Karate involves a lot of powerful thrusting movements that tighten the chest and lats. Over time this can lead to postural problems unless care is taken to both stretch the chest and lats and to condition the muscles that pull the shoulders down and back towards the spine. Over conditioned and tight chest muscles paired with under conditioned shoulder and back muscles can even contribute to shoulder instability and injury.
Here is an exercise that can help “realign” us and hopefully help to keep us injury free and happily punching for a long time to come; it is called the ball cobra:
Notice the slow tempo, try to perform it the same way.