It has been a contentious issue ever since England over-indulged their players in the 2005 Ashes series - lengthy substitutions in the field, so the quick bowlers, and anyone who fancies it, can have a rest.
Traditionally all players had to grin and bear the physical challenge of a
Test; they all had to field, however good or bad they were, and off-field recuperation was distinctly frowned on.
So finally, now, the amount of time that substitutes will spend on the field in international cricket is to be reduced. The ICC has recently altered its playing conditions relating to substitutes so that players will no longer be allowed to have a substitute while they are off the field for a "comfort break".
The Laws of Cricket have not changed and ICC’s tweak is a reaffirmation of what is laid down in any case. Under Law 2.1(c) a substitute fielder may act only for a player who is ill or injured or, at the umpire’s discretion, for "other wholly acceptable reasons". The ICC has stated in its playing regulations that these reasons must be extreme circumstances.
Law 2.1(c) clearly states that substitutes are not allowed if a fielder wants to change his shirt or his boots. The wicketkeeper, the umpires and, if they play well, the batsmen will stay on the field for a whole session without the need for a break.
Nutritionists and sports scientists encourage constant rehydration but drink breaks each hour provide up to a five-minute window for a comfort break, if needed. It is not fair for a fast bowler, who may not be the most agile fielder, to leave the field after a spell and be replaced by an athletic, specialist fielder.
There is confusion over whether a substitute is allowed to keep wicket.
Under the Laws a substitute is not allowed to act for the wicketkeeper - this is in line with the Law that a substitute cannot bat or bowl.
Wicketkeeping is a specialist discipline, like batting or bowling, and so is treated as such under the Laws. But, in professional cricket, there are certain playing conditions where this substitution is allowed.