Strain in two dimensions. As heart muscle is incompressible, systolic shortening has to be balanced by simultaneous wall thickening, and diastolic stretching by thinning, as shown in the upper panel. As the wall gets thinner by diastolic stretching, the transverse diameter of the left ventricle increases. As the wall stretching (and hence thinning) propagates from the base to the apex, the early filling phase can be illustrated as a sequence as shown in the lower panel. The slope of the thin line across illustrates the strain rate propagation.