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: Following placement of subcutaneous electrodes in each animal ' s left gastrocnemius muscle , rigidity was assessed by analyzing root - mean - square electromyographic activity .
### Output Text:
Following placement of subcutaneous electrodes in each animal ' s left gastrocnemius muscle , rigidity was assessed by analyzing root - mean - square electromyographic activity . 