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: Tibialis muscle mass was smaller in the P100 group than in the P10 or S groups .
### Output Text:
Tibialis muscle mass was smaller in the P100 group than in the P10 or S groups . 