The answer is nuanced.
The damage to the upper motor neurons themselves is completely irreversible. If you lose the ability to move a muscle entirely, it is gone for good. So PLS involves a base level of weakness that accrues and never goes backwards. The reason they clinically test strength with just one or two tries is because they are testing for the BEST your can put out. Slowly, that best becomes lower. That's your clinical strength score. For me, that's 0-3 in my legs, 2+ to 5 in my arms, 4 in my tongue/jaw, and a very difficult to evaluate level in my torso and neck.
But that doesn't exactly describe our functional ability. Functional ability is complicated. While for medical measurement clinical strength works great, it does not describe what you can DO very well.
The first complication is spasticity. Spasticity results from "go" signal that should be filtered running into the muscle that needs to relax for a motion to happen. (I read somwhere that 80% of your spine's job is telling your muscles NOT to do stuff... heh!) So even if you can't voluntarily use a muscle more than just a twitch, spasticity may cause it to contract anyway or just be stiff. Spasticity is often made better or worse by postures, and some of us got very, very lucky with how our posture and spasticity work. I learned to floor-to-chair transfer from a quad that could use posture to spaz his ab muscles into a position he could pull his body up. My torso spasticity kicks in most when I twist, so even though I can't sit up on my own traditionally, I can twist, my whole torso will go stiff, and push myself up with my arms almost upright before I sag again. Similarly, even my strength 1 (just twitch, no voluntary movement) mucles spazz my legs straight if I lever my weight over them. So despite having leg strengths that on paper should not allow standing, if I have the right equipment to lever my body up there, I can hold a standing position. What I can't do is voluntarily sit down or relax those muscles... to get down I have to lean back until my posture gets out of the spastic position and then my legs collapse and I fall.
There's also purely mechanical strengths of your bones and ligaments. Quads with totally paralyzed hands but just a little wrist strength can use a grip called tenodesis. You can experiment with this at home by letting your hand hang down relaxed, then pulling your hand up just by the wrist. You'll see your fingers curl. That's not muscle action at all - just mechanical action. C5/c6 quads use it to hold coke cans, pick things up, etc. Hand contracture done in a careful way can actaully make a decently strong tenodesis grip.
Technique is the third variable. Just because you can't do something in the normal fasion, because you lost that strength, doesn't mean you can't learn to use an alternative set of muscles to do it. Disability tends to go like this: "I can't do it at all!" "Oh, that guy can do it, how does he do it?" "Hey, this is easy once you practice, it just looks funny." The peak of functional loss is right after a new impairment, before you learn to get around the problem.
Environment is the fourth complication. There are many things you can functionally do at home, but only because you set your home up to maximize your abilities. You can be very competent at using the bathroom at home, but unable to do the more difficult transfer to a public bathroom's toilet. Grab bars, meds, everything you use at home is set up for you. Your utensils, food, remotes, everything you buy is based on what you can use. "In the world", it set at best to an arbitrary ADA standard that might do very little to address your needs.
Complications themselves are the fifth complication. I have strength 5 in my biceps, 2+ or 3- in my triceps, and 4- in the muscles right at the top right of my back. That is terrible for my shoulders. Joints expect close to equal strength on each side, and are pulled out of alignment when that isn't true. Arthritis and other damage will occur to joints used with less than full strength. Spasticity makes you that much more likely to pull a tendon or strain something. The pain and debility from those complications will wax and wane with activity and how careful you are.
Fatigue is of course a variable. We fatigue easily, and we demand more out of the muscles we can control than they are necessarily prepared to give. When you do all arm motions with your biceps, they get tired quickly. Neurological fatigue can also be a feature of PLS. My doc gives me provigil for it, which takes off the edge. What is a normal activity for someone else is often overdoing it for us. Like many people with chronic illnesses, we can push through today and wipe out our resources for the near future. A normal day can be like running a marathon.
Especially for those of us with the slow progression of PLS, another problem creeps up: mucles just strong enough to try, but not enough to win. I can only sit up in my chair for two hours or so, then its bed for the rest of the day. Full quads do better than I do! My doctor explained there are over one hundred muscles involved in just sitting, and mine aren't strong enough to do it right, but they won't stop trying. So sitting up fatigues me and puts me in a lot of pain, and will until something closer to paralysis sets in.
When it comes to it, sometimes we just have bad days. Normal, able bodied people do too. Our bad days just cause us to be even weaker, and it's more noticeable. Really, really bad days where your neurology is just wonky are called "exascerbations". PLSers and HSPers aren't supposed to get them, but 30% of us do anyway. Exascerbation symptoms wear off and we go back to our baseline strength. We have good days, where we get the most out of what we have left.
We also can't discount depression. The SPF claims most people with PLS and HSP are on antidepressants. After my diagnosis, it was the first thing my doctor talked to me about. Depression makes it's own physical symptoms. My hubby has major depression, and on his bad days he can't cut a pill without his hands shaking, or vacuum the apartment. His symptoms wax and wane like crazy, and if we end up with episodes of depression, our physicality will too.
But ultimately in PLS, your baseline just gets slowly worse or stays the same. It doesn't get better, it doesn't come and go. We can be so well adjusted that it surprises us... I didn't realize until my eye exam that I couldn't hold my head level enough to see through both holes simo, my head is permanently tilted one way or the other. We can look better or worse than that baseline based on environment, technique, how much energy we have, how enthused we are about our task, etc. So functional ability can change radically, but baseline strength only goes one direction: down.
