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Upper limb morbidities within the breast cancer population can interfere with completing daily life activities. Current knowledge of upper limb capabilities is limited; previous increases in muscle activation on the affected cancer side suggest this population works at a higher fraction of their capability. The purposes of this study were to describe upper limb capabilities and dysfunction of breast cancer survivors through muscle activation monitoring via surface electromyography and muscle-specific strength tests during functional tasks.Fifty survivors performed 88 dynamic tasks (divided into range of motion-reach or rotate, activities of daily life and work tasks). Muscle activation was examined for functional and strength testing tasks.Total muscle effort (summation of integrated electromyography across measured muscles) was up to 5.1% greater on the affected side during work tasks (p = 0.0258). Increased activations existed in posterior deltoid, supraspinatus, upper trapezius and serratus anterior (p < 0.05) for several tasks, including daily living tasks. Reduced activation occurred in affected pectoralis major sternal during all tasks (p < 0.0001–0.0032), and affected infraspinatus in all but daily living tasks (p = 0.0002–0.0328). The affected side infraspinatus, supraspinatus and upper trapezius muscles demonstrated significant reductions in targeted strength testing (p = 0.0001–0.0057).Both primary and secondary muscles (outside surgery and radiation fields) were affected. In general, this population works at higher levels of muscle effort for the affected side yet demonstrates weakness in strength testing, which may reflect tissue damage. Strengthening exercises for the posterior rotator cuff and upper trapezius may be the most beneficial.