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: While she was weak , 2 - Hz repetitive stimulation revealed a decrement without significant facilitation at rapid rates or after exercise , suggesting postsynaptic neuromuscular blockade .
### Output Text:
While she was weak , 2 - Hz repetitive stimulation revealed a decrement without significant facilitation at rapid rates or after exercise , suggesting postsynaptic neuromuscular blockade . 