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: Postural tremor was measured with the arm horizontally outstretched rest tremor with the arm supported by an armrest and finally tremor was measured after holding a 2 - kg weight until exhaustion .
### Output Text:
Postural tremor was measured with the arm horizontally outstretched rest tremor with the arm supported by an armrest and finally tremor was measured after holding a 2 - kg weight until exhaustion . 