World 400m hurdles champion Jana Pittman has broken down at training on Tuesday night and will miss the national championships in Sydney in a fortnight, her coach Phil King said.

King told AAP Pittman strained a thigh muscle during a session in Melbourne and will be out of action for at least three weeks.

Pittman, 21, has been battling injuries resulting from a misaligned pelvis which have forced her out of successive Telstra A-Series meets in Sydney and Brisbane.

King said Pittman had been diagnosed with a grade one tear of a thigh muscle after pulling up sore during tonight's session.

She was immediately taken for MRI scans, which revealed the problem.

"It's an injury which happened a week ago but it's been aggravated and there is bleeding there," King told AAP.

"She's definitely out of the nationals. But it's a minor strain and in three weeks' time, she'll be able to have all training options available to her.

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