Argh, could just be a smidgen of sarcopenia, just the natural process of aging when you don't use muscle enough and lose it over time, combined with possible lowered testosterone levels. Sarcopenia is thought to be "normal" though, or rather expected, unless you regularly contract the skeletal muscle in an intense manner to defy it, or even undo it. Sarcopenia is really just age-related atrophy, but can be slowed, or even reversed.
Well, unfortunately the issue is the best weapon for atrophy is the use of said fibers, and every fiber in a contraction is used to lift 100% of the weight possible. That just means that you have a 1 rep max as using more muscle than any other weight or movement, so that's gonna be the tricky part...Heavy weights, like going to no more than 6-8 reps per set...The more fibers you use, the more the muscles get the signal to hold on to the fibers, as they're needed...I DO know other muscle preservation techniques that are of some help, though, to start with for now.
Protein is numero uno, as muscles are more or less made of it, the reason could be lack of use or lack of nutrition, so either you're not using muscle that was there that used to be needed, or you're not fueling it properly so the body is disposing of it as the nutrition isn't supporting it, or both. Those are the two prime reasons to lose muscle in most cases, barring any other major affliction (cachexia is from cancer/AIDS, etc...and dystrophy is the deterioration of muscle, and can be a disease..."Muscular dystrophy" is actually a redundant term, as dystrophy mostly refers to muscle anyways, or the heart muscle, as technically it's a muscle too)...
Protein will help the most in regards to anything non-weight related. Also of assistance is the amino acid in particular (again, a component of protein as you know) L-Glutamine, it has the highest potency in muscle preservation/repair of any, and is the most abundant amino acid as well. In theory, supplementation isn't a requirement if you eat mass amounts of protein, but, a tasteless powder supplement of L-Glutamine is out there and I buy some from Walmart even for just a few bucks for a little tub.
As far as cardio, the best cardio for upper body is rowing, but that's not just something you can do without a machine or boat usually, unlike running or biking...Running and low intensity cardio can actually have atrophy affect, as the muscles over time adapt to a decrease in mass for efficiency purposes. You see this effect in Marathon runners, whereas sprinters almost see the opposite, they grow muscle. Running in and of itself helps mostly with endurance and fat oxidation (burning), and less with preservation..it can actually exacerbate the issue if you're not careful. The exception would be if you, like a sprinter, go heavier and harder and faster. Doing sprints or jogging with a weight vest/weighted back pack can have the effect. The reason is the difference between fast and slow twitch fibers.
The more you tell your body you want slow twitch use (slow, long running) the more it'll adapt to that need, likewise, the more you tell it it needs short bursts of contraction (fast twitch) the more it'll prepare/evolve for that need...ie: more fibers used at once, and more is saved, and you may even build muscle, as opposed to long jogs which can actually have the affect of atrophy over time.
If anything for now Drew, the protein/diet should help, and going as short yet hard with any exercise/weights that you can handle will help.
That 5 lb weight may seem measly, but if you did periods where you moved it as fast as you could for a couple minutes straight while jogging in place, it's better for muscle preservation than 30 minutes of a slow jog. Mixing the two should strike a good balance of fat burning and muscle preservation though, you'll have to find that balance and watch the effects to see if it tilts too far in either direction, from cardio to heavy, heavy lifting. The sweet spot in the middle ground. Try some bodyweight exercises if you can't do heavy weights just yet, too.
Oh, and as for form/use, the major muscles making up the most of the upper body are the back and chest, followed by shoulders, and finally followed by the arms. So curls are very minor compared to a motion that works the chest or back (a tennis type swing, parallel to the ground when standing is working the chest, while rowing or pulling down/pull up movements work the back, move the weight over and over with those motions until you're fatigued, it'll be a nice mix of cardio and muscle preservation).
