The test is performed by tapping the extensor digitorum muscle while the fingers are light or half flexed. A sudden contraction of the musculus extensor digitorum and extension of the fingers indicate a normal reflex.1
An absence of reflex can be an indicator for radiculopathy within the C6 and C7 or neuropathy within the deep branch of the radial nerve.
Extensor Carpi Radialis Longus, Extensor Digitorum Communis Reflex DTR MSR on YouTube https://www.youtube.com/watch?v=ezp_wt8YFJE ↩