From the perspective of "bone", firstly, it has the meaning of skeleton, that is to say, the structure of each word in regular script should be straight and generous, and it should not appear askew or cramped. Second, it mainly refers to the strength of each stroke itself, which must be strong and smooth, and there must be no weakness.
The strength of "tendon" is contained in the strokes rather than directly exposed, so "tendon" is more implicit than "bone".
The so-called "bones and muscles" mainly represent the power of a pen. Calligraphy appreciation, the strength of strokes is very particular. When we appreciate calligraphy works, if we look at the stroke order of each word, it is easy to imagine the calligrapher's writing state: whether to carefully describe the details of strokes or write confident points and lines with confidence?
For China's ancient literary tradition, implication represents a higher style. If it is too explicit and lacks meaning, it is easy to be considered that the realm is not high.
As far as calligraphy tradition is concerned, "Yan Jin" is more closely related to calligraphy in the early Tang Dynasty. It is generally believed that Yan Zhenqing directly absorbed a lot of nutrients from Chu Suiliang, and "Liugu" is more related to the inscriptions on regular script in Yan Zhenqing's later period, but it is somewhat different from the previous calligraphy tradition.
For calligraphy learning, it is relatively easy to trace back from Yan Zhenqing's calligraphy to the previous calligraphy tradition and transfer to cursive learning; If you practice Liu Gongquan's calligraphy from the beginning, it is easy to practice a neat structure, but if you want to learn cursive script, it is more difficult to appreciate the sense of flow in the strokes, and it may seem a little stiff at the joints and turning points of the strokes.
Summary: In any case, understanding the "bones and muscles" in strokes is the only way to learn calligraphy, and the strength of writing is the premise of further learning various fonts, so it is indeed a better way for beginners to start with Yan Liu's regular script.