Thumb Motion and Typing Forces during Text Messaging on a Mobile Phone

The objective of the study was to determine the thumb motion and typing forces when text messaging on a mobile phone. Thumb motion was captured using a 6-camera reflective marker based motion capture system. Typing forces were measured using four load cells in a force plate arrangement fitted into the phone. Thumb flexion-extension occurred mainly in the IP and MCP joints. The maximum flexion for IP joint was 20.2 degrees at key '*' and was 18.3 degrees at key '#' for MCP joint. Abduction-adduction of MCP and CMC joints were smaller than flexion-extension and thumb opposition. The maximum abduction for MCP and CMC joints were 7.2 degrees and 8.7 degrees respectively at key '*'. The maximum thumb opposition for MCP and CMC joints were 17.0 degrees at key '4' and 13.5 degrees at key '9' respectively. The incidence of peak force was shown to link to high flexion angle of the IP joint and in thumb opposition of the MCP joint. No incidence of peak forces was found in the right column of the keypad.