Testing times: Michael Clarke squats in the nets as he tests his back. Photo: Getty Images

Michael Clarke's back injury is serious, team physiotherapist Alex Kountouris has confirmed, but the Australian captain has decided to recommence his innings on the second day in Adelaide.

Clarke, who has had an injection to relieve the pain, shuffled into the nets before play on day two and decided afterwards he was capable of resuming his innings. He suffered a recurrence of his chronic back problem, which he has managed since he was a teenager, and was forced to retire hurt on 60 on Tuesday.

Bad back: Michael Clarke with team doctor Peter Brukner and physio Alex Kountouris. Photo: Getty Images

Kountouris said Clarke had suffered a relapse of the injury that struck him before the 2013 Ashes in England, which forced him to miss the entire Champions Trophy tournament. 

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But he said it was not directly related to the left hamstring problem he has carried over the past two months. 

"As everyone knows, he has got a chronic back injury and he dived to get back into the crease when David Warner was on 99, he felt it a little bit there and then just tried to get out of the way of a bouncer and it really grabbed," Kountouris said.

"Unfortunately it is quite a significant back injury. He hasn't had anything like this for 18 months. The last time he had this was in the Champions Trophy in England. It's poor timing and he is struggling at the moment."

Clarke suffered crippling back spasms and medical staff worked late into the night in an effort to free him up so he can bat again in the first Test. 

Cricket Australia is confronting questions about whether Clarke should have played on such little preparation, given he would have missed the series opener if it had started as originally scheduled last week. 

"The hamstring injury is on the other side. He has been carrying a left hamstring injury and this is his right lower back. This is his old injury, it's what he has had in the past with his back. I don't think it is directly related to his hamstring," Kountouris said. 

"You don't want to be a man down, so he is very determined to give it a go and see how he goes."

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