After the diagnosed I tried to gain back strength even though I was told not to .It didn't take long for me to find out how wrong I was . I realized that I was losing strength rapidly in the left arm instead of gaining it back
My neuro (and PT) told me that we build muscle by stressing the muscle, temporarily weakening it. In normal circumstances, the muscle rebounds stronger than before. But with ALS, when you stress and weaken a muscle, it remains in the weakened state. There is no rebound. So, instead of building muscle you are tearing it down, and hastening the body's deterioration.
Any muscles that are unaffected by ALS can still be strengthened by exercise, but you have to be careful, because sometimes the ALS process is going on in areas you are unaware of.