Analysis of deformations in terms of principal directions appears well suited for biological tissues that
present an underlying anatomical structure of fiber arrangement. We applied this concept here to study
deformation of the beating heart in vivo analyzing 30 subjects that underwent accurate three-dimensional
echocardiographic recording of the left ventricle. Results show that strain develops predominantly along
the principal direction with a much smaller transversal strain, indicating an underlying anisotropic, onedimensional contractile activity. The strain-line pattern closely resembles the helical anatomical structure
of the heart muscle. These findings demonstrate that cardiac contraction occurs along spatially variable
paths and suggest a potential clinical significance of the principal strain concept for the assessment of
mechanical cardiac function. The same concept can help in characterizing the relation between functional
and anatomical properties of biological tissues, as well as fiber-reinforced engineered materials.
