The problem I have always had with the analogy of a person holding up the books with their back is that a table doesn't have to do "work" to hold up the books, but a person does. They table isn't burning any calories.
The table has tension, though. Ad opposed to a table made of paper which would not exert force on the book--the book would overcome the table's tension and set the table in motion until the floor exerted an upward force.
yeah, so think of your muscles as temporarily pretending to act like wood (rigid), and it just so happens that humans have to expend energy in order to carry out this feat.