There has been great debate over the issue of hyperflexion or Rollkur in Dressage, with vets, scientists, riders and animal behaviourists suggesting that it causes pain, stress and physical damage to the horse.
A recent Dutch-US collaborative study examined video of horses competing in the Grand Prix at the 1992 Olympic Games and the 2008 World Cup Final. In 2008 horses were observed to be behind the vertical (BHV) in all paces whereas in 1992 this was only true for collected canter and collected trot.In Piaffe in 2008 the further BHV the horse was the higher the score.
So despite all the concern and a number of controlled studies, it appears that FEI Dressage judges not only tolerate horses being BHV but actually reward this with higher marks in some cases.
The abstract of this study can be found here: CLICK HERE