Question: “My left shoulder and chest have a muscle imbalance. I played baseball for many
years and pitched. I naturally did a lot of exercise to build my armstrength and neglected my left arm. What solution can you give me to balance my left side muscle (shoulder/chest)?”
Answer: It is inevitable that an athlete will experience some form of muscle imbalance or another. Baseball is one of the best examples of this. Many baseball related training regiments are ancient compared to the wealth of information that we have resulting in imbalances as well as shoulder injuries. In your case, a positive sign is that you don’t have any serious injuries to the right side. Rotator cuff injuries and imbalances are very common among pitchers. Assuming that there are no injuries to worry about, balancing the two sides should not be too hard. The first thing to look at is your posture. Since you are dominant on the right side, more likely than not, your right trapezius muscle will be significantly more tense than the left side. The right side of your chest will likely be more tense with your right upper back being loose.
The first thing I would work on is making sure that both sides are balances. Work on strengthening the right side of your upper back and stretching the chest. Also work on stretching the right side of the traps as well as strengthening the left side.
Secondly, I would work on stabilization exercises for both shoulders but specifically the right side. Pitching causes some extreme flexibility and stability is very important for someone who is going to weight train.
Finally, when training with weights, I would focus on dumbbell exercises as well as cable movements as long as each arm works independently. Machine exercises and bar exercises may exasperate the problem since you may be able to compensate for the imbalance by using the stronger arm.
Following these simple tips should bring the two sides closer together. Your right side will always be a little bit stronger but that is natural. Possibly a more important issue may deal with the muscle imbalance within the right shoulder. Here is a good article discussing that topic: