Verbal instructions given to individuals who are learning motor tasks have two types of attentional focuses, externally-focused and internally focused cues. Instructions with an external focus of attention refer to the effects of body movements instead of the movements themselves while internally focused cues directly address the movement of the body or specific body parts (Poolton, Maxwell, Masters, & Raab, 2005). Sam Leahey has good examples of internal and external cues for various exercises and movements in this article. Whether the use of externally-focused versus internally-focused cues is more efficacious in the learning of motor tasks is controversial in the research.
Like many topics in Kinesiology, I believe the answer to which attentional focus is better, internal or external, is it depends. What is the purpose of performing the targeted skill? If one is performing a squat, is he or she trying to move better, increase aesthetics (i.e., muscle size), or achieve a weightlifting PR? If a client is doing squats to move better, internal cues may be more effective for optimizing positioning of the knees, feet, and chest. In clients with aesthetic goals, the aim of squatting may be to increase gluteus hypertrophy. Research performed by Calatayud et al. (2016) demonstrated that an internal focus of attention on using certain muscles while performing a bench press increased muscle activity in the specific muscles on loads under 80% of 1RM. Lastly, when the goal of squatting is maximal strength, an external attentional focus is likely to result in better performance according to a study by Halperin, Williams, Martin, and Chapman (2015). They observed that participants with external focuses of attention exerted 9% more force on an isometric mid-thigh pulling exercise compared to participants with internal focuses.
In many studies, performance outcome measures are used to demonstrate the effectiveness of external attentional focus. In research by Wulf, Gartner, McConnel, and Schwarz (2002) task success is measured by how accurately participants hit target areas. The study by Poolton et al. (2005) judged success from accuracy on the putting task. Not all sports training and fitness activities have these kinds of outcome-based goals. Many who weightlift can display impressive amounts of strength despite under-recruiting certain muscles like the gluteal muscles or use of risky techniques. Internal cuing may be warranted to improve recruitment of proper muscles. Volleyball players can focus on jumping faster and higher all day, but if they exhibit knee valgus or other form flaws their success potential is limited and their injury risk is heightened. Internal cues may be necessary to bring a conscious focus back to a pattern that was learned incorrectly.
Calatayud, J., Vinstrup, J., Jakobsen, M. D., Sundstrup, E., Brandt, M…Anderson, L. L. (2016). Importance of mind-muscle connection during progressive resistance training. European Journal of Applied Physiology, 116(3): 527-533.
Halperin, I., Williams, K., Martin, D. T., & Chapman, D. W. (2015). The effects of attentional focusing instructions on force production during the isometric mid-thigh pull. Journal of Strength & Conditioning Research, DOI: 10.1519/JSC.0000000000001194
Poolton, J. M., Maxwell, J. P., Masters, R. S., & Raab, M. (2005). Benefits of an external focus of attention: Common coding or conscious processing? Journal of Sports Sciences, 24(1): 89-99.