This study examined the anatomical and functional relationships between the abductor pollicis longus (APL) and the abductor pollicis brevis (APB) muscles. The APL has 2 divisions, a distal superficial division and a more proximal deep one. A direct anatomical connection between the extrinsic and intrinsic muscles of the thumb is made up by the deep division of the APL, the APB and the trapezium. Electromyographic (EMG) recordings were made from these muscles to investigate differences and similarities in muscle activation. The EMG activity was recorded with intramuscular wire electrodes during isometric as well as dynamic contractions in different directions, both for the thumb and for the hand. The EMG activity of the right hand of 8 subjects was scaled relative to the mean EMG value at the maximum voluntary isometric contraction in order to compare relative muscle activity in various directions for different subjects. APB and APL were activated in a number of directions for the thumb as well as for the hand. Cooperation between these muscles is necessary to stabilise the trapezium to the carpus in movements of the thumb. APB is activated in movements of the hand to maintain the tension in the deep APL. The deep APL is activated in movements of the thumb to prevent undesired movements of the hand and the forearm. The APB and superficial APL are prime movers of the thumb.