During a post-mortem examination of a 55-year-old male found in a locked room, the forensic pathologist observes that the body exhibits generalized muscle stiffness that gradually diminishes when the joints are passively flexed. However, in the same body, the right arm shows pronounced flexion of all fingers and wrist with extreme rigidity that cannot be overcome by force. Which feature best distinguishes true rigor mortis from cadaveric spasm in this case?