The upper part of your chest muscle has fibers that are angled more than the lower part, which might mean they pull in slightly different ways and work differently when you move.

In dead human bodies, the muscle fibers in the upper part of the chest muscle are angled more sharply than those in the lower part — this difference might affect how the muscle pulls and generates strength in each area.

When people do flat bench press, the lower part of their chest muscle grows more than the upper part - and this is the opposite of what happens when they do incline bench press.

In the chest muscle, the upper part has longer fibers toward the bottom, and the lower part has its longest fibers in the middle — this might mean different parts of the muscle do different jobs.

When you do bench press on an incline, the upper part of your chest works harder. But when you do flat bench press, the middle-lower part of your chest works harder instead. Scientists measured this using muscle activity recordings.

When you change the angle of the bench press, it might make the different parts of your chest muscle work in uneven ways, potentially causing one part to activate differently than the other.