oh the fasciculations came after atrophy? I've had clinical weakness since March, but no one would notice yet. Though I cramp a lot
Here's the way it works. First, a motor nerve in your brain is destroyed. Since it no longer feeds a muscle, that nerve pathway and muscle become totally useless, paralyzed, limp, and won't do anything anymore. Keep in mind that actions in the foot or hand use any of nearly 100 muscles, so one muscle dead can be seen as "weakening" of the limb, when in fact it is actually partially paralyzed.
The unused muscles loss their "muscle tone" so they appear gone where before you had a stiff useful muscle.
Second, the nervous system might try to recruit new nerves. This causes twitches for a little while as the nerves attempt to rebuild. But it always fails. More muscles get paralyzed until your whole extremity is limp and useless. Once paralyzed, a muscle never comes back; it's gone forever.
In this way described above, the process is gradual. However, most people won't notice it gradually, they'll just fall over or drop coffee cups and the weakness appears to be sudden. Also, if your doctor doesn't see "obvious and total weakness" in a muscle, it isn't ALS. Clinically, the weakness is undeniable, as the particular muscle involved has totally lost its nerve and suddenly cannot activate.
After being paralyzed awhile, a whole bundle of useless muscles may appear to be a dent or atrophied. But this doesn't always become apparent. When my wife was totally paralyzed, her muscle tone still looked quite beautiful.
All of this process occurs without cramping or pain. There is no sensory sign since the disease only affects motor nerves, not sensory nerves. The pain that ALS patients complain of is typically joint pain after being immobile for quite some time, or bedsores.
So by the time you notice the weakness, your EMG is already dirty.