One thing that gym-goers are often baffled by is the huge loss of strength when they initially pick up something like a minor shoulder injury; wondering why their joint just buckles under a load they previous coped with fine, and the muscles appear to just go weak.
In reality, the muscles haven't suddenly lost all their strength - they may not actually be weak at all; the nervous system's ability to fire and react like it ordinarily does is the difference. The problem could be the muscle, tendon, nerve, joint, a biochemical thing.
What's interesting, and probably why it seems even more extreme and difficult to comprehend to those in a gym environment, is the stronger that person is, the weaker they will become, if something like this happens to them. Yes, you read that correctly...the stronger someone is, the weaker they will become. I could be bench pressing 3 times as much as the next guy, but, if we picked up the same rotator cuff strain that session, I could be equally as weak for that same lift.
This may not make a lot of sense, but consider that my nervous system has remembered and adapted to the stresses placed upon it over time, much in the same way my muscles have; suddenly, there's a spanner stuck in a cog of that neural pathway, and so things just stop functioning as they otherwise should, regardless of accrued muscle mass and strength in other movement patterns.
This lack of function around a joint can make initial diagnosis more difficult. Basic muscle tests are a good way to test function/ dysfunction of a joint, and a good physio can often diagnose the issue quickly, and, if it's something requiring further attention, refer you to a specialist.
The good news is that your training should have stood you in good stead, and the nervous system's memory, and previous adaption, should also aid and hasten rehabilitation, once things are back firing as they should.
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