There is no bending or flexing involved, only standing up straight and looking straight ahead.
Lets go through the measurements one by one.
While taking these measurements, you should be wearing the clothes you are going to be jumping in. Just tell me how you would like the fit and let me do the adjusting. This measurement should be between 17 and 21 inches on men and between 15 and 18 inches on women. Take this one across the back also. A good indicator of a correct measurement is that it should be around an inch of the A-2 measurement.