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: Stimulation often produces an improvement in the hyper - or dyskinetic upper limbs , but increases the " freezing " phenomenon in the lower limbs at the same time .
### Output Text:
Stimulation often produces an improvement in the hyper - or dyskinetic upper limbs , but increases the " freezing " phenomenon in the lower limbs at the same time . 