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 normalized reflex amplitude increased with an increase in velocity at a given displacement , but remained constant with different displacements at a given velocity .
### Output Text:
The normalized reflex amplitude increased with an increase in velocity at a given displacement , but remained constant with different displacements at a given velocity . 