One issue which has and continues to cause problems with my shoulders is a weak suprispinatus, especially in the left shoulder. This causes numerous issues including pains, winging, overcompensation by other muscles, nerve twinging etc.
One thing I found during rehab which I cannot understand and do now know what it could mean is: when doing shrug type movements, the right side feels fine and natural and I can feel the trap and rear delts working. But the left side: I can feel the trap working, but I feel almost a twinging/jumping reaction in the upper left pec (almost the feeling you get during electrical activation at a physio) and often this also extends up into the left side of the neck.
Any idea what this could be, what it means, what can be done about it?