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 peak twitch and tetanic tensions were less in the P100 group than in the P10 or S groups , however , tension per milligram of muscle mass was greater in the P100 group than in the S group .
### Output Text:
The evoked peak twitch and tetanic tensions were less in the P100 group than in the P10 or S groups , however , tension per milligram of muscle mass was greater in the P100 group than in the S group . 