LOIN AND BACK: BACK FAIRLY BROAD, MUSCLES SLIGHTLY ARCHED OVER THE LOIN.  

Note the loin muscles arching above the spine, as well as the shadow of the muscle at the tuck up. Tails are set low, as a continuation of the spine. The tuck up or waist is well defined, a result of correct structure and muscling. This is another factor of the double suspension gallop.