There was a very interesting study published in the Journal of Strength and Conditioning Research lately:
In this study, we have used the multimodular measuring system SMART. The system consisted of six infrared cameras, and a wireless module to measure muscle bioelectric activity. Additionally, the path of the barbell was measured with special device called the pantograph. Our study concerns the change in the structure of the flat bench press when the weight of the barbell is increased. The research on the bench press technique included both the causes of the motion: the internal structure of the movement as well as the external-kinematic structure showing the effects of the motion, i.e. all the characteristics of the movement. Twenty healthy, male recreational weight trainers with at least 1 year of lifting experience (the mean +/- SD = 3.3 +/- 1.6 years), were recruited for this study. The subjects had a mean body mass of 80.2 +/- 8.6 kg, an average height of 1.77 +/- 0.08 m, and their average age was 24.7 +/- 0.9 years old. In the measuring session, the participants performed consecutive sets of a single repetition of bench pressing with an increasing load (about 70, 80, 90, and 100% of their 1 repetition maximum - 1RM). The results showed a significant change in the phase structure of the bench press as the barbell weight was increased. While doing the bench press at a 100% 1RM load, the pectoralis major changes from being the prime mover to being the supportive-prime mover. At the same time, the role of the prime mover is taken on by the deltoideus anterior. The triceps brachii, in particular, clearly show a greater involvement.
So what exactly does this mean? Well, the authors of this study found that the more weight an individual bench presses, the less and less the chest remains as the "prime mover." This actually isn't all that surprising, if you think about it, but is important for a few reasons none-the-less.
The bench press is considered a "compound" or "core" movement because it displaces load across multiple muscle groups and joints. When you bench press, the weight gets displaced across not only the pectorals, but the deltoids, triceps, etc. The joints involved include the shoulders and elbows. Compound movements are generally able to move more load due to this loading scheme across the body.
What this study is telling us is that although the pectorals may be one of the larger muscle groups involved in pressing the weight, the workload across the chest seems to peak at around 70% of an individual's 1-rep max, and any loads greater than that, and approaching closer to 100% come from recruiting the "supporting cast" - deltoids, triceps, etc.
Why this finding is important is because the training plan for an individual should be tailored to his or her goals. An individual who wishes to get a bigger chest, for example, may be better off training at less than 70% of their 1-rep max in order to increase overall workout volume without failing out due to fatigue in either the deltoids or triceps.
Not only that, but this also tells us the importance of accessory training. An individual can train their chest at sub-optimal loads all they want, but if there isn't enough effort and time invested in proper triceps and deltoid mechanics, strength, and health, they may struggle to improve their bench press 1-rep max.
Just some food for thought.