Clinical Weakness—ALS is about failing, not feeling.
ALS is about failure—falling down, being unable to stand on your toes or heels, being unable to button your shirt, being unable to lift your hand, being unable to open a ziplock bag, etc. It is not about these things becoming more difficult. It is about these things being impossible… no matter how hard you try. If you can do normal things, but it is more difficult, you do not have ALS. If you used to be able to do 100 curls and now one arm can only do 50; that is not ALS. If you used to run 2 miles and now you can only run 1; that is not ALS. If you used to run 2 miles and now you can’t lift up one of your feet, you may have clinical weakness.
It really does happen that something stops working all of a sudden. It is generally one muscle so it will not be a whole limb, but the movement done by that muscle is suddenly gone. An example is a calf raise. It won't happen. Think of it like your wifi signal. You are surfing the net, then signal is lost and you can't do anything online no matter how hard you try or how long you wait for a page to load. This is what happens to a muscle in beginning ALS it has lost the signal from the nervous system that tells it to work. First it is one muscle, then another ,then another so the things you can’t do increase. This is why you see progressive weakness mentioned