A muscle has to complete its full action. Everyone thinks of the biceps as the muscle that bends the elbow. But part of its job is also to supinate the hand(turn the palm upward). If you turn the palm down and bend the elbow, you’re shutting off the biceps and using a muscle called the brachioradialis to bend the elbow. This is because you’re not allowing the biceps to complete its full action so it doesn’t do anything. This is why doing a pull-up is easier with your palms facing you. With muscles, it’s all or nothing. Muscles don’t compromise with you. Another example of this are the glutes(butt muscles). When you’re lying prone on your stomach and do a press up, not a push-up, a press-up(lifting up your upper body with your arms keeping your lower body on the ground), your butt is probably contracted. Part of the job of the gluteus maximus is to externally rotate the hip(point the toe out). So, when lying prone(on your stomach), if you point your toes in, this shuts off the glutes and they will not be recruited at all when doing a press up. After a stroke, I have no idea. This would still be the case for some muscles but maybe not for all because all the wires are different as a result of us having to rewire the brain.