  Microscopic spin interactions on a deformed Kagom\'{e} lattice of volborthite
are investigated through magnetoelastic couplings. A negative longitudinal
magnetostriction $\Delta L<0$ in the $b$ axis is observed, which depends on the
magnetization $M$ with a peculiar relation of $\Delta L/L \propto M^{1.3}$.
Based on the exchange striction model, it is argued that the negative
magnetostriction originates from a pantograph-like lattice change of the
Cu-O-Cu chain in the $b$ axis, and that the peculiar dependence arises from the
local spin correlation. This idea is supported by DFT+$U$ calculations
simulating the lattice change and a finite-size calculation of the spin
correlation, indicating that the recently proposed coupled-trimer model is a
plausible one.
