Stand up arms by your side take a couple of steps, stop, look down. Where are your thumbs pointing? More often than not they will be pointing diagonally across the body. This is indicative of medial (internal) rotation at the shoulder. This is something I see dozens of times a day as a personal trainer.
You might be thinking "Wow thanks Neil but what does that actually mean and should I care!?"
Allow me to elaborate, it basically means that the shoulder is rotated inwards towards the chest in the socket, whether that is something to care about is up to you. Let me explain the impact of this position though so you can decide. If you go to complete any frontal flexion (raising your arm in front of you) in an internally rotated position you will have significantly less room within the shoulder joint. This increases the risk of impingement, in other words you can cause pinching pain in the shoulder. Examples being bursitis, labrum aggravation and even rotator cuff tears.
So now you might be thinking "Alright alright enough with the doom and gloom, how do I fix it!?"
There are three points to address to fix this issue:
1. Stretching the tight muscles.
2. Strengthening the weak muscles.
3. Fix bad daily habits.
1. Stretching the tight muscles, the first step is to identify which ones are tight. This will vary on each individual but the usual suspects are: Pec major/minor, Biceps, Subscapularis, Teres major and Lats. (examples of how to stretch this coming soon) I would suggest getting warm and running through a few stretches to hit these muscles and feel which ones are particularly tight. In order to really develop elongation in the tight muscles hold the static stretches for 60-90 secs. I know that feels like a lifetime when you are doing the stretch but we must overcome stretch reflex to develop length in a static stretch.
2. Strengthening weak muscles, typically the weakness will be the external rotators in rotator cuff (infraspinatus, supraspinatus, teres minor), Rhomboids (major and minor), lower traps and even rear deltoid sometimes (examples to strengthen these coming soon). The intention here is to help externally rotate the shoulder and improve thoracic extension.
3. Daily habits, if you spend a lot of time at a desk or driving you undoubtedly spend a lot of time with your arms extended in front of the body. It is also common for the shoulders to slump forward a little in both situations. Working at a desk got me a treat. So how do you adjust these habits, I'd be foolish to ask you not to work at a desk and to stop driving but you can create feedback loops to adjust your habits.
If you sit in a high backed chair at your desk, sit up straight, pop a tennis ball between your shoulder blades and the chair. If you now hunch forward at all the ball will roll down your back between you and the chair, giving you immediate feedback you have lost your position.
Option two is using string, not literally. The bone at the top of your chest in the middle by the base of the neck, tap it. imagine that bone has a piece of string tied round it and the other end is straight up in the air pulling it up. By elevating the sternum you hold yourself in a better carry position. If you need to set yourself a reminder on your phone to go off every 5 minutes reminding you to keep your chest up this way. You'll soon have the habit dialed in.
This only scratches the surface of the biomechanics at play here, if you want me to go in further detail let me know!
I hope this helps,
B.A.L, Building Athletic Lifestyles
Teres major & Lats: https://www.yoganatomy.com/teres-major-muscle/
Rotator cuff: https://www.shoulderdoc.co.uk/article/384
rhomboids & Lower traps: http://www.kingofthegym.com/back-anatomy/