Gareth Southgate is determined to get to the bottom of the problem which is causing Jonathan Woodgate cramp.
The Middlesbrough defender has been substituted in three of his last four appearances due to an attack of cramp.
Boro boss Southgate insists the centre-half is not struggling with an injury, but is keen to solve the issue.
"It's a strange one," conceded Southgate to reporters, "but we must remember it's not an injury, it's cramp.
"At the moment we're doing various little tests to see what might be the root cause.
"Cramp is something where it could be one of four or five things causing it and we can only try and eliminate them one by one."
Robert Huth has been replacing Woodgate from the bench, and Southgate admits it is not an ideal scenario.
He added: "Once it sets in, it does make you struggle to perform at the highest level. Psychologically, he's pretty cheesed off with it.
"Robert's done fine when he's come on for Jonathan, but it's not ideal when you've got to change your defence late on in a game."