The purpose of this study was to observe changes in electromyographic activity appeared in shoulder and scapular muscles when one wears no wrist extension orthosis, short wrist extension orthosis and long wrist extension orthosis when performing two model assembling operations. The study subjects were 16 adults who use right hand and did not have any functional disorder on their wrist or the upper limbs without any orthopedic surgery. The study subjects were asked to do the two model assembling operations without wrist extension orthosis, with a short wrist extension orthosis and with a long wrist extension orthosis. As a result of repetitive assembling operation that requires shoulder movement, electromyographic activity in upper trapezius and serratus anterior increased significantly when the subjects wore the short and long wrist extension orthoses compared to none. When the subjects performed repetitive assembling operation that requires shoulder stability, electromyographic activity in upper trapezius, lower trapezius, serratus anterior, and anterior deltoid showed significant increase when they wore the short and long wrist extension orthoses compared to none. Especially, the upper trapezius showed significantly high electromyographic activity when they wore the long wrist extension orthosis compared to a short one.