In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: The evoked tensions correlated with muscle mass ( r ( 2 ) = 0 . 32 , P < 0 . 001 ) , however , not with expression of AChR .
### Output Text:
The evoked tensions correlated with muscle mass ( r ( 2 ) = 0 . 32 , P < 0 . 001 ) , however , not with expression of AChR . 