Script is produced through the combined operation of finger, wrist, and arm movements. The shape of each letter is produced by the fine movement of the fingers while movement at the wrist and forearm produce the horizontal movement across the page. Various muscles are involved in these movements. Other than muscles to abduct, adduct, flex, and extend the fingers, Flexor Carpi Ulnaris (FCU) and Flexor Carpi Radialis (FCR) have roles in wrist movement and Abductor Pollicis Brevis (APB) functions as an abductor of the thumb. The purpose of this study is to probe the importance and role of these three muscles in handwriting activity by studying the muscles' signal variability. Fifteen college students were selected as the subjects of this study. The subjects were required to perform writing task in two different forms: uppercase letters and lowercase letters. During writing, the muscles' activities due to wrist and thumb movement were recorded. The Root Mean Square (RMS) values were extracted and the variability of muscle activity during writing based on letter types were analysed. It has been found that the APB muscle showed higher variability than FCU and FCR muscles. This leads to the inference that the APB muscle was actively involved in dynamic process of handwriting whereas the FCR and FCU muscles were just act as the stabilizer. However, the differences in signal variability among the three muscles were not significant. This indicates that even though both wrist and thumb movements have different functions in handwriting activity, they are equally important. No significant difference in muscle activity between the two writing tasks was observed. This study concludes that the APB, FCR and FCU muscles can be considered when studying muscle performance due to handwriting activity. For that purpose, any letter case can be used in the writing task.

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