The proximal phalange of the thumb was used to measure thumb position/orientation relative to the forearm. However, the thumb is composed of more than just the proximal phalange and articulates about multiple joints and bones. As a result, the electrogoniometer in this study was measuring over three joints (the MCP-joint, the CMC-joint and the radiocarpeal joint), so it was possible that the measured position/orientation at the proximal phalange could have contributions from more than one joint.
However, it has been shown (Jonsson et al., 2007) that an electrogoniometer has validity for measuring simple thumb movements and that electrogoniometers can provide quantitative information on thumb movements during thumb intensive activities such as writing an SMS message on a mobile phone.
5. Conclusions
In summary, we found that the thumb, forearm and trapezius muscle activity during SMS messaging was relatively low. Relative to sitting, entering an SMS while standing increased trapezius muscle activity. Females, compared to males, typically had higher muscle activity levels and they tended to work in greater thumb abduction, to move their thumbs with higher velocities and to have fewer pauses in the thumb movements. Subjects with musculoskeletal symptoms had lower muscle activity levels in the abductor pollicis longus and tended to move their thumbs with higher velocities and to have fewer pauses in the thumb movements compared to those without symptoms.