Can martial arts training be considered a weapon in a court of law?

No, Martial Arts training is not a weapon.

However, many jurisdictions in the US do consider the use of hands and feet as enough to increase the charge from assault (aka, simple assault) to aggravated assault. This means the use of hands and feet during an assault (whether they are those of a professional or trained martial artist or a normal person) can be charged as a felony. Courts have been reluctant to rule that footwear is a dangerous weapon, though a pair of shoes certainly can increase the damage done by a kick.

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