This invention describes a novel photoplethysmographic system captures a joint shape change representing different positions of bones and tendons near the joint. At least one light source illuminates a joint interface and the light reflected from the joint location is captured by a photodiode sensor. Motions and positions of the bones surrounding the joint can be determined. One joint includes the knuckle. Finger lift-up motions, finger put-down motions, and finger bending events can be determined by monitoring a sensing area at the knuckle joint.
