Why Is My Dog Losing Muscle Mass And Having Trouble Walking?

Its frustrating not knowing if your dog is in pain or if something is wrong. I just recently adopted my current dog from a shelter. He was abused and neglected, which I knew. But right as I was walking out the door to bring him home the volunteer I had working with said " oh he also needs some xrays he has some leg problems it looks like. This disturbed me for a few reasons  but mainly two. The dog was going to be put down, that's a lot of the reason I initially adopted him, he had been at the shelter for 9 months, 6 months straight, and then adopted for about a month and brought back for 3 more until I got him. 9 months seems like an awfully long time for a dog that might be in pain or at least discomfort every day. I know that shelters are overcrowded and understaffed, and under funded but that just doesn't seem fair to me. I feel as though honestly that is neglectful. I asked them some more questions, like well is he hurting right now? All the time? What could cause this? Is it his hips?  They told me that it only seemed to hurt him after a lot of activity, and when he first woke up or got up from staying still. So yea all the time.  This also bothered me because I want to help every animal I can, but not one that may need very expensive operations or medications. I understand that accidents happen, and having a dog is expensive. I had wanted one of my own for 8 years, but 5000 dollars right off the bat to get new hips I did not possess. As ranty as this is, placing an animal with some one and not fully disclosing their health is not fair. I knew I couldn't leave him here now, but I was worried. What if they hadn't said anything? He is fine now and regaining his muscle mass but it still gives out sometimes a bit. My dog had lost the majority of the muscle mass in his back legs, but this was a result of being tied up so that he couldn't walk anywhere so from not using the muscles he lost them. Make sure he is getting plenty of exercise, not strenuous, just strengthening daily movement. Sorry short answer, long rant.