Why Is One Pec Bigger Than The Other?
While there are a variety of causes accounting for a discontinuity in pec size, a few main reasons stand out. This website is intended to provide some insight into the possible sources of pec unevenness and outline preventive measures.
The most common reason leading to one pec being bigger than the other is actually a pre-existing dissymmetry in the body. If you were to cut your body vertically down the middle, partitioning your body into a left half and a right half (ignoring the difference in pec size) you would find that the left half of your body is not symmetrical with the right half of your body in terms of skeletal structure and other non-muscular related factors. For example, one shoulder may naturally sit up higher than the other shoulder. Or perhaps the left shoulder is shifted further forward or further back than the right shoulder. This dissymmetry in the skeletal structure leads to a discontinuity in muscle size.
To illustrate this point of a dissymmetry in the skeletal structure leading to a difference in muscle size, feel free to participate in a short activity. Casually relax your left shoulder and drop it substantially lower than where it would otherwise rest, while at the same time raising your right shoulder substantially higher than it would comfortably rest. Now flex your chest muscles by attempting to touch your shoulders together. Hopefully you will be able to feel a more intense burn in the upper part of your right pectoral and the lower part of your left pectoral.
So, if you were to consistently go to the gym and work out this way (with your left shoulder far lower than your right shoulder), over time the upper part of your right pectoral would accumulate
more muscle mass than the lower part of your right pectoral, and the lower part of your left pectoral would accumulate more muscle mass than the upper part of your left pectoral. This inconsistency in muscle mass would cause your chest muscles to look lopsided. While this is an extreme example, even a small dissymmetry in the skeletal system can lead to a noticeable difference in pec size over time.
There are numerous types of skeletal dissymmetry that can lead to differences in muscle size. Consider someone bar benching on a flat bench press. When done properly, the bench press primarily works out the chest and triceps; however, the stress of the weight is not necessarily distributed evenly throughout the chest and triceps. For example, a skeletal dissymmetry may put more stress on the left triceps than on the left pectoral and more stress on the right pectoral than on the right triceps. This would cause the right chest muscle to look larger than the left chest muscle. Other types of skeletal dissymmetry cause more stress to the right shoulder, or left upper pec, or left lower pec, or right triceps. So, if you find that the muscles on the left side of your body are not even with the muscles on the right side of your body, this is likely because the bone structure on the left side of your body is not symmetrical with the bone structure on the right side of your body. In order to make your muscles even, you must first make your bone structure even.
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