Raul has been sleeping in a special oxygen room which simulates conditions of altitude in order to boost his endurance levels.
The technique was originally favoured by cyclists preparing for mountain climbs, and marathon runners such as Paula Radcliffe are also believed to have used the method. It is understood Raul spends one or two nights a week in the conditions, but that there is no mobile room for him to use prior to away games.
The room has a revolutionary air-conditioning system which pumps out air containing lower than normal levels of oxygen and slighter higher than normal levels of nitrogen. The result is an increase in red blood cells meaning oxygen reaches the muscles more quickly, delaying fatigue and speeding a player's recovery.