Applying an external force to a person's hyperextended fingertip produces electrical activity in the extensor digitorum communis, even if the person does not try to open their hand. Based on this, a finger extensor facilitation technique conducted by therapists was developed. In this study, we developed a finger extensor facilitation training device named iPARKO that imitates this technique. We examined the relationship between the metacarpophalangeal (MP) joint angle of the four fingers and the activities in the extensor digitorum communis resulting from active training using iPARKO. At the same time, the relationship between the MP joint angle and the reduced activities in the flexor digitorum superficialis was also examined. The experiments were conducted on five healthy subjects. It was found that as the MP joint approached its own maximum hyperextension position, the amount of activity of the extensor digitorum communis increased, and the amount of activity of the flexor digitorum superficialis decreased.