Hi, Jeff.
I'm a Physical Therapist, and my husband Hal was a runner and also lifted heavy weights prior to being diagnosed with ALS last August.
When you look for an answer about how much to exercise, and how to best slow down muscle loss, inevitably you find lots of conflicting advice. That's because everyone with ALS is different, and even within the individual, the specific way the disease is working changes over time. My understanding is that through whatever process, PALS' motor neurons are being made to die. We want to optimize the environment for the remaining motor neurons, to keep them functioning as long as possible. In addition, we want to keep the remaining muscle tissue as healthy as possible.
To do that, in the ideal situation, we exercise the healthy muscle enough to keep it from atrophying through disuse. We include stretches to maintain flexibility, as well as enough strength or functional usage to meet the muscle's current abilities. This is different from the way an healthy person who doesn't have ALS would go about doing a strength program. In the case of the person without ALS, the best way to maximize strength gains would be to go beyond the current level of the muscle. The muscle would be injured, and would send out signals - primarily chemical or hormonal - that would trigger responses resulting in repair and building of stronger muscle tissue. For PALS, the chemicals or the response systems are messed up in some way. So too much exercise can both injure the muscle and possibly accelerate the destruction of remaining muscle.
So you have to find a good level of exercises - that maintain what you have, both in ROM and strength. You can go beyond, gently, in ROM, to carefully increase flexibility, but it is probably best to be more careful when it comes to strength. If you can walk, continue to walk. But not so much that you are sore, stiff, and tired later. If you like to and can lift weights, do them at a level where you can do 25 reps or more, and don't go to complete exhaustion. And be ready and willing to lower the amount of weight you are lifting when you find the weight is becoming more challenging. Don't let any prior bodybuilder or athletic training experience lead you to ignore fatigue. This disease is progressive. The last thing you want to do is speed up the progression. Stay as healthy as you can, get lots of rest, and exercise in moderation. . .