An article by the Dystonia Medical Research Foundation states that hand dystonia is a focal dystonia (a movement disorder in which a person's muscles contract uncontrollably) characterized by excessive, involuntary muscle contractions in the fingers, hand, forearm, and sometimes shoulder. Symptoms usually appear when a person is doing a task that requires fine motor coordination of hand and arm muscles.
Hand dystonia includes a group of disorders called occupational hand dystonias that affect musicians, golfers, or typists. The symptoms often occur only during a specific activity requiring use of the hand, such as playing an instrument, golfing, or typing. Writer’s cramp is a dystonia that is present when a person is writing. Hand dystonia may occur as an isolated focal dystonia or be a part of generalized dystonia.
Researchers at the American Psychological Association found that humans tend to close their fists when they feel threatened or sense conflict. This reaction can also be an automatic response to anger the body exhibits. A fist bump is a gesture of greeting or affirmation in which two people lightly tap each other's clenched fist. A raised fist is a symbol of support and solidarity, and it’s also used to express unity, strength, defiance, or resistance.
A clenched fist has long been the symbol of someone holding on to a real or imagined possession, along with an unwillingness to share. The unwillingness to share should bring to mind the expression; you can hold more in an open hand than in a clenched fist.