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.
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!"
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.