Abstract: The biomechanics is the science of applying the mechanical principles and methods to quantitatively study mechanical problems in organisms. This paper points out that the constitutive relationship and the stress-growth relationship are the two core issues in the biomechanics. The meaning of the constitutive relationship can be explained by using three examples: the "blood is thicker than water", the "artery hardening" and the "bone under loading"; and the meaning of the stress-growth relationship can be explained by using other three examples: the "bone growth", the "hard Qigong" and the "hypertension and cardiovascular diseases".