The human body has a naturally occurring complement of muscle as determined in the first instance by genetics, gender and age. You can train the body to increase the amount of muscle by doing exercises that stimulate muscle growth. Even so, muscle growth in response to work will still depend on your gender, age and genes.
In bodybuilding and weight training, people who do not naturally carry, or easily enhance muscle, are often called "hardgainers." This sounds a little derogatory, but it is more a statement of fact. People with a lean rather than solid natural build are categorized scientifically as ectomorphs. The more muscled builds are mesomorphs. Those that carry more fat naturally might be endomorphs. But don't panic, there are many shades in between, and you are not destined to a life of a skinny ectomorph, although ectomorphs are probably never going to be Mr. Universe, steroids aside.
Whatever your propensity to carry, build and maintain muscle, here are some tips on how not to lose that muscle once you've gained it.
From around the time of 40 we start to lose muscle mass naturally, probably as the male hormone testosterone declines in concert with lower levels of physical activity. Natural loss my be in the order of 3% to 10% of muscle mass per decade after age 50 and perhaps even more for those in less than ideal health. Seniors can prevent or at least slow this natural state of loss by staying active.
Work out with weights two to three times each week, exercising all major muscle groups. Allow two days between workouts if possible. Seniors also need to ensure they eat well and get the recommended amount of protein for their activity level. About 0.5 gm per pound of body weight each day is about right. (1.1 gm/kg/bw/day)
2. Eat Right to Prevent Muscle Loss
While eating sufficient protein is important, and heavy trainers like athletes might need a little more protein than that above, eating sufficient energy is probably even more important. If you don't eat (and drink) sufficiently to maintain body weight in accord with how much energy you expend in day-to-day living, including physical activity, you will lose muscle and probably bone -- and fat of course. It can be a little tricky losing fat while maintaining muscle, but weight training certainly helps you hang onto that muscle in those circumstances.
If you're a sports athlete or serious recreational athlete, you need to determine an ideal weight for your activity, keep a watch on the scales, and adjust diet and exercise accordingly.
3. Train to Minimize Muscle Loss
Muscle has a nice store of available glucose -- and I don't mean just in glycogen storage, which is the first port of call for muscle energy. When you run out of the glucose stored in muscle, and blood glucose and liver glucose are also low, the body knows it can get more glucose from muscle protein to maintain the brain and other important organs. And that's exactly what it does: the hormone cortisol breaks down muscle into amino acids, then another hormone, glucagon, strips amino acids and turns the carbon skeleton into glucose. The body needs to do this to ensure a steady supply of glucose.
However, this is not good for muscle maintenance -- or muscle building for that matter. Don't train hard on an empty or fasted stomach consistently. If you do, take in an energy drink while you train to prevent this process called "gluconeogenesis" from occurring. There is also a risk of this with low-carbohdyrate diets.
Refuelling after exercise is also important. Taking in some protein and carbohydrate within an hour of your workout (and sufficiently beyond that to refuel) will help to ensure muscle maintenance, and even growth as you get an insulin spike.
4. Get Sufficient Sleep and Relaxation
Sleep is a time of rebuilding. Hormones like testosterone and human growth hormone set about rebuilding and repairing the organism. Restful sleep helps with this process. Make sure you get it. Emotional stress will induce catabolic stress hormones, which means more destruction of muscle if you're not careful.
5. Limit Alcohol Intake
This is not to say you can't have a drink, but excessive alcohol intake does not do your muscle load any good at all. Apart from all the other destructive effects of excessive alcohol consumption, boozing raises estrogen levels and knocks around your testosterone. More muscle loss.