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: Muscle tone was examined using a combined mechanoand electromyographic method , which measured simultaneously the muscle resistance ( MMG ) of the rat ' s hind foot to passive extension and flexion in the ankle joint and the electromyographic activity ( EMG ) of the antagonistic muscles of that joint : gastrocnemius and tibialis anterior .
### Output Text:
Muscle tone was examined using a combined mechanoand electromyographic method , which measured simultaneously the muscle resistance ( MMG ) of the rat ' s hind foot to passive extension and flexion in the ankle joint and the electromyographic activity ( EMG ) of the antagonistic muscles of that joint : gastrocnemius and tibialis anterior . 