Performance outcomes, such as distance rowed or kilometres run, can be influenced by the attentional focus an individual adopts. The present study tested how attentional focus during a weight training exercise influenced a direct measure of performance production (muscle activity) and heart rate. Participants executed bicep curls while adopting an associative, dissociative, or no specific attentional focus. Muscle activity, as measured by electromyographic recordings, and heart rate were lower in the associative condition than in the dissociative and control conditions. The participant's level of experience in weight training and the amount of weight lifted had little influence on this pattern of results. The results highlight that attentional focus is an important variable that can influence muscle activity, and ultimately training outcomes, during exercise.