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1.
Acting with bravado over email when in a face to face situation the emailer would be in a subservient position.

Related to beer muscles where the internet is substituted for alcohol.

Can also take the form of “message board muscles”
Example: a man in an email to his friends claims his wife does what she is told. In reality the man is a follower. Dave has email muscles when it comes to his wife.

by CMFuss May 01, 2008
 
2.
false bravado
When someone becomes "tough" and "combative" in an email, but never in person, or on the phone. A false show of digital power and might. A manifestation of their own sense of righteous indignation.
Email Muscles
Ninja 1 in an email format: "I asked you for that report last week you never responded, I'm losing confidence in you and your firm & will take my business else where."

Ninja 1 in a phone conversation: "Hey buddy, I know you have been busy, any chance of getting that report any time this week? No rush!"
by paranoidrain June 24, 2011