Most bodybuilders understand that the deltoids can be divided into three regions. Interestingly enough, research has shown that these regions can be further divided, with both the middle and posterior head having three distinct subregions. Therefore, the middle deltoid head plays a role in almost all shoulder exercises, but is mostly isolated in things like lateral raises and wide grip upright rows. The anterior and posterior delts are involved in shoulder flexion and extension, respectively. However, each one becomes more isolated as the hands move over the head; this range of motion reduces the contribution of other muscles.
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▪️ Source: Brown, J. M. M., Wickham, J. B., McAndrew, D. J., & Huang, X. F. (2007). Muscles within muscles: coordination of 19 muscle segments within three shoulder muscles during isometric motor tasks. Journal of Electromyography and Kinesiology, 17(1), 57-73.