The Dalitz-plot density of a sample of 3973 $K_{\mu 3}^{+}$ decays in flight at 1.8 GeV/c ($K^{+}\to {\pi }^0{\mu }^{+}{\nu }_{\mu }$) was studied to determine ${\lambda }_{+}$ and ${\lambda }_0$, the linear coefficients of the $t$ dependence of the vector and scalar form factors, where $t$ is the four-momentum transfer to the lepton pair. The ${\mu }^{+}$ was detected by scintillator hodoscopes, identified by thick-plate optical spark chambers and momentum analyzed in a magnetic spectrometer. The ${\pi }^0$ was detected by its decay into two $\gamma$'s which converted in a lead-optical-spark-chamber-scintillator sandwich detector. The measured coefficients were ${\lambda }_{+}=0.050\mathrm{}\pm 0.013$ and ${\lambda }_0=0.029\mathrm{}\pm 0.011$. The ${\chi }^2$ of the fit was 63 for 57 degrees of freedom.

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