It's not an easy job, holding still long enough for a drawing to be made. And the models who return to the same pose after breaks I especially admire.
All I can say is, they are either able to rise above the pain of being in the same position for long periods of time, or they are especially blessed with super powers of rest.
Here are a few more charcoal and chalk drawings to share with you, from sessions at Geatches.
(above left)
"Steady"
Charcoal and chalk on cotton paper 17.75"x11"
©2016 Diana Moses Botkin
(second sketch at left)
"At Ease"
Charcoal and chalk on cotton paper 17.75"x12"

(third sketch at left)
"Back Forms"
Charcoal and chalk on cotton paper 17.75"x12"
©2016 Diana Moses Botkin
(bottom sketch at left)
"Form Lights"
Charcoal and chalk on cotton paper 12x17.75"
©2016 Diana Moses Botkin