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The objective of this study was to investigate the influence of forearm, hand and thumb positions on electromyography (EMG) activity of the abductor pollicis longus (APL) and the extensor carpi ulnaris (ECU) muscles. A second objective was to study the role of these muscles in stabilizing the wrist during movements of the thumb. This knowledge is important in clinical assessment and reconstructive surgery. At a constant force of 5 N for the thumb and 20 N for the hand, EMG activity was recorded with intramuscular wire electrodes in isometric and dynamic contractions in different positions and movements in the thumb and the hand. EMG activity of the right hand of 7 subjects was scaled relative to the mean EMG value at maximum voluntary isometric contraction to compare relative muscle activity in the tests. The results show that the position of the forearm does influence activity of the APL, but not of the ECU. The deep part of the APL shows differences in EMG activity between the positions of the forearm in dorsal flexion of the hand and the superficial part of the APL in palmar flexion of the hand. The ECU and the deep part of the APL are very active during movements and isometric contractions of the thumb. It is suggested that these muscles are necessary to stabilize the wrist during movements of the thumb to prevent undesired movements of the hand and the forearm.