame. We consider the muon helicities in the LAB system, and study the sensitivity of the longitudinal polarization asymmetry to the flavour-mixing parameters in the Standard Model for stopped and in-flight decaying K + . A similar analysis is carried out for the decay K L → µ + µ − . We find that in both cases the asymmetry is diluted when increasing the energy of the decaying kaons.