cadaveric spasm 

cadaveric spasm of hand

FORENSICS

A spasm occurring at the time of death, e.g., clutching clumps of grass at a river bank in a drowning victim (image).

Cadaveric spasm is a controversial post-mortem finding and defined as virtually instantaneous rigor without a preceding flaccid period; it is thought to only occur on a substrate of intense physical or emotional activity just before the terminal event, possibly due to regional depletion of glycogen. It affects different muscle groups differently and may cause limb movement. It is often regarded as the stuff of murder novels. When a decedent is gripping a gun after supposedly killing himself, it is likely that the gun was “planted” and the death was homicidal rather than suicidal.

Synonyms Cataleptic rigidity, cadaveric rigidity, instantaneous rigidity, instantaneous rigor 

Reference https://en.wikipedia.org/wiki/Cadaveric_spasm