Shear strain measures how much a material layer deforms by dividing the sideways displacement by the layer height, giving a dimensionless ratio. The top moves 2 m over a height of 4 m, so the shear strain is 2/4 = 1/2 exactly for the stated toy geometry ledger; real soft tissues have nonlinear response, anisotropy, boundary-condition errors, and imaging/measurement uncertainty.
Shear strain begins as a visible displacement-height ratio. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Measure displacement and height
The top layer moves 2 m while the layer height is 4 m.
2m4m
Divide displacement by height
The shear strain is 1/2, an exact dimensionless ratio.