Can someone help me to understand this please:

The stretching tensor (D) is the symmetric part of the velocity gradient (L), and if I derive D from L with using the polar decomposition of the deformation gradient (F=RU) I get the relation:

D=0.5 R (Udot Uinv + Uinv Udot) R' ----(Eq.1) where Udot is the time derivative of U, Uinv is the inverse of U, and the prime indicates the tranpose of the matrix.