A device for detecting the posture a finger or forces applied to a finger, the finger having a fingernail illuminated by light, comprises at least one photodetector for measuring a change in light reflected by an area of the finger beneath the fingernail in response to the posture of the finger or forces applied to the finger. The photodetector provides a signal corresponding to the change in light reflected. The device also includes a processor for receiving the signal and determining whether the change corresponds to a specified condition. The photodetector may be enclosed in a housing and coupled to the fingernail.