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 maximum velocity of contractile element shortening increased from 1 . 68 + / - 0 . 11 ml / s to 2 . 18 + / - 0 . 21 muscle - lengths / s ( P less than 0 . 05 ) and is consistent with an increase in contractility .
### Output Text:
The maximum velocity of contractile element shortening increased from 1 . 68 + / - 0 . 11 ml / s to 2 . 18 + / - 0 . 21 muscle - lengths / s ( P less than 0 . 05 ) and is consistent with an increase in contractility . 