Human performance is limited by attention and automaticity. Attention is a conscious mental engagement in cognitive or motor task, while automaticity occurs when the same task is performed without requiring attention resources.
Theories of attention propose that people have difficulty doing several tasks at one time, due to the bottle-neck effect where information is filtered out, or an alternate theory that suggests attention limits are a result of a limited number of resources.
Attention limits human performance primarily due to the fact that we are limited by attention in terms of the number of activities we can undertake simultaneously. Attention encompasses the perceptual, cognitive and motor activities associated with performing skills. There is a trade off between these components which results in a human performance limitation.
Expertise occurs when an individual is able to circumvent human limitations such as attention. An expert will take activities which regular people need to think about and perform them with automaticity. This increases the number of activities they are able to undertake simultaneously, and allows them to perform at a higher level. This expertise may occur through cue association and the detection and correction of errors.
Specific to human performance in complex skills such as Olympic Weightlifting movements (sometimes called 'greasing the groove'), the persistent and repeated practice of movement will reduce the attentional requirement and increase automaticity and thus performance.