When it comes to the concept of Taijiquan's humble heart and solid belly, there are many interpretations on the Internet, everyone will have their own understanding of this concept, and I will give you a new interpretation of your dear friends for your reference.
I'll talk about the solid abdomen first, the abdomen is just above the pelvis, and the lumbar vertebrae on the back of the abdomen lack the support of the ribs in front of the thoracic vertebrae, the lumbar spine needs to be further strengthened in front of the abdominal muscles to maintain the human body upright, so the strength here needs to be adducted layer by layer on the horizontal plane, referring to the mandala pattern, the energy in the abdomen is gathered in a circle to the energy center to form a solid abdominal state, from the perspective of human anatomy, the abdominal cross-section from the inside to the outside, there are various internal organs, abdominal walls, various layers of abdominal muscles, and the deep layers have psoas quadratus muscles, etc, the posterior side of the erector spinae dorsi latissimus muscle, each layer of tissue layer by layer tightens its own tension, thereby promoting the vertical stability and flexibility of the lumbar spine. This is also the concept of core strength in the Western concept of exercise, similar to the concept of "Yi Shou Xia Dantian" in ancient Chinese Taoism, which is a solid belly.
The second "humility", the position of the heart is also the position of the Taoist dantian, the thoracic bony tissue is composed of the ribs in front of the posterior thoracic vertebrae, the stability here is more advantageous than the lumbar spine, and the human body has higher requirements for the flexibility of the thoracic cavity, because the narrow chest cavity is not conducive to the human body to breathe air. Therefore, in the position of the chest cavity, it is necessary to use the centrifugal contraction method of human tissue from the inside to the outside to support the chest cavity, and when the human myofascia is centrifugal contraction, the chest cavity muscle reaches a new length, so that the chest cavity is full of strength. Because the chest cavity opens, the lungs have more room to breathe, and the heart beats freely in the pericardium. Referring to the mandala pattern, the energy on the horizontal plane of the chest cavity is centrifugal and expands outward, which can also imagine why a man's mind can be wider than the sky, and the pattern of people can be stretched out step by step with their own experiences.
According to the above explanation, the energy state of the solid abdomen in the abdominal cross-section is the state of centripetal adduction, while in the thorax, the energy structure is the state of centrifugal outward scattering.