Not sure if this is true of all clubs, however, I know that when a player is signing he'll be assessed by club doctor, who will check joints, back, ankles, legs, etc. The player will then go for an MRI scan which will also cover the above plus heart and lung checks.
The MRI scan is only a basic check and not in depth therefore some heart problems can be missed. Plus a player will normally only get this done on signing, 2-3 years down the line, heart problems can develop which is what appears to have happened with Muamba
SUNDERLAND 'TIL I DIE. FTM