How to understand "those who are yang, the essence is to nourish the spirit, and the softness is to nourish the muscles"?
Yang Qi, the essence is to nourish the spirit, and the softness is to nourish the muscles" from the ancient book of traditional Chinese medicine "The Yellow Emperor's Neijing" "Su Wen Qi Tongtian Theory". "Essence is to nourish the spirit, and softness is to nourish the tendons" is understood as "to nourish the spirit, and to nourish the tendons is to be soft", that is, the yang energy and the spirit are shrewd and intelligent; Nourishing tendons is soft tendons and veins, and flexion and stretching are free. This phrase is a point in the theory of Chinese medicine that describes the two effects of yang energy.
First of all, "essence is nourishing the spirit" means that when yang energy is in a subtle, quiet state, it helps to nourish the spirit. In the theory of traditional Chinese medicine, God refers to the mental state and consciousness activities of a person, and the subtle state of Yang Qi can provide sufficient nutrients for the god to maintain a sober and energetic mental state.
Secondly, "softness nourishes tendons" means that when yang energy is in a soft and warm state, it helps to maintain tendons and veins. In traditional Chinese medicine, tendons refer to the soft tissues that connect bones and muscles, and the gentle state of yang energy can provide warmth and nutrients to the tendons, keeping them flexible and flexible, thus maintaining normal motor function. 
To sum up, this sentence emphasizes the important role of Yang Qi in maintaining the health of the human body's spirit and muscles. When Yang Qi is in a subtle and gentle state, it can provide sufficient nutrients and warmth to the human body, so that people can maintain a healthy spirit and flexible motor ability. Therefore, in TCM health preservation, it is very important to pay attention to the maintenance of yang qi.