The Back.

FIG  01

FIG  02

FIG  03

FIG  04

Fig. 04 shows the correct back, called the roach back.
This is a special characteristic of this breed.
The back should be short and strong and the shoulders must be broad, and rather narrow in loin.

The Hind Legs.

FIG  01

FIG  02

FIG  03

FIG  04

FIG  05

FIG  06

Fig. O3 and Fig. 06 shows the perfect hind legs.
The hindquaters should be heavy and muscular, the totall hindlegs should be longer than the fore legs. 
A correct angel of the hindquarters  produces good movement in gait.