Often isolation exercise is required to improve contractile ability of a muscle – strength, strength at weaker range, or a mix of both, before that is achieved, ‘normal’ movement, compound or functional, might be faulty. That’s because the body wants to move as efficiently as possible. If there are three muscles that ‘are supposed to’ create a movement, but one of them is injured, the body still wants to do the task/movement at hang, it doesn’t ‘care’ about one muscle missing, it will use the two available. This will achieve the task, but the lacking muscle’s action might create a slightly faulty movement pattern, which isn’t immediately obvious, but with time it will cause damage and pain. When it get’s to this point, until you find an exercise that isolates that lagging muscle, nothing else will improve the compound movement, you’ll just bad practice it. However, when the muscle is isolated and strengthened, then it will be easier to incorporate it into the correct compound one.
Little tip, test a compound exercise, like a squat, if for example one knee feels off, try an isolation exercise for Glute Med, then retest the squat, if there’s improvement, you hit the nail on the head. Just repeat it regularly, test, isolate, retest. The isolation exercise will temporarily activate the lagging muscle, the body will become aware that it is there ready to go, then incorporate it into the compound exercise. If done often enough it can solve the issue.