This is the way I see it and understand it.
In ALS, NOT all LMN die at the same time and same speed, they do die in stages.
Let’s say for example, when 60% of the LMN that nourish the muscles of your hand die, the rest of the LMN still “alive” try to do their own job and the job of the LMN that already died.
This causes a lot of stress to these motor neurons because they are OVERWORKING trying to make it up for the ones already dead.
This overwork then translates in muscle twitching [fasciculations] which we can see and feel.
Once the last of these motor neurons die, the group of affected muscles no longer receive any nourishment from the motor nerves and waste away (atrophy).
The speed on which all this happens is the one that determines how fast or how slow the ALS progress on people affected by it.
That’s why some PALS say this: “as long as you notice fasciculations, you still have life in those muscles”.
Hope this helps.
Regards,