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: This study tested the hypothesis that activation of nociceptive muscle afferent fibers would be linked to an increased excitability of the human jaw - stretch reflex and whether this process would be sensitive to length and velocity of the stretch .
### Output Text:
This study tested the hypothesis that activation of nociceptive muscle afferent fibers would be linked to an increased excitability of the human jaw - stretch reflex and whether this process would be sensitive to length and velocity of the stretch . 