Just want to point out that this is not genetic manipulation. Selective breeding has been done for as long as people have kept animals. It's how we have all the dog breeds, docile cattle and chickens that can feed whole families. Corn and many other vegetable crops came around the same way. The gene that caused these cows to be so large was a natural mutation, and there are a handful of humans with a similar trait. Look up muscle baby.
Where genetic manipulation will some day come in is when we people can start getting gene manipulating substances that can turn off that gene in us. From what I understand, the gene in those cows that's not working controls muscle degeneration, so their muscles don't break down when not being used. Imagine how helpful that could be to the elderly population. We may be able to save generations from becoming bedridden... and, once we have a superhuman elderly population, we may be able to use them to pull carts or till fields.
Another possibility would be a drug that inhibits that gene's function, which could be selectively used in small dosages to manage muscle loss.